[Crash-utility] [RFC PATCH 0/5] Improve stack unwind on ppc64

[Aditya Gupta]

The Problem:
============

Currently crash is unable to show function arguments and local variables, as
gdb can do. And functionality for moving between frames ('up'/'down') is not
working in crash.

Crash has 'gdb passthroughs' for things gdb can do, but the gdb passthroughs
'bt', 'frame', 'info locals', 'up', 'down' are not working either, due to
gdb not getting the register values from `crash_target::fetch_registers`,
which then uses `machdep->get_cpu_reg`, which is not implemented for PPC64

Proposed Solution:
==================

Fix the gdb passthroughs by implementing "machdep->get_cpu_reg" for PPC64.
This way, "gdb mode in crash" will support this feature for both ELF and
kdump-compressed vmcore formats, while "gdb" would only have supported ELF
format

Also, backtrace can be slightly different in gdb and crash (due to gdb
being able to print inline frames also). so it can cause confusion of 'bt'
working in crash context, but 'frame'/'up'/'down' working as
'gdb passthrough', showing different frames.
This has been explained in patch #4.

So to prevent confusion mentioned above, implement 'frame', 'up', 'down'
as commands in default crash mode also, instead of working via gdb
passthroughs which they do currently.

So, now in default mode, 'bt','frame','up','down' will be consistent with each
other.

Implications on Architectures:
====================================

No architecture other than PPC64 has been affected, other than in case of
'frame', 'up', 'down' commands

1. frame: As mentioned in patch #2, that frame will not be prohibited, and
   will print:

	crash> frame
	#0  <unavailable> in ?? ()

Instead of before prohibited message:

	crash> frame
	crash: prohibited gdb command: frame

2. up/down: These commands will now be run as native crash commands by
   default instead of showing 

	crash> up
	crash: ambiguous command: up (symbol and gdb command)
	crash> down
	crash: ambiguous command: down (symbol and gdb command)

On PPC64, the default mode ("crash mode") will not have ANY OTHER changes,
other than the 'frame', 'up', 'down' as mentioned above.

Major change will be in 'gdb mode' on PPC64, that it will print the frames, and
local variables, instead of failing with errors showing no frame, or showing
that couldn't get PC

Testing:
========

Git tree with this patch series applied: https://github.ibm.com/adityag/crash
(replace this link with github.com later)

To test 'frame'/'up'/'down' in crash (implemented in patch #3):

	crash> bt
	crash> frame
	crash> up
	crash> down
	crash> up 4

To test 'bt'/'frame'/'up'/'down'/'info locals' gdb passthroughs:

	crash> set gdb on
	gdb> thread 3 # or any other thread number to change context in gdb
	gdb> bt
	gdb> frame
	gdb> up
	gdb> down
	gdb> info locals

Known Issues:
=============

1. In gdb mode, 'info threads' might hang for few seconds, and print only 2
   threads
2. In gdb mode, 'bt' might fail to show backtrace in few vmcores collected
   from older kernels. This is a known issue due to register mismatch, and
   its fix has been accepted upstream:
Commit: https://github.com/linuxppc/linux/commit/b684c09f09e7a6af3794d4233ef785819e72db79

TODO:
=====

1. Introduce automatic thread selection in gdb mode, to select the crashing
   thread in gdb, eliminating the need to manually run "thread <id>" after
   switching to gdb mode. 


Aditya Gupta (5):
  add generic get_dumpfile_regs to read registers
  ppc64: fix gdb passthrough by implementing machdep->get_cpu_reg
  remove 'frame' from prohibited commands list
  implement 'frame', 'up', 'down' inside crash
  make cpu context change transparent to crash/gdb

 defs.h          | 135 ++++++++++++++++++++++++++
 gdb-10.2.patch  |  28 ++++++
 gdb_interface.c |   2 +-
 global_data.c   |   3 +
 help.c          |  34 +++++++
 kernel.c        | 183 +++++++++++++++++++++++++++++++++++
 ppc64.c         | 250 +++++++++++++++++++++++++++++++++++++++++++++++-
 task.c          |   1 +
 tools.c         |  12 ++-
 9 files changed, 641 insertions(+), 7 deletions(-)

-- 
2.41.0