Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Fix KGDB backtrace on ARM

Modify trapframe decoding to properly analyze trapframe.

Provide method for fixup_pc. It happens, that in some kernel
functions, the GDB stack frame decoder cannot determine both
func name and frame size. This is because these functions
either contain invalid instruction, or their format does
not match standard schema. Detect that scenarios and move
PC accordingly to jump into known function schema, which
GDB is able to parse.

Obtained from: Semihalf
Sponsored by: Juniper Networks
Reviewed by: kib, zbb
Differential Revision: https://reviews.freebsd.org/D5976

Change kbdb's kthr::cpu field into an int, to avoid gcc warnings about
comparing it with NOCPU, which became -1 recently. While here, avoid
using it for address calculations if it is negative.

Reviewed by: jhb, adrian
MFC after: 1 week

In kthr.c, obtain the address of the PCB for threads that were running
on a core, when the core was stopped, by calling kgdb_trgt_core_pcb().
This has 2 advantages:
1. We don't need to include a machine-specific header anymore and as
such kthr.c is truly machine independent. This allows the code to
be used in a cross-debugger.
2. We don't need to lookup stoppcbs in generic code when it's an
inherently target-spicific symbol. It does not exist for ia64.

Implement kgdb_trgt_core_pcb() for all architectures, except ia64, by
calling a common function called kgdb_trgt_stop_pcb(). This function
differs from kgdb_trgt_core_pcb() in that it gets the size of the PCB
structure as an argument and as such remains machine independent.

On ia64 the PCB for stopped cores is in the PCPU structure itself.
This for better scaling. The implementation of kgdb_trgt_core_pcb()
for ia64 uses the cpuid_to_pcpu[] array to to obtain the address of
the PCB structure.

Change kgdb_lookup() to resolve symbols via GDB instead of via libkvm(3).

Rework how kgdb manages kernel and vmcore files to be a bit more gdb-ish
so that kgdb can be used more like a normal gdb:
- Load the kernel via the standard 'exec' target and allow it to be changed
via the 'file' command.
- Instead of explicitly loading the kernel file as the mail symbol file
during startup, just pass it to gdb_main() as the executable file.
- Change the kld support (via shared libraries) to cache the address of
the linker_files and linker_kernel_file variables in addition to the
offsets of various members in 'struct linker_file'.
- When a new symbol file is loaded, recompute the addresses and offsets
used by the kld support code.
- When a new symbol file is loaded, recalculate the ofs_fix variable to
account for the different ways a trapframe can be passed to trap
frame handlers in i386. This is done by adding a MD
kgdb_trgt_new_objfile() hook that is empty on all but i386.
- Don't use the directory name of the kernel specified on the command
line to find kernel modules in the kld support code. Instead,
extract the filename of the current executable via exec_bfd. Now
the 'kernel' variable is private to main.c again.
- Make the 'add-kld' command explicitly fail if no executable is loaded.
- Make the support for vmcores a real core-dump target that opens the
kernel and vmcore on open and closes the kvm connection when closed, etc.
- The 'core' command can now be used to select a vmcore to use, either
a crash dump file or /dev/mem for live debugging.
- The 'detach' command can be used to detach from a vmcore w/o attaching
to a new one.
- kgdb no longer explicitly opens a core dump during startup and no longer
has to use an atexit() hook to close the kvm connection on shutdown.
- Symbols for kld's are automatically loaded anytime a core is opened.
Also, the unread portion of dmesg is dumped just as it was done on kgdb
startup previously.
- Don't require either a remote target or core dump if a kernel is specified.
You can now just run 'kgdb kernel' similar to running gdb on an executable
and later connect to a remote target or core dump.
- Use a more relaxed way to verify remote targets specified via -r.
Instead of explicitly allowing a few non-file target specifications,
just assume that if stat() on the arg and on "/dev/" + arg both fail
that is some non-file target and pass it to gdb.
- Don't use a custom interpreter. The existing kgdb_init() hook and the
target_new_objfile() hook give us sufficient hooks during startup to
setup kgdb-specific behavior now.
- Always add the 'proc', 'tid', and 'add-kld' commands on startup and not
just if we have a core dump. Currently the 'proc' and 'tid' commands do
not work for remote targets (I will fix at least 'tid' in the next round
of changes though). However, the 'add-kld' command works fine for
loading symbols for a kernel module on a remote target.
- Always setup the 'kld' shared library target operations instead of just
if we have a core dump. Although symbols for kernel modules are not
automatically loaded when connecting to a remote target, you can do
'info sharedlibrary' after connecting to the remote target and kgdb will
find all the modules. You can then use the 'sharedlibrary' command to
load symbols from the module files.
- Change kthr_init() to free the existing list of kthr objects before
generating a new one. This allows it to be invoked multiple times
w/o leaking memory.

MFC after: 1 week

Remove a stale prototype I missed when converting the kld support over to
hooking into gdb's shared library infrastructure.

- Rework the kld support to hook into GDB's shared library support.
kgdb(8) now treats kld's as shared libraries relative to the kernel
"binary". Thus, you can use 'info sharedlibrary' to list the kld's
along with 'sharedlibrary' and 'nosharedlibrary' to manage symbol
loading and unloading. Note that there isn't an easy way to force GDB
to use a specific path for a shared library. However, you can use
'nosharedlibrary' to unload all the klds and then use 'sharedlibrary'
to load specific klds where it gets the kld correct and use
'add-kld' for the kld's where the default open behavior doesn't work.
klds opened via 'sharedlibrary' (and during startup) do have their
sections listed in 'info files'.
- Change the 'add-kld' command to use filename completion to complete its
argument.

Don't close the kernel bfd object during startup. Instead, leave it open
and build a section table from the kernel file so that 'info files' output
for kgdb now matches the usage of gdb on a regular file with the exception
that we don't list sections for memory in the crash dump.

Add support for automatically loading symbols for kld's on startup:
- Add a new 'kgdb_auto_load_klds()' routine which is invoked during
startup that walks the list of linker files and tries to find a matching
kld on disk for each non-kernel kld. If a kld file is found, then it
is added as if the 'add-kld' command is invoked. One change from
'add-kld' is that this method attempts to use the 'pathname' from the
linker_file structure first to try to load the file. If that fails
it then looks in the kernel directory followed by the directories in
the module path.
- Move the kld file suffix handling into a separate routine so that it
can be called standalone and to reduce duplicate code in find_kld_path().
- Cache the offsets of members of 'struct linker_file' during startup
instead of computing them for each 'add-kld'.
- Use GDB's target_read_string() instead of direct KVM access.
- Add all resident sections from a kld by using bfd_map_over_sections() to
build the section list rather than just adding symbols for ".text",
".data", ".bss", and ".rodata".
- Change the 'add-kld' command to do a y/n prompt before adding the
symbols when run interactively to match 'add-symbol-file'.

MFC after: 1 week

Move the code for working with kld's out into its own file.

Add a new 'add-kld <kld>' command to kgdb to make it easier to analyze
crash dumps with kernel modules. The command is basically a wrapper
around add-symbol-file except that it uses the kernel linker data
structures and the ELF section headers of the kld to calculate the
section addresses add-symbol-file needs.

The 'kld' parameter may either be an absolute path or a relative path.
kgdb looks for the kld in several locations checking for variants with
".symbols" or ".debug" suffixes in each location. The first location it
tries is just opening the specified path (this handles absolute paths and
looks for the kld relative to the current directory otherwise). Next
it tries to find the module in the same directory of the kernel image
being used. If that fails it extracts the kern.module_path from the
kernel being debugged and looks in each of those paths.

The upshot is that for the common cases of debugging /boot/kernel/kernel
where the module is in either /boot/kernel or /boot/modules one can merely
do 'add-kld foo.ko'.

MFC after: 1 week

Teach kgdb how to handle double fault frames on i386:
- Save td_oncpu in 'struct kthr' so the i386 target code can see which CPU
a thread is running on.
- Add a new frame unwinder for double fault frames. This unwinder is used
when "dblfault_handler" is encountered in the stack. It uses the CPU of
the current thread to lookup the base address of the TSS used for the
double fault from the GDT. It then fetches the various registers out
of the TSS similar to how the current trapframe unwinder fetches
registers out of the trapframe.

MFC after: 3 days

Remove extern int verbose declaration. It is declared static in the
only file it is used in.

Rename lookup() to kgdb_lookup() and make it global (for use in trgt_i386.c).

No objections from: marcel

Add a kluge to allow kgdb(1) to inject its own frame sniffer in the
list of frame sniffers so that trapframes can be detected. The kluge
is needed because this version of gdb only supports appending a
sniffer to the list of sniffers and the moment kgdb gets a chance to
add its own frame sniffer, the target's default frame sniffer is
already in the list. Since the default frame sniffer claims any
frame thrown at it, kgdb's frame sniffer never gets to smell (a
process much akin to tasting, but with lesser chance of hurling :-)

This commit adds dummy frame sniffers that never claim a frame and
as such don't fix anything yet. However, we now have frame sniffers
and they are being called, so it's just a matter of adding meat to
the bones and we'll be able to properly unwind across trapframes.

MFC after: 1 week

Attempt to make kgdb little more useful and easy to use. Properly initialize
it to recognise what ABI to use on amd64 (and possibly others) platform.
Display PID and process name as a part of the 'info threads' output, TIDs
alone are too confusing. Introduce new commmands 'tid <tid>' and 'proc <pid>'
to accompany gdb's default 'thread <thread num>' to make the task of switching
between different contexts easier.

Add the beginnings of kernel debugging support. the kgdb(1) tool
is basicly a shell on top of libgdb that knows about kernel threads,
kernel modules and kvm(3). As the word "beginnings" implies, not
all of the features have been implemented yet. The tool is useful
and I'd like feedback on the taken route.

The simplest way to debug a kernel core file is:
kgdb -n 0

This opens /var/crash/vmcore.0 with the corresponding kernel in
the object directory (kernel.debug is used if it exists).

Typical things that need to be added are:
o Auto loading of kernel modules,
o Handling of trapframes so that backtraces can be taken across
them,
o Some fancy commands to extract useful information out of a core
file,
o Various (probably many) other things.