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/linux-master/scripts/coccinelle/api/ | ||
H A D | stream_open.cocci | diff 3975b097 Wed Jun 19 11:26:56 MDT 2019 Kirill Smelkov <kirr@nexedi.com> *: convert stream-like files -> stream_open, even if they use noop_llseek This patch continues 10dce8af3422 (fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock) and c5bf68fe0c86 (*: convert stream-like files from nonseekable_open -> stream_open) and teaches steam_open.cocci to consider files as being stream-like not only if they have .llseek=no_llseek, but also if they have .llseek=noop_llseek. This is safe to do: the comment about noop_llseek says This is an implementation of ->llseek useable for the rare special case when userspace expects the seek to succeed but the (device) file is actually not able to perform the seek. In this case you use noop_llseek() instead of falling back to the default implementation of ->llseek. and in general noop_llseek was massively added to drivers in 6038f373a3dc (llseek: automatically add .llseek fop) when changing default for NULL .llseek from NOP to no_llseek with the idea to avoid breaking compatibility, if maybe some user-space program was using lseek on a device without caring about the result, but caring if it was an error or not. Amended semantic patch produces two changes when applied tree-wide: drivers/hid/hid-sensor-custom.c:690:8-24: WARNING: hid_sensor_custom_fops: .read() has stream semantic; safe to change nonseekable_open -> stream_open. drivers/input/mousedev.c:564:1-17: ERROR: mousedev_fops: .read() can deadlock .write(); change nonseekable_open -> stream_open to fix. Cc: Julia Lawall <Julia.Lawall@lip6.fr> Cc: Jan Blunck <jblunck@suse.de> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Jiri Kosina <jikos@kernel.org> Cc: Jonathan Cameron <jic23@kernel.org> Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com> Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com> Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> 10dce8af Tue Mar 26 16:20:43 MDT 2019 Kirill Smelkov <kirr@nexedi.com> fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX") added locking for file.f_pos access and in particular made concurrent read and write not possible - now both those functions take f_pos lock for the whole run, and so if e.g. a read is blocked waiting for data, write will deadlock waiting for that read to complete. This caused regression for stream-like files where previously read and write could run simultaneously, but after that patch could not do so anymore. See e.g. commit 581d21a2d02a ("xenbus: fix deadlock on writes to /proc/xen/xenbus") which fixes such regression for particular case of /proc/xen/xenbus. The patch that added f_pos lock in 2014 did so to guarantee POSIX thread safety for read/write/lseek and added the locking to file descriptors of all regular files. In 2014 that thread-safety problem was not new as it was already discussed earlier in 2006. However even though 2006'th version of Linus's patch was adding f_pos locking "only for files that are marked seekable with FMODE_LSEEK (thus avoiding the stream-like objects like pipes and sockets)", the 2014 version - the one that actually made it into the tree as 9c225f2655e3 - is doing so irregardless of whether a file is seekable or not. See https://lore.kernel.org/lkml/53022DB1.4070805@gmail.com/ https://lwn.net/Articles/180387 https://lwn.net/Articles/180396 for historic context. The reason that it did so is, probably, that there are many files that are marked non-seekable, but e.g. their read implementation actually depends on knowing current position to correctly handle the read. Some examples: kernel/power/user.c snapshot_read fs/debugfs/file.c u32_array_read fs/fuse/control.c fuse_conn_waiting_read + ... drivers/hwmon/asus_atk0110.c atk_debugfs_ggrp_read arch/s390/hypfs/inode.c hypfs_read_iter ... Despite that, many nonseekable_open users implement read and write with pure stream semantics - they don't depend on passed ppos at all. And for those cases where read could wait for something inside, it creates a situation similar to xenbus - the write could be never made to go until read is done, and read is waiting for some, potentially external, event, for potentially unbounded time -> deadlock. Besides xenbus, there are 14 such places in the kernel that I've found with semantic patch (see below): drivers/xen/evtchn.c:667:8-24: ERROR: evtchn_fops: .read() can deadlock .write() drivers/isdn/capi/capi.c:963:8-24: ERROR: capi_fops: .read() can deadlock .write() drivers/input/evdev.c:527:1-17: ERROR: evdev_fops: .read() can deadlock .write() drivers/char/pcmcia/cm4000_cs.c:1685:7-23: ERROR: cm4000_fops: .read() can deadlock .write() net/rfkill/core.c:1146:8-24: ERROR: rfkill_fops: .read() can deadlock .write() drivers/s390/char/fs3270.c:488:1-17: ERROR: fs3270_fops: .read() can deadlock .write() drivers/usb/misc/ldusb.c:310:1-17: ERROR: ld_usb_fops: .read() can deadlock .write() drivers/hid/uhid.c:635:1-17: ERROR: uhid_fops: .read() can deadlock .write() net/batman-adv/icmp_socket.c:80:1-17: ERROR: batadv_fops: .read() can deadlock .write() drivers/media/rc/lirc_dev.c:198:1-17: ERROR: lirc_fops: .read() can deadlock .write() drivers/leds/uleds.c:77:1-17: ERROR: uleds_fops: .read() can deadlock .write() drivers/input/misc/uinput.c:400:1-17: ERROR: uinput_fops: .read() can deadlock .write() drivers/infiniband/core/user_mad.c:985:7-23: ERROR: umad_fops: .read() can deadlock .write() drivers/gnss/core.c:45:1-17: ERROR: gnss_fops: .read() can deadlock .write() In addition to the cases above another regression caused by f_pos locking is that now FUSE filesystems that implement open with FOPEN_NONSEEKABLE flag, can no longer implement bidirectional stream-like files - for the same reason as above e.g. read can deadlock write locking on file.f_pos in the kernel. FUSE's FOPEN_NONSEEKABLE was added in 2008 in a7c1b990f715 ("fuse: implement nonseekable open") to support OSSPD. OSSPD implements /dev/dsp in userspace with FOPEN_NONSEEKABLE flag, with corresponding read and write routines not depending on current position at all, and with both read and write being potentially blocking operations: See https://github.com/libfuse/osspd https://lwn.net/Articles/308445 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1406 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1438-L1477 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1479-L1510 Corresponding libfuse example/test also describes FOPEN_NONSEEKABLE as "somewhat pipe-like files ..." with read handler not using offset. However that test implements only read without write and cannot exercise the deadlock scenario: https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L124-L131 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L146-L163 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L209-L216 I've actually hit the read vs write deadlock for real while implementing my FUSE filesystem where there is /head/watch file, for which open creates separate bidirectional socket-like stream in between filesystem and its user with both read and write being later performed simultaneously. And there it is semantically not easy to split the stream into two separate read-only and write-only channels: https://lab.nexedi.com/kirr/wendelin.core/blob/f13aa600/wcfs/wcfs.go#L88-169 Let's fix this regression. The plan is: 1. We can't change nonseekable_open to include &~FMODE_ATOMIC_POS - doing so would break many in-kernel nonseekable_open users which actually use ppos in read/write handlers. 2. Add stream_open() to kernel to open stream-like non-seekable file descriptors. Read and write on such file descriptors would never use nor change ppos. And with that property on stream-like files read and write will be running without taking f_pos lock - i.e. read and write could be running simultaneously. 3. With semantic patch search and convert to stream_open all in-kernel nonseekable_open users for which read and write actually do not depend on ppos and where there is no other methods in file_operations which assume @offset access. 4. Add FOPEN_STREAM to fs/fuse/ and open in-kernel file-descriptors via steam_open if that bit is present in filesystem open reply. It was tempting to change fs/fuse/ open handler to use stream_open instead of nonseekable_open on just FOPEN_NONSEEKABLE flags, but grepping through Debian codesearch shows users of FOPEN_NONSEEKABLE, and in particular GVFS which actually uses offset in its read and write handlers https://codesearch.debian.net/search?q=-%3Enonseekable+%3D https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1080 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1247-1346 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1399-1481 so if we would do such a change it will break a real user. 5. Add stream_open and FOPEN_STREAM handling to stable kernels starting from v3.14+ (the kernel where 9c225f2655 first appeared). This will allow to patch OSSPD and other FUSE filesystems that provide stream-like files to return FOPEN_STREAM | FOPEN_NONSEEKABLE in their open handler and this way avoid the deadlock on all kernel versions. This should work because fs/fuse/ ignores unknown open flags returned from a filesystem and so passing FOPEN_STREAM to a kernel that is not aware of this flag cannot hurt. In turn the kernel that is not aware of FOPEN_STREAM will be < v3.14 where just FOPEN_NONSEEKABLE is sufficient to implement streams without read vs write deadlock. This patch adds stream_open, converts /proc/xen/xenbus to it and adds semantic patch to automatically locate in-kernel places that are either required to be converted due to read vs write deadlock, or that are just safe to be converted because read and write do not use ppos and there are no other funky methods in file_operations. Regarding semantic patch I've verified each generated change manually - that it is correct to convert - and each other nonseekable_open instance left - that it is either not correct to convert there, or that it is not converted due to current stream_open.cocci limitations. The script also does not convert files that should be valid to convert, but that currently have .llseek = noop_llseek or generic_file_llseek for unknown reason despite file being opened with nonseekable_open (e.g. drivers/input/mousedev.c) Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Yongzhi Pan <panyongzhi@gmail.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: David Vrabel <david.vrabel@citrix.com> Cc: Juergen Gross <jgross@suse.com> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Tejun Heo <tj@kernel.org> Cc: Kirill Tkhai <ktkhai@virtuozzo.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Christoph Hellwig <hch@lst.de> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Julia Lawall <Julia.Lawall@lip6.fr> Cc: Nikolaus Rath <Nikolaus@rath.org> Cc: Han-Wen Nienhuys <hanwen@google.com> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> 10dce8af Tue Mar 26 16:20:43 MDT 2019 Kirill Smelkov <kirr@nexedi.com> fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX") added locking for file.f_pos access and in particular made concurrent read and write not possible - now both those functions take f_pos lock for the whole run, and so if e.g. a read is blocked waiting for data, write will deadlock waiting for that read to complete. This caused regression for stream-like files where previously read and write could run simultaneously, but after that patch could not do so anymore. See e.g. commit 581d21a2d02a ("xenbus: fix deadlock on writes to /proc/xen/xenbus") which fixes such regression for particular case of /proc/xen/xenbus. The patch that added f_pos lock in 2014 did so to guarantee POSIX thread safety for read/write/lseek and added the locking to file descriptors of all regular files. In 2014 that thread-safety problem was not new as it was already discussed earlier in 2006. However even though 2006'th version of Linus's patch was adding f_pos locking "only for files that are marked seekable with FMODE_LSEEK (thus avoiding the stream-like objects like pipes and sockets)", the 2014 version - the one that actually made it into the tree as 9c225f2655e3 - is doing so irregardless of whether a file is seekable or not. See https://lore.kernel.org/lkml/53022DB1.4070805@gmail.com/ https://lwn.net/Articles/180387 https://lwn.net/Articles/180396 for historic context. The reason that it did so is, probably, that there are many files that are marked non-seekable, but e.g. their read implementation actually depends on knowing current position to correctly handle the read. Some examples: kernel/power/user.c snapshot_read fs/debugfs/file.c u32_array_read fs/fuse/control.c fuse_conn_waiting_read + ... drivers/hwmon/asus_atk0110.c atk_debugfs_ggrp_read arch/s390/hypfs/inode.c hypfs_read_iter ... Despite that, many nonseekable_open users implement read and write with pure stream semantics - they don't depend on passed ppos at all. And for those cases where read could wait for something inside, it creates a situation similar to xenbus - the write could be never made to go until read is done, and read is waiting for some, potentially external, event, for potentially unbounded time -> deadlock. Besides xenbus, there are 14 such places in the kernel that I've found with semantic patch (see below): drivers/xen/evtchn.c:667:8-24: ERROR: evtchn_fops: .read() can deadlock .write() drivers/isdn/capi/capi.c:963:8-24: ERROR: capi_fops: .read() can deadlock .write() drivers/input/evdev.c:527:1-17: ERROR: evdev_fops: .read() can deadlock .write() drivers/char/pcmcia/cm4000_cs.c:1685:7-23: ERROR: cm4000_fops: .read() can deadlock .write() net/rfkill/core.c:1146:8-24: ERROR: rfkill_fops: .read() can deadlock .write() drivers/s390/char/fs3270.c:488:1-17: ERROR: fs3270_fops: .read() can deadlock .write() drivers/usb/misc/ldusb.c:310:1-17: ERROR: ld_usb_fops: .read() can deadlock .write() drivers/hid/uhid.c:635:1-17: ERROR: uhid_fops: .read() can deadlock .write() net/batman-adv/icmp_socket.c:80:1-17: ERROR: batadv_fops: .read() can deadlock .write() drivers/media/rc/lirc_dev.c:198:1-17: ERROR: lirc_fops: .read() can deadlock .write() drivers/leds/uleds.c:77:1-17: ERROR: uleds_fops: .read() can deadlock .write() drivers/input/misc/uinput.c:400:1-17: ERROR: uinput_fops: .read() can deadlock .write() drivers/infiniband/core/user_mad.c:985:7-23: ERROR: umad_fops: .read() can deadlock .write() drivers/gnss/core.c:45:1-17: ERROR: gnss_fops: .read() can deadlock .write() In addition to the cases above another regression caused by f_pos locking is that now FUSE filesystems that implement open with FOPEN_NONSEEKABLE flag, can no longer implement bidirectional stream-like files - for the same reason as above e.g. read can deadlock write locking on file.f_pos in the kernel. FUSE's FOPEN_NONSEEKABLE was added in 2008 in a7c1b990f715 ("fuse: implement nonseekable open") to support OSSPD. OSSPD implements /dev/dsp in userspace with FOPEN_NONSEEKABLE flag, with corresponding read and write routines not depending on current position at all, and with both read and write being potentially blocking operations: See https://github.com/libfuse/osspd https://lwn.net/Articles/308445 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1406 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1438-L1477 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1479-L1510 Corresponding libfuse example/test also describes FOPEN_NONSEEKABLE as "somewhat pipe-like files ..." with read handler not using offset. However that test implements only read without write and cannot exercise the deadlock scenario: https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L124-L131 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L146-L163 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L209-L216 I've actually hit the read vs write deadlock for real while implementing my FUSE filesystem where there is /head/watch file, for which open creates separate bidirectional socket-like stream in between filesystem and its user with both read and write being later performed simultaneously. And there it is semantically not easy to split the stream into two separate read-only and write-only channels: https://lab.nexedi.com/kirr/wendelin.core/blob/f13aa600/wcfs/wcfs.go#L88-169 Let's fix this regression. The plan is: 1. We can't change nonseekable_open to include &~FMODE_ATOMIC_POS - doing so would break many in-kernel nonseekable_open users which actually use ppos in read/write handlers. 2. Add stream_open() to kernel to open stream-like non-seekable file descriptors. Read and write on such file descriptors would never use nor change ppos. And with that property on stream-like files read and write will be running without taking f_pos lock - i.e. read and write could be running simultaneously. 3. With semantic patch search and convert to stream_open all in-kernel nonseekable_open users for which read and write actually do not depend on ppos and where there is no other methods in file_operations which assume @offset access. 4. Add FOPEN_STREAM to fs/fuse/ and open in-kernel file-descriptors via steam_open if that bit is present in filesystem open reply. It was tempting to change fs/fuse/ open handler to use stream_open instead of nonseekable_open on just FOPEN_NONSEEKABLE flags, but grepping through Debian codesearch shows users of FOPEN_NONSEEKABLE, and in particular GVFS which actually uses offset in its read and write handlers https://codesearch.debian.net/search?q=-%3Enonseekable+%3D https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1080 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1247-1346 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1399-1481 so if we would do such a change it will break a real user. 5. Add stream_open and FOPEN_STREAM handling to stable kernels starting from v3.14+ (the kernel where 9c225f2655 first appeared). This will allow to patch OSSPD and other FUSE filesystems that provide stream-like files to return FOPEN_STREAM | FOPEN_NONSEEKABLE in their open handler and this way avoid the deadlock on all kernel versions. This should work because fs/fuse/ ignores unknown open flags returned from a filesystem and so passing FOPEN_STREAM to a kernel that is not aware of this flag cannot hurt. In turn the kernel that is not aware of FOPEN_STREAM will be < v3.14 where just FOPEN_NONSEEKABLE is sufficient to implement streams without read vs write deadlock. This patch adds stream_open, converts /proc/xen/xenbus to it and adds semantic patch to automatically locate in-kernel places that are either required to be converted due to read vs write deadlock, or that are just safe to be converted because read and write do not use ppos and there are no other funky methods in file_operations. Regarding semantic patch I've verified each generated change manually - that it is correct to convert - and each other nonseekable_open instance left - that it is either not correct to convert there, or that it is not converted due to current stream_open.cocci limitations. The script also does not convert files that should be valid to convert, but that currently have .llseek = noop_llseek or generic_file_llseek for unknown reason despite file being opened with nonseekable_open (e.g. drivers/input/mousedev.c) Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Yongzhi Pan <panyongzhi@gmail.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: David Vrabel <david.vrabel@citrix.com> Cc: Juergen Gross <jgross@suse.com> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Tejun Heo <tj@kernel.org> Cc: Kirill Tkhai <ktkhai@virtuozzo.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Christoph Hellwig <hch@lst.de> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Julia Lawall <Julia.Lawall@lip6.fr> Cc: Nikolaus Rath <Nikolaus@rath.org> Cc: Han-Wen Nienhuys <hanwen@google.com> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
/linux-master/drivers/xen/xenbus/ | ||
H A D | xenbus_dev_frontend.c | diff 51cf07a7 Mon Apr 15 10:06:51 MDT 2019 Mao Wenan <maowenan@huawei.com> xenbus: drop useless LIST_HEAD in xenbus_write_watch() and xenbus_file_write() Drop LIST_HEAD where the variable it declares is never used. The declarations were introduced with the file, but the declared variables were not used. Fixes: 1107ba885e469 ("xen: add xenfs to allow usermode <-> Xen interaction") Signed-off-by: Mao Wenan <maowenan@huawei.com> Reviewed-by: Juergen Gross <jgross@suse.com> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com> diff 10dce8af Tue Mar 26 16:20:43 MDT 2019 Kirill Smelkov <kirr@nexedi.com> fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX") added locking for file.f_pos access and in particular made concurrent read and write not possible - now both those functions take f_pos lock for the whole run, and so if e.g. a read is blocked waiting for data, write will deadlock waiting for that read to complete. This caused regression for stream-like files where previously read and write could run simultaneously, but after that patch could not do so anymore. See e.g. commit 581d21a2d02a ("xenbus: fix deadlock on writes to /proc/xen/xenbus") which fixes such regression for particular case of /proc/xen/xenbus. The patch that added f_pos lock in 2014 did so to guarantee POSIX thread safety for read/write/lseek and added the locking to file descriptors of all regular files. In 2014 that thread-safety problem was not new as it was already discussed earlier in 2006. However even though 2006'th version of Linus's patch was adding f_pos locking "only for files that are marked seekable with FMODE_LSEEK (thus avoiding the stream-like objects like pipes and sockets)", the 2014 version - the one that actually made it into the tree as 9c225f2655e3 - is doing so irregardless of whether a file is seekable or not. See https://lore.kernel.org/lkml/53022DB1.4070805@gmail.com/ https://lwn.net/Articles/180387 https://lwn.net/Articles/180396 for historic context. The reason that it did so is, probably, that there are many files that are marked non-seekable, but e.g. their read implementation actually depends on knowing current position to correctly handle the read. Some examples: kernel/power/user.c snapshot_read fs/debugfs/file.c u32_array_read fs/fuse/control.c fuse_conn_waiting_read + ... drivers/hwmon/asus_atk0110.c atk_debugfs_ggrp_read arch/s390/hypfs/inode.c hypfs_read_iter ... Despite that, many nonseekable_open users implement read and write with pure stream semantics - they don't depend on passed ppos at all. And for those cases where read could wait for something inside, it creates a situation similar to xenbus - the write could be never made to go until read is done, and read is waiting for some, potentially external, event, for potentially unbounded time -> deadlock. Besides xenbus, there are 14 such places in the kernel that I've found with semantic patch (see below): drivers/xen/evtchn.c:667:8-24: ERROR: evtchn_fops: .read() can deadlock .write() drivers/isdn/capi/capi.c:963:8-24: ERROR: capi_fops: .read() can deadlock .write() drivers/input/evdev.c:527:1-17: ERROR: evdev_fops: .read() can deadlock .write() drivers/char/pcmcia/cm4000_cs.c:1685:7-23: ERROR: cm4000_fops: .read() can deadlock .write() net/rfkill/core.c:1146:8-24: ERROR: rfkill_fops: .read() can deadlock .write() drivers/s390/char/fs3270.c:488:1-17: ERROR: fs3270_fops: .read() can deadlock .write() drivers/usb/misc/ldusb.c:310:1-17: ERROR: ld_usb_fops: .read() can deadlock .write() drivers/hid/uhid.c:635:1-17: ERROR: uhid_fops: .read() can deadlock .write() net/batman-adv/icmp_socket.c:80:1-17: ERROR: batadv_fops: .read() can deadlock .write() drivers/media/rc/lirc_dev.c:198:1-17: ERROR: lirc_fops: .read() can deadlock .write() drivers/leds/uleds.c:77:1-17: ERROR: uleds_fops: .read() can deadlock .write() drivers/input/misc/uinput.c:400:1-17: ERROR: uinput_fops: .read() can deadlock .write() drivers/infiniband/core/user_mad.c:985:7-23: ERROR: umad_fops: .read() can deadlock .write() drivers/gnss/core.c:45:1-17: ERROR: gnss_fops: .read() can deadlock .write() In addition to the cases above another regression caused by f_pos locking is that now FUSE filesystems that implement open with FOPEN_NONSEEKABLE flag, can no longer implement bidirectional stream-like files - for the same reason as above e.g. read can deadlock write locking on file.f_pos in the kernel. FUSE's FOPEN_NONSEEKABLE was added in 2008 in a7c1b990f715 ("fuse: implement nonseekable open") to support OSSPD. OSSPD implements /dev/dsp in userspace with FOPEN_NONSEEKABLE flag, with corresponding read and write routines not depending on current position at all, and with both read and write being potentially blocking operations: See https://github.com/libfuse/osspd https://lwn.net/Articles/308445 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1406 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1438-L1477 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1479-L1510 Corresponding libfuse example/test also describes FOPEN_NONSEEKABLE as "somewhat pipe-like files ..." with read handler not using offset. However that test implements only read without write and cannot exercise the deadlock scenario: https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L124-L131 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L146-L163 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L209-L216 I've actually hit the read vs write deadlock for real while implementing my FUSE filesystem where there is /head/watch file, for which open creates separate bidirectional socket-like stream in between filesystem and its user with both read and write being later performed simultaneously. And there it is semantically not easy to split the stream into two separate read-only and write-only channels: https://lab.nexedi.com/kirr/wendelin.core/blob/f13aa600/wcfs/wcfs.go#L88-169 Let's fix this regression. The plan is: 1. We can't change nonseekable_open to include &~FMODE_ATOMIC_POS - doing so would break many in-kernel nonseekable_open users which actually use ppos in read/write handlers. 2. Add stream_open() to kernel to open stream-like non-seekable file descriptors. Read and write on such file descriptors would never use nor change ppos. And with that property on stream-like files read and write will be running without taking f_pos lock - i.e. read and write could be running simultaneously. 3. With semantic patch search and convert to stream_open all in-kernel nonseekable_open users for which read and write actually do not depend on ppos and where there is no other methods in file_operations which assume @offset access. 4. Add FOPEN_STREAM to fs/fuse/ and open in-kernel file-descriptors via steam_open if that bit is present in filesystem open reply. It was tempting to change fs/fuse/ open handler to use stream_open instead of nonseekable_open on just FOPEN_NONSEEKABLE flags, but grepping through Debian codesearch shows users of FOPEN_NONSEEKABLE, and in particular GVFS which actually uses offset in its read and write handlers https://codesearch.debian.net/search?q=-%3Enonseekable+%3D https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1080 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1247-1346 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1399-1481 so if we would do such a change it will break a real user. 5. Add stream_open and FOPEN_STREAM handling to stable kernels starting from v3.14+ (the kernel where 9c225f2655 first appeared). This will allow to patch OSSPD and other FUSE filesystems that provide stream-like files to return FOPEN_STREAM | FOPEN_NONSEEKABLE in their open handler and this way avoid the deadlock on all kernel versions. This should work because fs/fuse/ ignores unknown open flags returned from a filesystem and so passing FOPEN_STREAM to a kernel that is not aware of this flag cannot hurt. In turn the kernel that is not aware of FOPEN_STREAM will be < v3.14 where just FOPEN_NONSEEKABLE is sufficient to implement streams without read vs write deadlock. This patch adds stream_open, converts /proc/xen/xenbus to it and adds semantic patch to automatically locate in-kernel places that are either required to be converted due to read vs write deadlock, or that are just safe to be converted because read and write do not use ppos and there are no other funky methods in file_operations. Regarding semantic patch I've verified each generated change manually - that it is correct to convert - and each other nonseekable_open instance left - that it is either not correct to convert there, or that it is not converted due to current stream_open.cocci limitations. The script also does not convert files that should be valid to convert, but that currently have .llseek = noop_llseek or generic_file_llseek for unknown reason despite file being opened with nonseekable_open (e.g. drivers/input/mousedev.c) Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Yongzhi Pan <panyongzhi@gmail.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: David Vrabel <david.vrabel@citrix.com> Cc: Juergen Gross <jgross@suse.com> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Tejun Heo <tj@kernel.org> Cc: Kirill Tkhai <ktkhai@virtuozzo.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Christoph Hellwig <hch@lst.de> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Julia Lawall <Julia.Lawall@lip6.fr> Cc: Nikolaus Rath <Nikolaus@rath.org> Cc: Han-Wen Nienhuys <hanwen@google.com> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> diff 10dce8af Tue Mar 26 16:20:43 MDT 2019 Kirill Smelkov <kirr@nexedi.com> fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX") added locking for file.f_pos access and in particular made concurrent read and write not possible - now both those functions take f_pos lock for the whole run, and so if e.g. a read is blocked waiting for data, write will deadlock waiting for that read to complete. This caused regression for stream-like files where previously read and write could run simultaneously, but after that patch could not do so anymore. See e.g. commit 581d21a2d02a ("xenbus: fix deadlock on writes to /proc/xen/xenbus") which fixes such regression for particular case of /proc/xen/xenbus. The patch that added f_pos lock in 2014 did so to guarantee POSIX thread safety for read/write/lseek and added the locking to file descriptors of all regular files. In 2014 that thread-safety problem was not new as it was already discussed earlier in 2006. However even though 2006'th version of Linus's patch was adding f_pos locking "only for files that are marked seekable with FMODE_LSEEK (thus avoiding the stream-like objects like pipes and sockets)", the 2014 version - the one that actually made it into the tree as 9c225f2655e3 - is doing so irregardless of whether a file is seekable or not. See https://lore.kernel.org/lkml/53022DB1.4070805@gmail.com/ https://lwn.net/Articles/180387 https://lwn.net/Articles/180396 for historic context. The reason that it did so is, probably, that there are many files that are marked non-seekable, but e.g. their read implementation actually depends on knowing current position to correctly handle the read. Some examples: kernel/power/user.c snapshot_read fs/debugfs/file.c u32_array_read fs/fuse/control.c fuse_conn_waiting_read + ... drivers/hwmon/asus_atk0110.c atk_debugfs_ggrp_read arch/s390/hypfs/inode.c hypfs_read_iter ... Despite that, many nonseekable_open users implement read and write with pure stream semantics - they don't depend on passed ppos at all. And for those cases where read could wait for something inside, it creates a situation similar to xenbus - the write could be never made to go until read is done, and read is waiting for some, potentially external, event, for potentially unbounded time -> deadlock. Besides xenbus, there are 14 such places in the kernel that I've found with semantic patch (see below): drivers/xen/evtchn.c:667:8-24: ERROR: evtchn_fops: .read() can deadlock .write() drivers/isdn/capi/capi.c:963:8-24: ERROR: capi_fops: .read() can deadlock .write() drivers/input/evdev.c:527:1-17: ERROR: evdev_fops: .read() can deadlock .write() drivers/char/pcmcia/cm4000_cs.c:1685:7-23: ERROR: cm4000_fops: .read() can deadlock .write() net/rfkill/core.c:1146:8-24: ERROR: rfkill_fops: .read() can deadlock .write() drivers/s390/char/fs3270.c:488:1-17: ERROR: fs3270_fops: .read() can deadlock .write() drivers/usb/misc/ldusb.c:310:1-17: ERROR: ld_usb_fops: .read() can deadlock .write() drivers/hid/uhid.c:635:1-17: ERROR: uhid_fops: .read() can deadlock .write() net/batman-adv/icmp_socket.c:80:1-17: ERROR: batadv_fops: .read() can deadlock .write() drivers/media/rc/lirc_dev.c:198:1-17: ERROR: lirc_fops: .read() can deadlock .write() drivers/leds/uleds.c:77:1-17: ERROR: uleds_fops: .read() can deadlock .write() drivers/input/misc/uinput.c:400:1-17: ERROR: uinput_fops: .read() can deadlock .write() drivers/infiniband/core/user_mad.c:985:7-23: ERROR: umad_fops: .read() can deadlock .write() drivers/gnss/core.c:45:1-17: ERROR: gnss_fops: .read() can deadlock .write() In addition to the cases above another regression caused by f_pos locking is that now FUSE filesystems that implement open with FOPEN_NONSEEKABLE flag, can no longer implement bidirectional stream-like files - for the same reason as above e.g. read can deadlock write locking on file.f_pos in the kernel. FUSE's FOPEN_NONSEEKABLE was added in 2008 in a7c1b990f715 ("fuse: implement nonseekable open") to support OSSPD. OSSPD implements /dev/dsp in userspace with FOPEN_NONSEEKABLE flag, with corresponding read and write routines not depending on current position at all, and with both read and write being potentially blocking operations: See https://github.com/libfuse/osspd https://lwn.net/Articles/308445 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1406 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1438-L1477 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1479-L1510 Corresponding libfuse example/test also describes FOPEN_NONSEEKABLE as "somewhat pipe-like files ..." with read handler not using offset. However that test implements only read without write and cannot exercise the deadlock scenario: https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L124-L131 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L146-L163 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L209-L216 I've actually hit the read vs write deadlock for real while implementing my FUSE filesystem where there is /head/watch file, for which open creates separate bidirectional socket-like stream in between filesystem and its user with both read and write being later performed simultaneously. And there it is semantically not easy to split the stream into two separate read-only and write-only channels: https://lab.nexedi.com/kirr/wendelin.core/blob/f13aa600/wcfs/wcfs.go#L88-169 Let's fix this regression. The plan is: 1. We can't change nonseekable_open to include &~FMODE_ATOMIC_POS - doing so would break many in-kernel nonseekable_open users which actually use ppos in read/write handlers. 2. Add stream_open() to kernel to open stream-like non-seekable file descriptors. Read and write on such file descriptors would never use nor change ppos. And with that property on stream-like files read and write will be running without taking f_pos lock - i.e. read and write could be running simultaneously. 3. With semantic patch search and convert to stream_open all in-kernel nonseekable_open users for which read and write actually do not depend on ppos and where there is no other methods in file_operations which assume @offset access. 4. Add FOPEN_STREAM to fs/fuse/ and open in-kernel file-descriptors via steam_open if that bit is present in filesystem open reply. It was tempting to change fs/fuse/ open handler to use stream_open instead of nonseekable_open on just FOPEN_NONSEEKABLE flags, but grepping through Debian codesearch shows users of FOPEN_NONSEEKABLE, and in particular GVFS which actually uses offset in its read and write handlers https://codesearch.debian.net/search?q=-%3Enonseekable+%3D https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1080 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1247-1346 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1399-1481 so if we would do such a change it will break a real user. 5. Add stream_open and FOPEN_STREAM handling to stable kernels starting from v3.14+ (the kernel where 9c225f2655 first appeared). This will allow to patch OSSPD and other FUSE filesystems that provide stream-like files to return FOPEN_STREAM | FOPEN_NONSEEKABLE in their open handler and this way avoid the deadlock on all kernel versions. This should work because fs/fuse/ ignores unknown open flags returned from a filesystem and so passing FOPEN_STREAM to a kernel that is not aware of this flag cannot hurt. In turn the kernel that is not aware of FOPEN_STREAM will be < v3.14 where just FOPEN_NONSEEKABLE is sufficient to implement streams without read vs write deadlock. This patch adds stream_open, converts /proc/xen/xenbus to it and adds semantic patch to automatically locate in-kernel places that are either required to be converted due to read vs write deadlock, or that are just safe to be converted because read and write do not use ppos and there are no other funky methods in file_operations. Regarding semantic patch I've verified each generated change manually - that it is correct to convert - and each other nonseekable_open instance left - that it is either not correct to convert there, or that it is not converted due to current stream_open.cocci limitations. The script also does not convert files that should be valid to convert, but that currently have .llseek = noop_llseek or generic_file_llseek for unknown reason despite file being opened with nonseekable_open (e.g. drivers/input/mousedev.c) Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Yongzhi Pan <panyongzhi@gmail.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: David Vrabel <david.vrabel@citrix.com> Cc: Juergen Gross <jgross@suse.com> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Tejun Heo <tj@kernel.org> Cc: Kirill Tkhai <ktkhai@virtuozzo.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Christoph Hellwig <hch@lst.de> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Julia Lawall <Julia.Lawall@lip6.fr> Cc: Nikolaus Rath <Nikolaus@rath.org> Cc: Han-Wen Nienhuys <hanwen@google.com> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> diff fe7acdbe Sat Dec 10 11:29:50 MST 2011 Bastian Blank <waldi@debian.org> xen/xenbus-frontend: Make error message more clear Add the work frontend to the error message because we now also have a backend device. Signed-off-by: Bastian Blank <waldi@debian.org> Acked-by: Ian Campbell <ian.campbell@citrix.com> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> 2fb3683e Sat Dec 10 11:29:47 MST 2011 Bastian Blank <waldi@debian.org> xen: Add xenbus device driver Access to xenbus is currently handled via xenfs. This adds a device driver for xenbus and makes xenfs use this code. Signed-off-by: Bastian Blank <waldi@debian.org> Acked-by: Ian Campbell <ian.campbell@citrix.com> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> |
/linux-master/fs/ | ||
H A D | read_write.c | diff 89cbd4c0 Thu Aug 10 19:43:59 MDT 2023 Yang Li <yang.lee@linux.alibaba.com> fs: Fix one kernel-doc comment Fix one kernel-doc comment to silence the warning: fs/read_write.c:88: warning: Function parameter or member 'maxsize' not described in 'generic_file_llseek_size' Signed-off-by: Yang Li <yang.lee@linux.alibaba.com> Reviewed-by: Randy Dunlap <rdunlap@infradead.org> Message-Id: <20230811014359.4960-1-yang.lee@linux.alibaba.com> Signed-off-by: Christian Brauner <brauner@kernel.org> diff 10bc8e4a Thu Nov 17 13:52:49 MST 2022 Amir Goldstein <amir73il@gmail.com> vfs: fix copy_file_range() averts filesystem freeze protection Commit 868f9f2f8e00 ("vfs: fix copy_file_range() regression in cross-fs copies") removed fallback to generic_copy_file_range() for cross-fs cases inside vfs_copy_file_range(). To preserve behavior of nfsd and ksmbd server-side-copy, the fallback to generic_copy_file_range() was added in nfsd and ksmbd code, but that call is missing sb_start_write(), fsnotify hooks and more. Ideally, nfsd and ksmbd would pass a flag to vfs_copy_file_range() that will take care of the fallback, but that code would be subtle and we got vfs_copy_file_range() logic wrong too many times already. Instead, add a flag to explicitly request vfs_copy_file_range() to perform only generic_copy_file_range() and let nfsd and ksmbd use this flag only in the fallback path. This choise keeps the logic changes to minimum in the non-nfsd/ksmbd code paths to reduce the risk of further regressions. Fixes: 868f9f2f8e00 ("vfs: fix copy_file_range() regression in cross-fs copies") Tested-by: Namjae Jeon <linkinjeon@kernel.org> Tested-by: Luis Henriques <lhenriques@suse.de> Signed-off-by: Amir Goldstein <amir73il@gmail.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> diff bdeb77bc Sat Jul 16 22:37:10 MDT 2022 Andrei Vagin <avagin@gmail.com> fs: sendfile handles O_NONBLOCK of out_fd sendfile has to return EAGAIN if out_fd is nonblocking and the write into it would block. Here is a small reproducer for the problem: #define _GNU_SOURCE /* See feature_test_macros(7) */ #include <fcntl.h> #include <stdio.h> #include <unistd.h> #include <errno.h> #include <sys/stat.h> #include <sys/types.h> #include <sys/sendfile.h> #define FILE_SIZE (1UL << 30) int main(int argc, char **argv) { int p[2], fd; if (pipe2(p, O_NONBLOCK)) return 1; fd = open(argv[1], O_RDWR | O_TMPFILE, 0666); if (fd < 0) return 1; ftruncate(fd, FILE_SIZE); if (sendfile(p[1], fd, 0, FILE_SIZE) == -1) { fprintf(stderr, "FAIL\n"); } if (sendfile(p[1], fd, 0, FILE_SIZE) != -1 || errno != EAGAIN) { fprintf(stderr, "FAIL\n"); } return 0; } It worked before b964bf53e540, it is stuck after b964bf53e540, and it works again with this fix. This regression occurred because do_splice_direct() calls pipe_write that handles O_NONBLOCK. Here is a trace log from the reproducer: 1) | __x64_sys_sendfile64() { 1) | do_sendfile() { 1) | __fdget() 1) | rw_verify_area() 1) | __fdget() 1) | rw_verify_area() 1) | do_splice_direct() { 1) | rw_verify_area() 1) | splice_direct_to_actor() { 1) | do_splice_to() { 1) | rw_verify_area() 1) | generic_file_splice_read() 1) + 74.153 us | } 1) | direct_splice_actor() { 1) | iter_file_splice_write() { 1) | __kmalloc() 1) 0.148 us | pipe_lock(); 1) 0.153 us | splice_from_pipe_next.part.0(); 1) 0.162 us | page_cache_pipe_buf_confirm(); ... 16 times 1) 0.159 us | page_cache_pipe_buf_confirm(); 1) | vfs_iter_write() { 1) | do_iter_write() { 1) | rw_verify_area() 1) | do_iter_readv_writev() { 1) | pipe_write() { 1) | mutex_lock() 1) 0.153 us | mutex_unlock(); 1) 1.368 us | } 1) 1.686 us | } 1) 5.798 us | } 1) 6.084 us | } 1) 0.174 us | kfree(); 1) 0.152 us | pipe_unlock(); 1) + 14.461 us | } 1) + 14.783 us | } 1) 0.164 us | page_cache_pipe_buf_release(); ... 16 times 1) 0.161 us | page_cache_pipe_buf_release(); 1) | touch_atime() 1) + 95.854 us | } 1) + 99.784 us | } 1) ! 107.393 us | } 1) ! 107.699 us | } Link: https://lkml.kernel.org/r/20220415005015.525191-1-avagin@gmail.com Fixes: b964bf53e540 ("teach sendfile(2) to handle send-to-pipe directly") Signed-off-by: Andrei Vagin <avagin@gmail.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> diff 407e9c63 Thu Oct 15 10:21:17 MDT 2020 Darrick J. Wong <darrick.wong@oracle.com> vfs: move the generic write and copy checks out of mm The generic write check helpers also don't have much to do with the page cache, so move them to the vfs. Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> diff 438ab720 Thu Apr 11 15:31:57 MDT 2019 Kirill Smelkov <kirr@nexedi.com> vfs: pass ppos=NULL to .read()/.write() of FMODE_STREAM files This amends commit 10dce8af3422 ("fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock") in how position is passed into .read()/.write() handler for stream-like files: Rasmus noticed that we currently pass 0 as position and ignore any position change if that is done by a file implementation. This papers over bugs if ppos is used in files that declare themselves as being stream-like as such bugs will go unnoticed. Even if a file implementation is correctly converted into using stream_open, its read/write later could be changed to use ppos and even though that won't be working correctly, that bug might go unnoticed without someone doing wrong behaviour analysis. It is thus better to pass ppos=NULL into read/write for stream-like files as that don't give any chance for ppos usage bugs because it will oops if ppos is ever used inside .read() or .write(). Note 1: rw_verify_area, new_sync_{read,write} needs to be updated because they are called by vfs_read/vfs_write & friends before file_operations .read/.write . Note 2: if file backend uses new-style .read_iter/.write_iter, position is still passed into there as non-pointer kiocb.ki_pos . Currently stream_open.cocci (semantic patch added by 10dce8af3422) ignores files whose file_operations has *_iter methods. Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> diff 438ab720 Thu Apr 11 15:31:57 MDT 2019 Kirill Smelkov <kirr@nexedi.com> vfs: pass ppos=NULL to .read()/.write() of FMODE_STREAM files This amends commit 10dce8af3422 ("fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock") in how position is passed into .read()/.write() handler for stream-like files: Rasmus noticed that we currently pass 0 as position and ignore any position change if that is done by a file implementation. This papers over bugs if ppos is used in files that declare themselves as being stream-like as such bugs will go unnoticed. Even if a file implementation is correctly converted into using stream_open, its read/write later could be changed to use ppos and even though that won't be working correctly, that bug might go unnoticed without someone doing wrong behaviour analysis. It is thus better to pass ppos=NULL into read/write for stream-like files as that don't give any chance for ppos usage bugs because it will oops if ppos is ever used inside .read() or .write(). Note 1: rw_verify_area, new_sync_{read,write} needs to be updated because they are called by vfs_read/vfs_write & friends before file_operations .read/.write . Note 2: if file backend uses new-style .read_iter/.write_iter, position is still passed into there as non-pointer kiocb.ki_pos . Currently stream_open.cocci (semantic patch added by 10dce8af3422) ignores files whose file_operations has *_iter methods. Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> diff 10dce8af Tue Mar 26 16:20:43 MDT 2019 Kirill Smelkov <kirr@nexedi.com> fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX") added locking for file.f_pos access and in particular made concurrent read and write not possible - now both those functions take f_pos lock for the whole run, and so if e.g. a read is blocked waiting for data, write will deadlock waiting for that read to complete. This caused regression for stream-like files where previously read and write could run simultaneously, but after that patch could not do so anymore. See e.g. commit 581d21a2d02a ("xenbus: fix deadlock on writes to /proc/xen/xenbus") which fixes such regression for particular case of /proc/xen/xenbus. The patch that added f_pos lock in 2014 did so to guarantee POSIX thread safety for read/write/lseek and added the locking to file descriptors of all regular files. In 2014 that thread-safety problem was not new as it was already discussed earlier in 2006. However even though 2006'th version of Linus's patch was adding f_pos locking "only for files that are marked seekable with FMODE_LSEEK (thus avoiding the stream-like objects like pipes and sockets)", the 2014 version - the one that actually made it into the tree as 9c225f2655e3 - is doing so irregardless of whether a file is seekable or not. See https://lore.kernel.org/lkml/53022DB1.4070805@gmail.com/ https://lwn.net/Articles/180387 https://lwn.net/Articles/180396 for historic context. The reason that it did so is, probably, that there are many files that are marked non-seekable, but e.g. their read implementation actually depends on knowing current position to correctly handle the read. Some examples: kernel/power/user.c snapshot_read fs/debugfs/file.c u32_array_read fs/fuse/control.c fuse_conn_waiting_read + ... drivers/hwmon/asus_atk0110.c atk_debugfs_ggrp_read arch/s390/hypfs/inode.c hypfs_read_iter ... Despite that, many nonseekable_open users implement read and write with pure stream semantics - they don't depend on passed ppos at all. And for those cases where read could wait for something inside, it creates a situation similar to xenbus - the write could be never made to go until read is done, and read is waiting for some, potentially external, event, for potentially unbounded time -> deadlock. Besides xenbus, there are 14 such places in the kernel that I've found with semantic patch (see below): drivers/xen/evtchn.c:667:8-24: ERROR: evtchn_fops: .read() can deadlock .write() drivers/isdn/capi/capi.c:963:8-24: ERROR: capi_fops: .read() can deadlock .write() drivers/input/evdev.c:527:1-17: ERROR: evdev_fops: .read() can deadlock .write() drivers/char/pcmcia/cm4000_cs.c:1685:7-23: ERROR: cm4000_fops: .read() can deadlock .write() net/rfkill/core.c:1146:8-24: ERROR: rfkill_fops: .read() can deadlock .write() drivers/s390/char/fs3270.c:488:1-17: ERROR: fs3270_fops: .read() can deadlock .write() drivers/usb/misc/ldusb.c:310:1-17: ERROR: ld_usb_fops: .read() can deadlock .write() drivers/hid/uhid.c:635:1-17: ERROR: uhid_fops: .read() can deadlock .write() net/batman-adv/icmp_socket.c:80:1-17: ERROR: batadv_fops: .read() can deadlock .write() drivers/media/rc/lirc_dev.c:198:1-17: ERROR: lirc_fops: .read() can deadlock .write() drivers/leds/uleds.c:77:1-17: ERROR: uleds_fops: .read() can deadlock .write() drivers/input/misc/uinput.c:400:1-17: ERROR: uinput_fops: .read() can deadlock .write() drivers/infiniband/core/user_mad.c:985:7-23: ERROR: umad_fops: .read() can deadlock .write() drivers/gnss/core.c:45:1-17: ERROR: gnss_fops: .read() can deadlock .write() In addition to the cases above another regression caused by f_pos locking is that now FUSE filesystems that implement open with FOPEN_NONSEEKABLE flag, can no longer implement bidirectional stream-like files - for the same reason as above e.g. read can deadlock write locking on file.f_pos in the kernel. FUSE's FOPEN_NONSEEKABLE was added in 2008 in a7c1b990f715 ("fuse: implement nonseekable open") to support OSSPD. OSSPD implements /dev/dsp in userspace with FOPEN_NONSEEKABLE flag, with corresponding read and write routines not depending on current position at all, and with both read and write being potentially blocking operations: See https://github.com/libfuse/osspd https://lwn.net/Articles/308445 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1406 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1438-L1477 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1479-L1510 Corresponding libfuse example/test also describes FOPEN_NONSEEKABLE as "somewhat pipe-like files ..." with read handler not using offset. However that test implements only read without write and cannot exercise the deadlock scenario: https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L124-L131 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L146-L163 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L209-L216 I've actually hit the read vs write deadlock for real while implementing my FUSE filesystem where there is /head/watch file, for which open creates separate bidirectional socket-like stream in between filesystem and its user with both read and write being later performed simultaneously. And there it is semantically not easy to split the stream into two separate read-only and write-only channels: https://lab.nexedi.com/kirr/wendelin.core/blob/f13aa600/wcfs/wcfs.go#L88-169 Let's fix this regression. The plan is: 1. We can't change nonseekable_open to include &~FMODE_ATOMIC_POS - doing so would break many in-kernel nonseekable_open users which actually use ppos in read/write handlers. 2. Add stream_open() to kernel to open stream-like non-seekable file descriptors. Read and write on such file descriptors would never use nor change ppos. And with that property on stream-like files read and write will be running without taking f_pos lock - i.e. read and write could be running simultaneously. 3. With semantic patch search and convert to stream_open all in-kernel nonseekable_open users for which read and write actually do not depend on ppos and where there is no other methods in file_operations which assume @offset access. 4. Add FOPEN_STREAM to fs/fuse/ and open in-kernel file-descriptors via steam_open if that bit is present in filesystem open reply. It was tempting to change fs/fuse/ open handler to use stream_open instead of nonseekable_open on just FOPEN_NONSEEKABLE flags, but grepping through Debian codesearch shows users of FOPEN_NONSEEKABLE, and in particular GVFS which actually uses offset in its read and write handlers https://codesearch.debian.net/search?q=-%3Enonseekable+%3D https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1080 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1247-1346 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1399-1481 so if we would do such a change it will break a real user. 5. Add stream_open and FOPEN_STREAM handling to stable kernels starting from v3.14+ (the kernel where 9c225f2655 first appeared). This will allow to patch OSSPD and other FUSE filesystems that provide stream-like files to return FOPEN_STREAM | FOPEN_NONSEEKABLE in their open handler and this way avoid the deadlock on all kernel versions. This should work because fs/fuse/ ignores unknown open flags returned from a filesystem and so passing FOPEN_STREAM to a kernel that is not aware of this flag cannot hurt. In turn the kernel that is not aware of FOPEN_STREAM will be < v3.14 where just FOPEN_NONSEEKABLE is sufficient to implement streams without read vs write deadlock. This patch adds stream_open, converts /proc/xen/xenbus to it and adds semantic patch to automatically locate in-kernel places that are either required to be converted due to read vs write deadlock, or that are just safe to be converted because read and write do not use ppos and there are no other funky methods in file_operations. Regarding semantic patch I've verified each generated change manually - that it is correct to convert - and each other nonseekable_open instance left - that it is either not correct to convert there, or that it is not converted due to current stream_open.cocci limitations. The script also does not convert files that should be valid to convert, but that currently have .llseek = noop_llseek or generic_file_llseek for unknown reason despite file being opened with nonseekable_open (e.g. drivers/input/mousedev.c) Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Yongzhi Pan <panyongzhi@gmail.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: David Vrabel <david.vrabel@citrix.com> Cc: Juergen Gross <jgross@suse.com> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Tejun Heo <tj@kernel.org> Cc: Kirill Tkhai <ktkhai@virtuozzo.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Christoph Hellwig <hch@lst.de> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Julia Lawall <Julia.Lawall@lip6.fr> Cc: Nikolaus Rath <Nikolaus@rath.org> Cc: Han-Wen Nienhuys <hanwen@google.com> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> diff 10dce8af Tue Mar 26 16:20:43 MDT 2019 Kirill Smelkov <kirr@nexedi.com> fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX") added locking for file.f_pos access and in particular made concurrent read and write not possible - now both those functions take f_pos lock for the whole run, and so if e.g. a read is blocked waiting for data, write will deadlock waiting for that read to complete. This caused regression for stream-like files where previously read and write could run simultaneously, but after that patch could not do so anymore. See e.g. commit 581d21a2d02a ("xenbus: fix deadlock on writes to /proc/xen/xenbus") which fixes such regression for particular case of /proc/xen/xenbus. The patch that added f_pos lock in 2014 did so to guarantee POSIX thread safety for read/write/lseek and added the locking to file descriptors of all regular files. In 2014 that thread-safety problem was not new as it was already discussed earlier in 2006. However even though 2006'th version of Linus's patch was adding f_pos locking "only for files that are marked seekable with FMODE_LSEEK (thus avoiding the stream-like objects like pipes and sockets)", the 2014 version - the one that actually made it into the tree as 9c225f2655e3 - is doing so irregardless of whether a file is seekable or not. See https://lore.kernel.org/lkml/53022DB1.4070805@gmail.com/ https://lwn.net/Articles/180387 https://lwn.net/Articles/180396 for historic context. The reason that it did so is, probably, that there are many files that are marked non-seekable, but e.g. their read implementation actually depends on knowing current position to correctly handle the read. Some examples: kernel/power/user.c snapshot_read fs/debugfs/file.c u32_array_read fs/fuse/control.c fuse_conn_waiting_read + ... drivers/hwmon/asus_atk0110.c atk_debugfs_ggrp_read arch/s390/hypfs/inode.c hypfs_read_iter ... Despite that, many nonseekable_open users implement read and write with pure stream semantics - they don't depend on passed ppos at all. And for those cases where read could wait for something inside, it creates a situation similar to xenbus - the write could be never made to go until read is done, and read is waiting for some, potentially external, event, for potentially unbounded time -> deadlock. Besides xenbus, there are 14 such places in the kernel that I've found with semantic patch (see below): drivers/xen/evtchn.c:667:8-24: ERROR: evtchn_fops: .read() can deadlock .write() drivers/isdn/capi/capi.c:963:8-24: ERROR: capi_fops: .read() can deadlock .write() drivers/input/evdev.c:527:1-17: ERROR: evdev_fops: .read() can deadlock .write() drivers/char/pcmcia/cm4000_cs.c:1685:7-23: ERROR: cm4000_fops: .read() can deadlock .write() net/rfkill/core.c:1146:8-24: ERROR: rfkill_fops: .read() can deadlock .write() drivers/s390/char/fs3270.c:488:1-17: ERROR: fs3270_fops: .read() can deadlock .write() drivers/usb/misc/ldusb.c:310:1-17: ERROR: ld_usb_fops: .read() can deadlock .write() drivers/hid/uhid.c:635:1-17: ERROR: uhid_fops: .read() can deadlock .write() net/batman-adv/icmp_socket.c:80:1-17: ERROR: batadv_fops: .read() can deadlock .write() drivers/media/rc/lirc_dev.c:198:1-17: ERROR: lirc_fops: .read() can deadlock .write() drivers/leds/uleds.c:77:1-17: ERROR: uleds_fops: .read() can deadlock .write() drivers/input/misc/uinput.c:400:1-17: ERROR: uinput_fops: .read() can deadlock .write() drivers/infiniband/core/user_mad.c:985:7-23: ERROR: umad_fops: .read() can deadlock .write() drivers/gnss/core.c:45:1-17: ERROR: gnss_fops: .read() can deadlock .write() In addition to the cases above another regression caused by f_pos locking is that now FUSE filesystems that implement open with FOPEN_NONSEEKABLE flag, can no longer implement bidirectional stream-like files - for the same reason as above e.g. read can deadlock write locking on file.f_pos in the kernel. FUSE's FOPEN_NONSEEKABLE was added in 2008 in a7c1b990f715 ("fuse: implement nonseekable open") to support OSSPD. OSSPD implements /dev/dsp in userspace with FOPEN_NONSEEKABLE flag, with corresponding read and write routines not depending on current position at all, and with both read and write being potentially blocking operations: See https://github.com/libfuse/osspd https://lwn.net/Articles/308445 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1406 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1438-L1477 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1479-L1510 Corresponding libfuse example/test also describes FOPEN_NONSEEKABLE as "somewhat pipe-like files ..." with read handler not using offset. However that test implements only read without write and cannot exercise the deadlock scenario: https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L124-L131 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L146-L163 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L209-L216 I've actually hit the read vs write deadlock for real while implementing my FUSE filesystem where there is /head/watch file, for which open creates separate bidirectional socket-like stream in between filesystem and its user with both read and write being later performed simultaneously. And there it is semantically not easy to split the stream into two separate read-only and write-only channels: https://lab.nexedi.com/kirr/wendelin.core/blob/f13aa600/wcfs/wcfs.go#L88-169 Let's fix this regression. The plan is: 1. We can't change nonseekable_open to include &~FMODE_ATOMIC_POS - doing so would break many in-kernel nonseekable_open users which actually use ppos in read/write handlers. 2. Add stream_open() to kernel to open stream-like non-seekable file descriptors. Read and write on such file descriptors would never use nor change ppos. And with that property on stream-like files read and write will be running without taking f_pos lock - i.e. read and write could be running simultaneously. 3. With semantic patch search and convert to stream_open all in-kernel nonseekable_open users for which read and write actually do not depend on ppos and where there is no other methods in file_operations which assume @offset access. 4. Add FOPEN_STREAM to fs/fuse/ and open in-kernel file-descriptors via steam_open if that bit is present in filesystem open reply. It was tempting to change fs/fuse/ open handler to use stream_open instead of nonseekable_open on just FOPEN_NONSEEKABLE flags, but grepping through Debian codesearch shows users of FOPEN_NONSEEKABLE, and in particular GVFS which actually uses offset in its read and write handlers https://codesearch.debian.net/search?q=-%3Enonseekable+%3D https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1080 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1247-1346 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1399-1481 so if we would do such a change it will break a real user. 5. Add stream_open and FOPEN_STREAM handling to stable kernels starting from v3.14+ (the kernel where 9c225f2655 first appeared). This will allow to patch OSSPD and other FUSE filesystems that provide stream-like files to return FOPEN_STREAM | FOPEN_NONSEEKABLE in their open handler and this way avoid the deadlock on all kernel versions. This should work because fs/fuse/ ignores unknown open flags returned from a filesystem and so passing FOPEN_STREAM to a kernel that is not aware of this flag cannot hurt. In turn the kernel that is not aware of FOPEN_STREAM will be < v3.14 where just FOPEN_NONSEEKABLE is sufficient to implement streams without read vs write deadlock. This patch adds stream_open, converts /proc/xen/xenbus to it and adds semantic patch to automatically locate in-kernel places that are either required to be converted due to read vs write deadlock, or that are just safe to be converted because read and write do not use ppos and there are no other funky methods in file_operations. Regarding semantic patch I've verified each generated change manually - that it is correct to convert - and each other nonseekable_open instance left - that it is either not correct to convert there, or that it is not converted due to current stream_open.cocci limitations. The script also does not convert files that should be valid to convert, but that currently have .llseek = noop_llseek or generic_file_llseek for unknown reason despite file being opened with nonseekable_open (e.g. drivers/input/mousedev.c) Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Yongzhi Pan <panyongzhi@gmail.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: David Vrabel <david.vrabel@citrix.com> Cc: Juergen Gross <jgross@suse.com> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Tejun Heo <tj@kernel.org> Cc: Kirill Tkhai <ktkhai@virtuozzo.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Christoph Hellwig <hch@lst.de> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Julia Lawall <Julia.Lawall@lip6.fr> Cc: Nikolaus Rath <Nikolaus@rath.org> Cc: Han-Wen Nienhuys <hanwen@google.com> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> diff eca3654e Mon Oct 29 17:42:10 MDT 2018 Darrick J. Wong <darrick.wong@oracle.com> vfs: enable remap callers that can handle short operations Plumb in a remap flag that enables the filesystem remap handler to shorten remapping requests for callers that can handle it. Now copy_file_range can report partial success (in case we run up against alignment problems, resource limits, etc.). We also enable CAN_SHORTEN for fideduperange to maintain existing userspace-visible behavior where xfs/btrfs shorten the dedupe range to avoid stale post-eof data exposure. Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Reviewed-by: Amir Goldstein <amir73il@gmail.com> Signed-off-by: Dave Chinner <david@fromorbit.com> diff 85c95f20 Mon Sep 10 17:21:18 MDT 2018 Mark Fasheh <mfasheh@suse.de> vfs: dedupe should return EPERM if permission is not granted Right now we return EINVAL if a process does not have permission to dedupe a file. This was an oversight on my part. EPERM gives a true description of the nature of our error, and EINVAL is already used for the case that the filesystem does not support dedupe. Signed-off-by: Mark Fasheh <mfasheh@suse.de> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Acked-by: David Sterba <dsterba@suse.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> |
H A D | open.c | diff 83bc1d29 Mon Oct 09 09:37:10 MDT 2023 Amir Goldstein <amir73il@gmail.com> fs: get mnt_writers count for an open backing file's real path A writeable mapped backing file can perform writes to the real inode. Therefore, the real path mount must be kept writable so long as the writable map exists. This may not be strictly needed for ovelrayfs private upper mount, but it is correct to take the mnt_writers count in the vfs helper. Signed-off-by: Amir Goldstein <amir73il@gmail.com> Link: https://lore.kernel.org/r/20231009153712.1566422-2-amir73il@gmail.com Signed-off-by: Christian Brauner <brauner@kernel.org> diff 09da082b Tue Jul 11 10:16:04 MDT 2023 Alexey Gladkov <legion@kernel.org> fs: Add fchmodat2() On the userspace side fchmodat(3) is implemented as a wrapper function which implements the POSIX-specified interface. This interface differs from the underlying kernel system call, which does not have a flags argument. Most implementations require procfs [1][2]. There doesn't appear to be a good userspace workaround for this issue but the implementation in the kernel is pretty straight-forward. The new fchmodat2() syscall allows to pass the AT_SYMLINK_NOFOLLOW flag, unlike existing fchmodat. [1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/fchmodat.c;h=17eca54051ee28ba1ec3f9aed170a62630959143;hb=a492b1e5ef7ab50c6fdd4e4e9879ea5569ab0a6c#l35 [2] https://git.musl-libc.org/cgit/musl/tree/src/stat/fchmodat.c?id=718f363bc2067b6487900eddc9180c84e7739f80#n28 Co-developed-by: Palmer Dabbelt <palmer@sifive.com> Signed-off-by: Palmer Dabbelt <palmer@sifive.com> Signed-off-by: Alexey Gladkov <legion@kernel.org> Acked-by: Arnd Bergmann <arnd@arndb.de> Message-Id: <f2a846ef495943c5d101011eebcf01179d0c7b61.1689092120.git.legion@kernel.org> [brauner: pre reviews, do flag conversion in do_fchmodat() directly] Signed-off-by: Christian Brauner <brauner@kernel.org> diff a0fc452a Sat Aug 05 10:11:58 MDT 2023 Aleksa Sarai <cyphar@cyphar.com> open: make RESOLVE_CACHED correctly test for O_TMPFILE O_TMPFILE is actually __O_TMPFILE|O_DIRECTORY. This means that the old fast-path check for RESOLVE_CACHED would reject all users passing O_DIRECTORY with -EAGAIN, when in fact the intended test was to check for __O_TMPFILE. Cc: stable@vger.kernel.org # v5.12+ Fixes: 99668f618062 ("fs: expose LOOKUP_CACHED through openat2() RESOLVE_CACHED") Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> Message-Id: <20230806-resolve_cached-o_tmpfile-v1-1-7ba16308465e@cyphar.com> Signed-off-by: Christian Brauner <brauner@kernel.org> diff abf08576 Thu Jan 12 16:49:10 MST 2023 Christian Brauner <brauner@kernel.org> fs: port vfs_*() helpers to struct mnt_idmap Convert to struct mnt_idmap. Last cycle we merged the necessary infrastructure in 256c8aed2b42 ("fs: introduce dedicated idmap type for mounts"). This is just the conversion to struct mnt_idmap. Currently we still pass around the plain namespace that was attached to a mount. This is in general pretty convenient but it makes it easy to conflate namespaces that are relevant on the filesystem with namespaces that are relevent on the mount level. Especially for non-vfs developers without detailed knowledge in this area this can be a potential source for bugs. Once the conversion to struct mnt_idmap is done all helpers down to the really low-level helpers will take a struct mnt_idmap argument instead of two namespace arguments. This way it becomes impossible to conflate the two eliminating the possibility of any bugs. All of the vfs and all filesystems only operate on struct mnt_idmap. Acked-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org> diff 99668f61 Thu Dec 17 09:19:10 MST 2020 Jens Axboe <axboe@kernel.dk> fs: expose LOOKUP_CACHED through openat2() RESOLVE_CACHED Now that we support non-blocking path resolution internally, expose it via openat2() in the struct open_how ->resolve flags. This allows applications using openat2() to limit path resolution to the extent that it is already cached. If the lookup cannot be satisfied in a non-blocking manner, openat2(2) will return -1/-EAGAIN. Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Jens Axboe <axboe@kernel.dk> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> diff 35cb6d54 Fri Dec 13 11:10:11 MST 2019 Jens Axboe <axboe@kernel.dk> fs: make build_open_flags() available internally This is a prep patch for supporting non-blocking open from io_uring. Signed-off-by: Jens Axboe <axboe@kernel.dk> diff d7852fbd Thu Jul 11 10:54:40 MDT 2019 Linus Torvalds <torvalds@linux-foundation.org> access: avoid the RCU grace period for the temporary subjective credentials It turns out that 'access()' (and 'faccessat()') can cause a lot of RCU work because it installs a temporary credential that gets allocated and freed for each system call. The allocation and freeing overhead is mostly benign, but because credentials can be accessed under the RCU read lock, the freeing involves a RCU grace period. Which is not a huge deal normally, but if you have a lot of access() calls, this causes a fair amount of seconday damage: instead of having a nice alloc/free patterns that hits in hot per-CPU slab caches, you have all those delayed free's, and on big machines with hundreds of cores, the RCU overhead can end up being enormous. But it turns out that all of this is entirely unnecessary. Exactly because access() only installs the credential as the thread-local subjective credential, the temporary cred pointer doesn't actually need to be RCU free'd at all. Once we're done using it, we can just free it synchronously and avoid all the RCU overhead. So add a 'non_rcu' flag to 'struct cred', which can be set by users that know they only use it in non-RCU context (there are other potential users for this). We can make it a union with the rcu freeing list head that we need for the RCU case, so this doesn't need any extra storage. Note that this also makes 'get_current_cred()' clear the new non_rcu flag, in case we have filesystems that take a long-term reference to the cred and then expect the RCU delayed freeing afterwards. It's not entirely clear that this is required, but it makes for clear semantics: the subjective cred remains non-RCU as long as you only access it synchronously using the thread-local accessors, but you _can_ use it as a generic cred if you want to. It is possible that we should just remove the whole RCU markings for ->cred entirely. Only ->real_cred is really supposed to be accessed through RCU, and the long-term cred copies that nfs uses might want to explicitly re-enable RCU freeing if required, rather than have get_current_cred() do it implicitly. But this is a "minimal semantic changes" change for the immediate problem. Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by: Eric Dumazet <edumazet@google.com> Acked-by: Paul E. McKenney <paulmck@linux.ibm.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Jan Glauber <jglauber@marvell.com> Cc: Jiri Kosina <jikos@kernel.org> Cc: Jayachandran Chandrasekharan Nair <jnair@marvell.com> Cc: Greg KH <greg@kroah.com> Cc: Kees Cook <keescook@chromium.org> Cc: David Howells <dhowells@redhat.com> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> diff 438ab720 Thu Apr 11 15:31:57 MDT 2019 Kirill Smelkov <kirr@nexedi.com> vfs: pass ppos=NULL to .read()/.write() of FMODE_STREAM files This amends commit 10dce8af3422 ("fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock") in how position is passed into .read()/.write() handler for stream-like files: Rasmus noticed that we currently pass 0 as position and ignore any position change if that is done by a file implementation. This papers over bugs if ppos is used in files that declare themselves as being stream-like as such bugs will go unnoticed. Even if a file implementation is correctly converted into using stream_open, its read/write later could be changed to use ppos and even though that won't be working correctly, that bug might go unnoticed without someone doing wrong behaviour analysis. It is thus better to pass ppos=NULL into read/write for stream-like files as that don't give any chance for ppos usage bugs because it will oops if ppos is ever used inside .read() or .write(). Note 1: rw_verify_area, new_sync_{read,write} needs to be updated because they are called by vfs_read/vfs_write & friends before file_operations .read/.write . Note 2: if file backend uses new-style .read_iter/.write_iter, position is still passed into there as non-pointer kiocb.ki_pos . Currently stream_open.cocci (semantic patch added by 10dce8af3422) ignores files whose file_operations has *_iter methods. Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> diff 438ab720 Thu Apr 11 15:31:57 MDT 2019 Kirill Smelkov <kirr@nexedi.com> vfs: pass ppos=NULL to .read()/.write() of FMODE_STREAM files This amends commit 10dce8af3422 ("fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock") in how position is passed into .read()/.write() handler for stream-like files: Rasmus noticed that we currently pass 0 as position and ignore any position change if that is done by a file implementation. This papers over bugs if ppos is used in files that declare themselves as being stream-like as such bugs will go unnoticed. Even if a file implementation is correctly converted into using stream_open, its read/write later could be changed to use ppos and even though that won't be working correctly, that bug might go unnoticed without someone doing wrong behaviour analysis. It is thus better to pass ppos=NULL into read/write for stream-like files as that don't give any chance for ppos usage bugs because it will oops if ppos is ever used inside .read() or .write(). Note 1: rw_verify_area, new_sync_{read,write} needs to be updated because they are called by vfs_read/vfs_write & friends before file_operations .read/.write . Note 2: if file backend uses new-style .read_iter/.write_iter, position is still passed into there as non-pointer kiocb.ki_pos . Currently stream_open.cocci (semantic patch added by 10dce8af3422) ignores files whose file_operations has *_iter methods. Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> diff 10dce8af Tue Mar 26 16:20:43 MDT 2019 Kirill Smelkov <kirr@nexedi.com> fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX") added locking for file.f_pos access and in particular made concurrent read and write not possible - now both those functions take f_pos lock for the whole run, and so if e.g. a read is blocked waiting for data, write will deadlock waiting for that read to complete. This caused regression for stream-like files where previously read and write could run simultaneously, but after that patch could not do so anymore. See e.g. commit 581d21a2d02a ("xenbus: fix deadlock on writes to /proc/xen/xenbus") which fixes such regression for particular case of /proc/xen/xenbus. The patch that added f_pos lock in 2014 did so to guarantee POSIX thread safety for read/write/lseek and added the locking to file descriptors of all regular files. In 2014 that thread-safety problem was not new as it was already discussed earlier in 2006. However even though 2006'th version of Linus's patch was adding f_pos locking "only for files that are marked seekable with FMODE_LSEEK (thus avoiding the stream-like objects like pipes and sockets)", the 2014 version - the one that actually made it into the tree as 9c225f2655e3 - is doing so irregardless of whether a file is seekable or not. See https://lore.kernel.org/lkml/53022DB1.4070805@gmail.com/ https://lwn.net/Articles/180387 https://lwn.net/Articles/180396 for historic context. The reason that it did so is, probably, that there are many files that are marked non-seekable, but e.g. their read implementation actually depends on knowing current position to correctly handle the read. Some examples: kernel/power/user.c snapshot_read fs/debugfs/file.c u32_array_read fs/fuse/control.c fuse_conn_waiting_read + ... drivers/hwmon/asus_atk0110.c atk_debugfs_ggrp_read arch/s390/hypfs/inode.c hypfs_read_iter ... Despite that, many nonseekable_open users implement read and write with pure stream semantics - they don't depend on passed ppos at all. And for those cases where read could wait for something inside, it creates a situation similar to xenbus - the write could be never made to go until read is done, and read is waiting for some, potentially external, event, for potentially unbounded time -> deadlock. Besides xenbus, there are 14 such places in the kernel that I've found with semantic patch (see below): drivers/xen/evtchn.c:667:8-24: ERROR: evtchn_fops: .read() can deadlock .write() drivers/isdn/capi/capi.c:963:8-24: ERROR: capi_fops: .read() can deadlock .write() drivers/input/evdev.c:527:1-17: ERROR: evdev_fops: .read() can deadlock .write() drivers/char/pcmcia/cm4000_cs.c:1685:7-23: ERROR: cm4000_fops: .read() can deadlock .write() net/rfkill/core.c:1146:8-24: ERROR: rfkill_fops: .read() can deadlock .write() drivers/s390/char/fs3270.c:488:1-17: ERROR: fs3270_fops: .read() can deadlock .write() drivers/usb/misc/ldusb.c:310:1-17: ERROR: ld_usb_fops: .read() can deadlock .write() drivers/hid/uhid.c:635:1-17: ERROR: uhid_fops: .read() can deadlock .write() net/batman-adv/icmp_socket.c:80:1-17: ERROR: batadv_fops: .read() can deadlock .write() drivers/media/rc/lirc_dev.c:198:1-17: ERROR: lirc_fops: .read() can deadlock .write() drivers/leds/uleds.c:77:1-17: ERROR: uleds_fops: .read() can deadlock .write() drivers/input/misc/uinput.c:400:1-17: ERROR: uinput_fops: .read() can deadlock .write() drivers/infiniband/core/user_mad.c:985:7-23: ERROR: umad_fops: .read() can deadlock .write() drivers/gnss/core.c:45:1-17: ERROR: gnss_fops: .read() can deadlock .write() In addition to the cases above another regression caused by f_pos locking is that now FUSE filesystems that implement open with FOPEN_NONSEEKABLE flag, can no longer implement bidirectional stream-like files - for the same reason as above e.g. read can deadlock write locking on file.f_pos in the kernel. FUSE's FOPEN_NONSEEKABLE was added in 2008 in a7c1b990f715 ("fuse: implement nonseekable open") to support OSSPD. OSSPD implements /dev/dsp in userspace with FOPEN_NONSEEKABLE flag, with corresponding read and write routines not depending on current position at all, and with both read and write being potentially blocking operations: See https://github.com/libfuse/osspd https://lwn.net/Articles/308445 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1406 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1438-L1477 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1479-L1510 Corresponding libfuse example/test also describes FOPEN_NONSEEKABLE as "somewhat pipe-like files ..." with read handler not using offset. However that test implements only read without write and cannot exercise the deadlock scenario: https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L124-L131 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L146-L163 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L209-L216 I've actually hit the read vs write deadlock for real while implementing my FUSE filesystem where there is /head/watch file, for which open creates separate bidirectional socket-like stream in between filesystem and its user with both read and write being later performed simultaneously. And there it is semantically not easy to split the stream into two separate read-only and write-only channels: https://lab.nexedi.com/kirr/wendelin.core/blob/f13aa600/wcfs/wcfs.go#L88-169 Let's fix this regression. The plan is: 1. We can't change nonseekable_open to include &~FMODE_ATOMIC_POS - doing so would break many in-kernel nonseekable_open users which actually use ppos in read/write handlers. 2. Add stream_open() to kernel to open stream-like non-seekable file descriptors. Read and write on such file descriptors would never use nor change ppos. And with that property on stream-like files read and write will be running without taking f_pos lock - i.e. read and write could be running simultaneously. 3. With semantic patch search and convert to stream_open all in-kernel nonseekable_open users for which read and write actually do not depend on ppos and where there is no other methods in file_operations which assume @offset access. 4. Add FOPEN_STREAM to fs/fuse/ and open in-kernel file-descriptors via steam_open if that bit is present in filesystem open reply. It was tempting to change fs/fuse/ open handler to use stream_open instead of nonseekable_open on just FOPEN_NONSEEKABLE flags, but grepping through Debian codesearch shows users of FOPEN_NONSEEKABLE, and in particular GVFS which actually uses offset in its read and write handlers https://codesearch.debian.net/search?q=-%3Enonseekable+%3D https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1080 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1247-1346 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1399-1481 so if we would do such a change it will break a real user. 5. Add stream_open and FOPEN_STREAM handling to stable kernels starting from v3.14+ (the kernel where 9c225f2655 first appeared). This will allow to patch OSSPD and other FUSE filesystems that provide stream-like files to return FOPEN_STREAM | FOPEN_NONSEEKABLE in their open handler and this way avoid the deadlock on all kernel versions. This should work because fs/fuse/ ignores unknown open flags returned from a filesystem and so passing FOPEN_STREAM to a kernel that is not aware of this flag cannot hurt. In turn the kernel that is not aware of FOPEN_STREAM will be < v3.14 where just FOPEN_NONSEEKABLE is sufficient to implement streams without read vs write deadlock. This patch adds stream_open, converts /proc/xen/xenbus to it and adds semantic patch to automatically locate in-kernel places that are either required to be converted due to read vs write deadlock, or that are just safe to be converted because read and write do not use ppos and there are no other funky methods in file_operations. Regarding semantic patch I've verified each generated change manually - that it is correct to convert - and each other nonseekable_open instance left - that it is either not correct to convert there, or that it is not converted due to current stream_open.cocci limitations. The script also does not convert files that should be valid to convert, but that currently have .llseek = noop_llseek or generic_file_llseek for unknown reason despite file being opened with nonseekable_open (e.g. drivers/input/mousedev.c) Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Yongzhi Pan <panyongzhi@gmail.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: David Vrabel <david.vrabel@citrix.com> Cc: Juergen Gross <jgross@suse.com> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Tejun Heo <tj@kernel.org> Cc: Kirill Tkhai <ktkhai@virtuozzo.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Christoph Hellwig <hch@lst.de> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Julia Lawall <Julia.Lawall@lip6.fr> Cc: Nikolaus Rath <Nikolaus@rath.org> Cc: Han-Wen Nienhuys <hanwen@google.com> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> diff 10dce8af Tue Mar 26 16:20:43 MDT 2019 Kirill Smelkov <kirr@nexedi.com> fs: stream_open - opener for stream-like files so that read and write can run simultaneously without deadlock Commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX") added locking for file.f_pos access and in particular made concurrent read and write not possible - now both those functions take f_pos lock for the whole run, and so if e.g. a read is blocked waiting for data, write will deadlock waiting for that read to complete. This caused regression for stream-like files where previously read and write could run simultaneously, but after that patch could not do so anymore. See e.g. commit 581d21a2d02a ("xenbus: fix deadlock on writes to /proc/xen/xenbus") which fixes such regression for particular case of /proc/xen/xenbus. The patch that added f_pos lock in 2014 did so to guarantee POSIX thread safety for read/write/lseek and added the locking to file descriptors of all regular files. In 2014 that thread-safety problem was not new as it was already discussed earlier in 2006. However even though 2006'th version of Linus's patch was adding f_pos locking "only for files that are marked seekable with FMODE_LSEEK (thus avoiding the stream-like objects like pipes and sockets)", the 2014 version - the one that actually made it into the tree as 9c225f2655e3 - is doing so irregardless of whether a file is seekable or not. See https://lore.kernel.org/lkml/53022DB1.4070805@gmail.com/ https://lwn.net/Articles/180387 https://lwn.net/Articles/180396 for historic context. The reason that it did so is, probably, that there are many files that are marked non-seekable, but e.g. their read implementation actually depends on knowing current position to correctly handle the read. Some examples: kernel/power/user.c snapshot_read fs/debugfs/file.c u32_array_read fs/fuse/control.c fuse_conn_waiting_read + ... drivers/hwmon/asus_atk0110.c atk_debugfs_ggrp_read arch/s390/hypfs/inode.c hypfs_read_iter ... Despite that, many nonseekable_open users implement read and write with pure stream semantics - they don't depend on passed ppos at all. And for those cases where read could wait for something inside, it creates a situation similar to xenbus - the write could be never made to go until read is done, and read is waiting for some, potentially external, event, for potentially unbounded time -> deadlock. Besides xenbus, there are 14 such places in the kernel that I've found with semantic patch (see below): drivers/xen/evtchn.c:667:8-24: ERROR: evtchn_fops: .read() can deadlock .write() drivers/isdn/capi/capi.c:963:8-24: ERROR: capi_fops: .read() can deadlock .write() drivers/input/evdev.c:527:1-17: ERROR: evdev_fops: .read() can deadlock .write() drivers/char/pcmcia/cm4000_cs.c:1685:7-23: ERROR: cm4000_fops: .read() can deadlock .write() net/rfkill/core.c:1146:8-24: ERROR: rfkill_fops: .read() can deadlock .write() drivers/s390/char/fs3270.c:488:1-17: ERROR: fs3270_fops: .read() can deadlock .write() drivers/usb/misc/ldusb.c:310:1-17: ERROR: ld_usb_fops: .read() can deadlock .write() drivers/hid/uhid.c:635:1-17: ERROR: uhid_fops: .read() can deadlock .write() net/batman-adv/icmp_socket.c:80:1-17: ERROR: batadv_fops: .read() can deadlock .write() drivers/media/rc/lirc_dev.c:198:1-17: ERROR: lirc_fops: .read() can deadlock .write() drivers/leds/uleds.c:77:1-17: ERROR: uleds_fops: .read() can deadlock .write() drivers/input/misc/uinput.c:400:1-17: ERROR: uinput_fops: .read() can deadlock .write() drivers/infiniband/core/user_mad.c:985:7-23: ERROR: umad_fops: .read() can deadlock .write() drivers/gnss/core.c:45:1-17: ERROR: gnss_fops: .read() can deadlock .write() In addition to the cases above another regression caused by f_pos locking is that now FUSE filesystems that implement open with FOPEN_NONSEEKABLE flag, can no longer implement bidirectional stream-like files - for the same reason as above e.g. read can deadlock write locking on file.f_pos in the kernel. FUSE's FOPEN_NONSEEKABLE was added in 2008 in a7c1b990f715 ("fuse: implement nonseekable open") to support OSSPD. OSSPD implements /dev/dsp in userspace with FOPEN_NONSEEKABLE flag, with corresponding read and write routines not depending on current position at all, and with both read and write being potentially blocking operations: See https://github.com/libfuse/osspd https://lwn.net/Articles/308445 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1406 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1438-L1477 https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1479-L1510 Corresponding libfuse example/test also describes FOPEN_NONSEEKABLE as "somewhat pipe-like files ..." with read handler not using offset. However that test implements only read without write and cannot exercise the deadlock scenario: https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L124-L131 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L146-L163 https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L209-L216 I've actually hit the read vs write deadlock for real while implementing my FUSE filesystem where there is /head/watch file, for which open creates separate bidirectional socket-like stream in between filesystem and its user with both read and write being later performed simultaneously. And there it is semantically not easy to split the stream into two separate read-only and write-only channels: https://lab.nexedi.com/kirr/wendelin.core/blob/f13aa600/wcfs/wcfs.go#L88-169 Let's fix this regression. The plan is: 1. We can't change nonseekable_open to include &~FMODE_ATOMIC_POS - doing so would break many in-kernel nonseekable_open users which actually use ppos in read/write handlers. 2. Add stream_open() to kernel to open stream-like non-seekable file descriptors. Read and write on such file descriptors would never use nor change ppos. And with that property on stream-like files read and write will be running without taking f_pos lock - i.e. read and write could be running simultaneously. 3. With semantic patch search and convert to stream_open all in-kernel nonseekable_open users for which read and write actually do not depend on ppos and where there is no other methods in file_operations which assume @offset access. 4. Add FOPEN_STREAM to fs/fuse/ and open in-kernel file-descriptors via steam_open if that bit is present in filesystem open reply. It was tempting to change fs/fuse/ open handler to use stream_open instead of nonseekable_open on just FOPEN_NONSEEKABLE flags, but grepping through Debian codesearch shows users of FOPEN_NONSEEKABLE, and in particular GVFS which actually uses offset in its read and write handlers https://codesearch.debian.net/search?q=-%3Enonseekable+%3D https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1080 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1247-1346 https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1399-1481 so if we would do such a change it will break a real user. 5. Add stream_open and FOPEN_STREAM handling to stable kernels starting from v3.14+ (the kernel where 9c225f2655 first appeared). This will allow to patch OSSPD and other FUSE filesystems that provide stream-like files to return FOPEN_STREAM | FOPEN_NONSEEKABLE in their open handler and this way avoid the deadlock on all kernel versions. This should work because fs/fuse/ ignores unknown open flags returned from a filesystem and so passing FOPEN_STREAM to a kernel that is not aware of this flag cannot hurt. In turn the kernel that is not aware of FOPEN_STREAM will be < v3.14 where just FOPEN_NONSEEKABLE is sufficient to implement streams without read vs write deadlock. This patch adds stream_open, converts /proc/xen/xenbus to it and adds semantic patch to automatically locate in-kernel places that are either required to be converted due to read vs write deadlock, or that are just safe to be converted because read and write do not use ppos and there are no other funky methods in file_operations. Regarding semantic patch I've verified each generated change manually - that it is correct to convert - and each other nonseekable_open instance left - that it is either not correct to convert there, or that it is not converted due to current stream_open.cocci limitations. The script also does not convert files that should be valid to convert, but that currently have .llseek = noop_llseek or generic_file_llseek for unknown reason despite file being opened with nonseekable_open (e.g. drivers/input/mousedev.c) Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Yongzhi Pan <panyongzhi@gmail.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: David Vrabel <david.vrabel@citrix.com> Cc: Juergen Gross <jgross@suse.com> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Tejun Heo <tj@kernel.org> Cc: Kirill Tkhai <ktkhai@virtuozzo.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Christoph Hellwig <hch@lst.de> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Julia Lawall <Julia.Lawall@lip6.fr> Cc: Nikolaus Rath <Nikolaus@rath.org> Cc: Han-Wen Nienhuys <hanwen@google.com> Signed-off-by: Kirill Smelkov <kirr@nexedi.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
/linux-master/include/linux/ | ||
H A D | fs.h | diff ddd65e19 Sat Mar 23 10:11:19 MDT 2024 Christian Brauner <brauner@kernel.org> block: handle BLK_OPEN_RESTRICT_WRITES correctly Last kernel release we introduce CONFIG_BLK_DEV_WRITE_MOUNTED. By default this option is set. When it is set the long-standing behavior of being able to write to mounted block devices is enabled. But in order to guard against unintended corruption by writing to the block device buffer cache CONFIG_BLK_DEV_WRITE_MOUNTED can be turned off. In that case it isn't possible to write to mounted block devices anymore. A filesystem may open its block devices with BLK_OPEN_RESTRICT_WRITES which disallows concurrent BLK_OPEN_WRITE access. When we still had the bdev handle around we could recognize BLK_OPEN_RESTRICT_WRITES because the mode was passed around. Since we managed to get rid of the bdev handle we changed that logic to recognize BLK_OPEN_RESTRICT_WRITES based on whether the file was opened writable and writes to that block device are blocked. That logic doesn't work because we do allow BLK_OPEN_RESTRICT_WRITES to be specified without BLK_OPEN_WRITE. Fix the detection logic and use an FMODE_* bit. We could've also abused O_EXCL as an indicator that BLK_OPEN_RESTRICT_WRITES has been requested. For userspace open paths O_EXCL will never be retained but for internal opens where we open files that are never installed into a file descriptor table this is fine. But it would be a gamble that this doesn't cause bugs. Note that BLK_OPEN_RESTRICT_WRITES is an internal only flag that cannot directly be raised by userspace. It is implicitly raised during mounting. Passes xftests and blktests with CONFIG_BLK_DEV_WRITE_MOUNTED set and unset. Link: https://lore.kernel.org/r/ZfyyEwu9Uq5Pgb94@casper.infradead.org Link: https://lore.kernel.org/r/20240323-zielbereich-mittragen-6fdf14876c3e@brauner Fixes: 321de651fa56 ("block: don't rely on BLK_OPEN_RESTRICT_WRITES when yielding write access") Reviewed-by: Yu Kuai <yukuai3@huawei.com> Reviewed-by: Jan Kara <jack@suse.cz> Reported-by: Matthew Wilcox <willy@infradead.org> Signed-off-by: Christian Brauner <brauner@kernel.org> diff f3a60882 Thu Feb 08 10:47:35 MST 2024 Christian Brauner <brauner@kernel.org> bdev: open block device as files Add two new helpers to allow opening block devices as files. This is not the final infrastructure. This still opens the block device before opening a struct a file. Until we have removed all references to struct bdev_handle we can't switch the order: * Introduce blk_to_file_flags() to translate from block specific to flags usable to pen a new file. * Introduce bdev_file_open_by_{dev,path}(). * Introduce temporary sb_bdev_handle() helper to retrieve a struct bdev_handle from a block device file and update places that directly reference struct bdev_handle to rely on it. * Don't count block device openes against the number of open files. A bdev_file_open_by_{dev,path}() file is never installed into any file descriptor table. One idea that came to mind was to use kernel_tmpfile_open() which would require us to pass a path and it would then call do_dentry_open() going through the regular fops->open::blkdev_open() path. But then we're back to the problem of routing block specific flags such as BLK_OPEN_RESTRICT_WRITES through the open path and would have to waste FMODE_* flags every time we add a new one. With this we can avoid using a flag bit and we have more leeway in how we open block devices from bdev_open_by_{dev,path}(). Link: https://lore.kernel.org/r/20240123-vfs-bdev-file-v2-1-adbd023e19cc@kernel.org Signed-off-by: Christian Brauner <brauner@kernel.org> diff e4e8b47a Mon Jan 15 10:30:46 MST 2018 Max Kellermann <mk@cm4all.com> fs: fix umask on NFS with CONFIG_FS_POSIX_ACL=n Make IS_POSIXACL() return false if POSIX ACL support is disabled. Never skip applying the umask in namei.c and never bother to do any ACL specific checks if the filesystem falsely indicates it has ACLs enabled when the feature is completely disabled in the kernel. This fixes a problem where the umask is always ignored in the NFS client when compiled without CONFIG_FS_POSIX_ACL. This is a 4 year old regression caused by commit 013cdf1088d723 which itself was not completely wrong, but failed to consider all the side effects by misdesigned VFS code. Prior to that commit, there were two places where the umask could be applied, for example when creating a directory: 1. in the VFS layer in SYSCALL_DEFINE3(mkdirat), but only if !IS_POSIXACL() 2. again (unconditionally) in nfs3_proc_mkdir() The first one does not apply, because even without CONFIG_FS_POSIX_ACL, the NFS client sets SB_POSIXACL in nfs_fill_super(). After that commit, (2.) was replaced by: 2b. in posix_acl_create(), called by nfs3_proc_mkdir() There's one branch in posix_acl_create() which applies the umask; however, without CONFIG_FS_POSIX_ACL, posix_acl_create() is an empty dummy function which does not apply the umask. The approach chosen by this patch is to make IS_POSIXACL() always return false when POSIX ACL support is disabled, so the umask always gets applied by the VFS layer. This is consistent with the (regular) behavior of posix_acl_create(): that function returns early if IS_POSIXACL() is false, before applying the umask. Therefore, posix_acl_create() is responsible for applying the umask if there is ACL support enabled in the file system (SB_POSIXACL), and the VFS layer is responsible for all other cases (no SB_POSIXACL or no CONFIG_FS_POSIX_ACL). Signed-off-by: Max Kellermann <mk@cm4all.com> Link: https://lore.kernel.org/r/151603744662.29035.4910161264124875658.stgit@rabbit.intern.cm-ag Signed-off-by: Christian Brauner <brauner@kernel.org> diff 38bcdd38 Thu Aug 10 16:08:27 MDT 2023 Christoph Hellwig <hch@lst.de> fs: remove get_super get_super is unused now, remove it. Signed-off-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Christian Brauner <brauner@kernel.org> Reviewed-by: Josef Bacik <josef@toxicpanda.com> Message-Id: <20230811100828.1897174-17-hch@lst.de> Signed-off-by: Christian Brauner <brauner@kernel.org> diff 880b9577 Mon Jul 17 10:00:09 MDT 2023 Darrick J. Wong <djwong@kernel.org> fs: distinguish between user initiated freeze and kernel initiated freeze Userspace can freeze a filesystem using the FIFREEZE ioctl or by suspending the block device; this state persists until userspace thaws the filesystem with the FITHAW ioctl or resuming the block device. Since commit 18e9e5104fcd ("Introduce freeze_super and thaw_super for the fsfreeze ioctl") we only allow the first freeze command to succeed. The kernel may decide that it is necessary to freeze a filesystem for its own internal purposes, such as suspends in progress, filesystem fsck activities, or quiescing a device prior to removal. Userspace thaw commands must never break a kernel freeze, and kernel thaw commands shouldn't undo userspace's freeze command. Introduce a couple of freeze holder flags and wire it into the sb_writers state. One kernel and one userspace freeze are allowed to coexist at the same time; the filesystem will not thaw until both are lifted. I wonder if the f2fs/gfs2 code should be using a kernel freeze here, but for now we'll use FREEZE_HOLDER_USERSPACE to preserve existing behaviors. Cc: mcgrof@kernel.org Cc: jack@suse.cz Cc: hch@infradead.org Cc: ruansy.fnst@fujitsu.com Signed-off-by: Darrick J. Wong <djwong@kernel.org> Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Jan Kara <jack@suse.cz> diff 9b6304c1 Wed Jul 05 12:58:10 MDT 2023 Jeff Layton <jlayton@kernel.org> fs: add ctime accessors infrastructure struct timespec64 has unused bits in the tv_nsec field that can be used for other purposes. In future patches, we're going to change how the inode->i_ctime is accessed in certain inodes in order to make use of them. In order to do that safely though, we'll need to eradicate raw accesses of the inode->i_ctime field from the kernel. Add new accessor functions for the ctime that we use to replace them. Reviewed-by: Jan Kara <jack@suse.cz> Reviewed-by: Luis Chamberlain <mcgrof@kernel.org> Signed-off-by: Jeff Layton <jlayton@kernel.org> Reviewed-by: Damien Le Moal <dlemoal@kernel.org> Message-Id: <20230705185812.579118-2-jlayton@kernel.org> Signed-off-by: Christian Brauner <brauner@kernel.org> diff 44fff0fa Thu Jun 01 08:59:01 MDT 2023 Christoph Hellwig <hch@lst.de> fs: factor out a direct_write_fallback helper Add a helper dealing with handling the syncing of a buffered write fallback for direct I/O. Link: https://lkml.kernel.org/r/20230601145904.1385409-10-hch@lst.de Signed-off-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Damien Le Moal <dlemoal@kernel.org> Reviewed-by: Miklos Szeredi <mszeredi@redhat.com> Reviewed-by: Darrick J. Wong <djwong@kernel.org> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Andreas Gruenbacher <agruenba@redhat.com> Cc: Anna Schumaker <anna@kernel.org> Cc: Chao Yu <chao@kernel.org> Cc: Christian Brauner <brauner@kernel.org> Cc: Hannes Reinecke <hare@suse.de> Cc: Ilya Dryomov <idryomov@gmail.com> Cc: Jaegeuk Kim <jaegeuk@kernel.org> Cc: Jens Axboe <axboe@kernel.dk> Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Theodore Ts'o <tytso@mit.edu> Cc: Trond Myklebust <trond.myklebust@hammerspace.com> Cc: Xiubo Li <xiubli@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> diff 2bfc6685 Wed Jun 07 12:19:10 MDT 2023 David Howells <dhowells@redhat.com> splice, net: Add a splice_eof op to file-ops and socket-ops Add an optional method, ->splice_eof(), to allow splice to indicate the premature termination of a splice to struct file_operations and struct proto_ops. This is called if sendfile() or splice() encounters all of the following conditions inside splice_direct_to_actor(): (1) the user did not set SPLICE_F_MORE (splice only), and (2) an EOF condition occurred (->splice_read() returned 0), and (3) we haven't read enough to fulfill the request (ie. len > 0 still), and (4) we have already spliced at least one byte. A further patch will modify the behaviour of SPLICE_F_MORE to always be passed to the actor if either the user set it or we haven't yet read sufficient data to fulfill the request. Suggested-by: Linus Torvalds <torvalds@linux-foundation.org> Link: https://lore.kernel.org/r/CAHk-=wh=V579PDYvkpnTobCLGczbgxpMgGmmhqiTyE34Cpi5Gg@mail.gmail.com/ Signed-off-by: David Howells <dhowells@redhat.com> Reviewed-by: Jakub Kicinski <kuba@kernel.org> cc: Jens Axboe <axboe@kernel.dk> cc: Christoph Hellwig <hch@lst.de> cc: Al Viro <viro@zeniv.linux.org.uk> cc: Matthew Wilcox <willy@infradead.org> cc: Jan Kara <jack@suse.cz> cc: Jeff Layton <jlayton@kernel.org> cc: David Hildenbrand <david@redhat.com> cc: Christian Brauner <brauner@kernel.org> cc: Chuck Lever <chuck.lever@oracle.com> cc: Boris Pismenny <borisp@nvidia.com> cc: John Fastabend <john.fastabend@gmail.com> cc: linux-mm@kvack.org Signed-off-by: Jakub Kicinski <kuba@kernel.org> diff 576215cf Thu May 25 08:17:10 MDT 2023 Jan Kara <jack@suse.cz> fs: Drop wait_unfrozen wait queue wait_unfrozen waitqueue is used only in quota code to wait for filesystem to become unfrozen. In that place we can just use sb_start_write() - sb_end_write() pair to achieve the same. So just remove the waitqueue. Reviewed-by: Christian Brauner <brauner@kernel.org> Message-Id: <20230525141710.7595-1-jack@suse.cz> Signed-off-by: Jan Kara <jack@suse.cz> diff 9828ed3f Thu May 25 10:32:25 MDT 2023 Linus Torvalds <torvalds@linux-foundation.org> module: error out early on concurrent load of the same module file It turns out that udev under certain circumstances will concurrently try to load the same modules over-and-over excessively. This isn't a kernel bug, but it ends up affecting the kernel, to the point that under certain circumstances we can fail to boot, because the kernel uses a lot of memory to read all the module data all at once. Note that it isn't a memory leak, it's just basically a thundering herd problem happening at bootup with a lot of CPUs, with the worst cases then being pretty bad. Admittedly the worst situations are somewhat contrived: lots and lots of CPUs, not a lot of memory, and KASAN enabled to make it all slower and as such (unintentionally) exacerbate the problem. Luis explains: [1] "My best assessment of the situation is that each CPU in udev ends up triggering a load of duplicate set of modules, not just one, but *a lot*. Not sure what heuristics udev uses to load a set of modules per CPU." Petr Pavlu chimes in: [2] "My understanding is that udev workers are forked. An initial kmod context is created by the main udevd process but no sharing happens after the fork. It means that the mentioned memory pool logic doesn't really kick in. Multiple parallel load requests come from multiple udev workers, for instance, each handling an udev event for one CPU device and making the exactly same requests as all others are doing at the same time. The optimization idea would be to recognize these duplicate requests at the udevd/kmod level and converge them" Note that module loading has tried to mitigate this issue before, see for example commit 064f4536d139 ("module: avoid allocation if module is already present and ready"), which has a few ASCII graphs on memory use due to this same issue. However, while that noticed that the module was already loaded, and exited with an error early before spending any more time on setting up the module, it didn't handle the case of multiple concurrent module loads all being active - but not complete - at the same time. Yes, one of them will eventually win the race and finalize its copy, and the others will then notice that the module already exists and error out, but while this all happens, we have tons of unnecessary concurrent work being done. Again, the real fix is for udev to not do that (maybe it should use threads instead of fork, and have actual shared data structures and not cause duplicate work). That real fix is apparently not trivial. But it turns out that the kernel already has a pretty good model for dealing with concurrent access to the same file: the i_writecount of the inode. In fact, the module loading already indirectly uses 'i_writecount' , because 'kernel_file_read()' will in fact do ret = deny_write_access(file); if (ret) return ret; ... allow_write_access(file); around the read of the file data. We do not allow concurrent writes to the file, and return -ETXTBUSY if the file was open for writing at the same time as the module data is loaded from it. And the solution to the reader concurrency problem is to simply extend this "no concurrent writers" logic to simply be "exclusive access". Note that "exclusive" in this context isn't really some absolute thing: it's only exclusion from writers and from other "special readers" that do this writer denial. So we simply introduce a variation of that "deny_write_access()" logic that not only denies write access, but also requires that this is the _only_ such access that denies write access. Which means that you can't start loading a module that is already being loaded as a module by somebody else, or you will get the same -ETXTBSY error that you would get if there were writers around. [ It also means that you can't try to load a currently executing executable as a module, for the same reason: executables do that same "deny_write_access()" thing, and that's obviously where the whole ETXTBSY logic traditionally came from. This is not a problem for kernel modules, since the set of normal executable files and kernel module files is entirely disjoint. ] This new function is called "exclusive_deny_write_access()", and the implementation is trivial, in that it's just an atomic decrement of i_writecount if it was 0 before. To use that new exclusivity check, all we then do is wrap the module loading with that exclusive_deny_write_access()() / allow_write_access() pair. The actual patch is a bit bigger than that, because we want to surround not just the "load file data" part, but the whole module setup, to get maximum exclusion. So this ends up splitting up "finit_module()" into a few helper functions to make it all very clear and legible. In Luis' test-case (bringing up 255 vcpu's in a virtual machine [3]), the "wasted vmalloc" space (ie module data read into a vmalloc'ed area in order to be loaded as a module, but then discarded because somebody else loaded the same module instead) dropped from 1.8GiB to 474kB. Yes, that's gigabytes to kilobytes. It doesn't drop completely to zero, because even with this change, you can still end up having completely serial pointless module loads, where one udev process has loaded a module fully (and thus the kernel has released that exclusive lock on the module file), and then another udev process tries to load the same module again. So while we cannot fully get rid of the fundamental bug in user space, we _can_ get rid of the excessive concurrent thundering herd effect. A couple of final side notes on this all: - This tweak only affects the "finit_module()" system call, which gives the kernel a file descriptor with the module data. You can also just feed the module data as raw data from user space with "init_module()" (note the lack of 'f' at the beginning), and obviously for that case we do _not_ have any "exclusive read" logic. So if you absolutely want to do things wrong in user space, and try to load the same module multiple times, and error out only later when the kernel ends up saying "you can't load the same module name twice", you can still do that. And in fact, some distros will do exactly that, because they will uncompress the kernel module data in user space before feeding it to the kernel (mainly because they haven't started using the new kernel side decompression yet). So this is not some absolute "you can't do concurrent loads of the same module". It's literally just a very simple heuristic that will catch it early in case you try to load the exact same module file at the same time, and in that case avoid a potentially nasty situation. - There is another user of "deny_write_access()": the verity code that enables fs-verity on a file (the FS_IOC_ENABLE_VERITY ioctl). If you use fs-verity and you care about verifying the kernel modules (which does make sense), you should do it *before* loading said kernel module. That may sound obvious, but now the implementation basically requires it. Because if you try to do it concurrently, the kernel may refuse to load the module file that is being set up by the fs-verity code. - This all will obviously mean that if you insist on loading the same module in parallel, only one module load will succeed, and the others will return with an error. That was true before too, but what is different is that the -ETXTBSY error can be returned *before* the success case of another process fully loading and instantiating the module. Again, that might sound obvious, and it is indeed the whole point of the whole change: we are much quicker to notice the whole "you're already in the process of loading this module". So it's very much intentional, but it does mean that if you just spray the kernel with "finit_module()", and expect that the module is immediately loaded afterwards without checking the return value, you are doing something horribly horribly wrong. I'd like to say that that would never happen, but the whole _reason_ for this commit is that udev is currently doing something horribly horribly wrong, so ... Link: https://lore.kernel.org/all/ZEGopJ8VAYnE7LQ2@bombadil.infradead.org/ [1] Link: https://lore.kernel.org/all/23bd0ce6-ef78-1cd8-1f21-0e706a00424a@suse.com/ [2] Link: https://lore.kernel.org/lkml/ZG%2Fa+nrt4%2FAAUi5z@bombadil.infradead.org/ [3] Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Lucas De Marchi <lucas.demarchi@intel.com> Cc: Petr Pavlu <petr.pavlu@suse.com> Tested-by: Luis Chamberlain <mcgrof@kernel.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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