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
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