pidfs: remove config option

As Linus suggested this enables pidfs unconditionally. A key property to
retain is the ability to compare pidfds by inode number (cf. [1]).
That's extremely helpful just as comparing namespace file descriptors by
inode number is. They are used in a variety of scenarios where they need
to be compared, e.g., when receiving a pidfd via SO_PEERPIDFD from a
socket to trivially authenticate a the sender and various other
use-cases.

For 64bit systems this is pretty trivial to do. For 32bit it's slightly
more annoying as we discussed but we simply add a dumb ida based
allocator that gets used on 32bit. This gives the same guarantees about
inode numbers on 64bit without any overflow risk. Practically, we'll
never run into overflow issues because we're constrained by the number
of processes that can exist on 32bit and by the number of open files
that can exist on a 32bit system. On 64bit none of this matters and
things are very simple.

If 32bit also needs the uniqueness guarantee they can simply parse the
contents of /proc/<pid>/fd/<nr>. The uniqueness guarantees have a
variety of use-cases. One of the most obvious ones is that they will
make pidfiles (or "pidfdfiles", I guess) reliable as the unique
identifier can be placed into there that won't be reycled. Also a
frequent request.

Note, I took the chance and simplified path_from_stashed() even further.
Instead of passing the inode number explicitly to path_from_stashed() we
let the filesystem handle that internally. So path_from_stashed() ends
up even simpler than it is now. This is also a good solution allowing
the cleanup code to be clean and consistent between 32bit and 64bit. The
cleanup path in prepare_anon_dentry() is also switched around so we put
the inode before the dentry allocation. This means we only have to call
the cleanup handler for the filesystem's inode data once and can rely
->evict_inode() otherwise.

Aside from having to have a bit of extra code for 32bit it actually ends
up a nice cleanup for path_from_stashed() imho.

Tested on both 32 and 64bit including error injection.

Link: https://github.com/systemd/systemd/pull/31713 [1]
Link: https://lore.kernel.org/r/20240312-dingo-sehnlich-b3ecc35c6de7@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

pidfs: convert to path_from_stashed() helper

Moving pidfds from the anonymous inode infrastructure to a separate tiny
in-kernel filesystem similar to sockfs, pipefs, and anon_inodefs causes
selinux denials and thus various userspace components that make heavy
use of pidfds to fail as pidfds used anon_inode_getfile() which aren't
subject to any LSM hooks. But dentry_open() is and that would cause
regressions.

The failures that are seen are selinux denials. But the core failure is
dbus-broker. That cascades into other services failing that depend on
dbus-broker. For example, when dbus-broker fails to start polkit and all
the others won't be able to work because they depend on dbus-broker.

The reason for dbus-broker failing is because it doesn't handle failures
for SO_PEERPIDFD correctly. Last kernel release we introduced
SO_PEERPIDFD (and SCM_PIDFD). SO_PEERPIDFD allows dbus-broker and polkit
and others to receive a pidfd for the peer of an AF_UNIX socket. This is
the first time in the history of Linux that we can safely authenticate
clients in a race-free manner.

dbus-broker immediately made use of this but messed up the error
checking. It only allowed EINVAL as a valid failure for SO_PEERPIDFD.
That's obviously problematic not just because of LSM denials but because
of seccomp denials that would prevent SO_PEERPIDFD from working; or any
other new error code from there.

So this is catching a flawed implementation in dbus-broker as well. It
has to fallback to the old pid-based authentication when SO_PEERPIDFD
doesn't work no matter the reasons otherwise it'll always risk such
failures. So overall that LSM denial should not have caused dbus-broker
to fail. It can never assume that a feature released one kernel ago like
SO_PEERPIDFD can be assumed to be available.

So, the next fix separate from the selinux policy update is to try and
fix dbus-broker at [3]. That should make it into Fedora as well. In
addition the selinux reference policy should also be updated. See [4]
for that. If Selinux is in enforcing mode in userspace and it encounters
anything that it doesn't know about it will deny it by default. And the
policy is entirely in userspace including declaring new types for stuff
like nsfs or pidfs to allow it.

For now we continue to raise S_PRIVATE on the inode if it's a pidfs
inode which means things behave exactly like before.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2265630
Link: https://github.com/fedora-selinux/selinux-policy/pull/2050
Link: https://github.com/bus1/dbus-broker/pull/343 [3]
Link: https://github.com/SELinuxProject/refpolicy/pull/762 [4]
Reported-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20240222190334.GA412503@dev-arch.thelio-3990X
Link: https://lore.kernel.org/r/20240218-neufahrzeuge-brauhaus-fb0eb6459771@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

pidfd: add pidfs

This moves pidfds from the anonymous inode infrastructure to a tiny
pseudo filesystem. This has been on my todo for quite a while as it will
unblock further work that we weren't able to do simply because of the
very justified limitations of anonymous inodes. Moving pidfds to a tiny
pseudo filesystem allows:

* statx() on pidfds becomes useful for the first time.
* pidfds can be compared simply via statx() and then comparing inode
numbers.
* pidfds have unique inode numbers for the system lifetime.
* struct pid is now stashed in inode->i_private instead of
file->private_data. This means it is now possible to introduce
concepts that operate on a process once all file descriptors have been
closed. A concrete example is kill-on-last-close.
* file->private_data is freed up for per-file options for pidfds.
* Each struct pid will refer to a different inode but the same struct
pid will refer to the same inode if it's opened multiple times. In
contrast to now where each struct pid refers to the same inode. Even
if we were to move to anon_inode_create_getfile() which creates new
inodes we'd still be associating the same struct pid with multiple
different inodes.

The tiny pseudo filesystem is not visible anywhere in userspace exactly
like e.g., pipefs and sockfs. There's no lookup, there's no complex
inode operations, nothing. Dentries and inodes are always deleted when
the last pidfd is closed.

We allocate a new inode for each struct pid and we reuse that inode for
all pidfds. We use iget_locked() to find that inode again based on the
inode number which isn't recycled. We allocate a new dentry for each
pidfd that uses the same inode. That is similar to anonymous inodes
which reuse the same inode for thousands of dentries. For pidfds we're
talking way less than that. There usually won't be a lot of concurrent
openers of the same struct pid. They can probably often be counted on
two hands. I know that systemd does use separate pidfd for the same
struct pid for various complex process tracking issues. So I think with
that things actually become way simpler. Especially because we don't
have to care about lookup. Dentries and inodes continue to be always
deleted.

The code is entirely optional and fairly small. If it's not selected we
fallback to anonymous inodes. Heavily inspired by nsfs which uses a
similar stashing mechanism just for namespaces.

Link: https://lore.kernel.org/r/20240213-vfs-pidfd_fs-v1-2-f863f58cfce1@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>