On 12/5/23 13:41, Tao Liu wrote:
Hi Aditya,
On Tue, Dec 5, 2023 at 12:24 PM Aditya Gupta <adityag@xxxxxxxxxxxxx> wrote:
On Tue, Dec 05, 2023 at 10:25:58AM +0800, Tao Liu wrote:
Hi Aditya,
On Mon, Dec 4, 2023 at 11:47 PM Aditya Gupta <adityag@xxxxxxxxxxxxx> wrote:
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
This way other features of 'gdb', such as seeing
backtraces/registers/variables/arguments/local variables, moving up and
down stack frames, can be used with any ppc64 vmcore, irrespective of
being ELF format or kdump-compressed format.
< ...snip... >
Fixing GDB passthroughs on other architectures
==============================================
Much of the work for making gdb passthroughs like 'gdb bt', 'gdb
thread', 'gdb info locals' etc. has been done by the patches introducing
'machdep->get_cpu_reg' and this series fixing some issues in that.
Other architectures should be able to fix these gdb functionalities by
simply implementing 'machdep->get_cpu_reg (cpu, regno, ...)'.
The reasoning behind that has been explained with a diagram in commit
description of patch #1
I will assist with my findings/observations fixing it on ppc64 whenever needed.
< ...snip... >
Thanks a lot for the PPC64 solution. However do you have any plans for
supporting stack unwinding on other ARCHs, such as x86_64/86/arm64...
etc?
I think it is an important feature for crash to support stack frame
up/down and local variable inspecting. So I'd like to take
x86_64/64/arm etc works based on your v3 work if they are not on your
to-do list. Do you have any suggestions?
Thanks for showing the interest, please feel free to implement it for x86_64/arm
also.
Sounds good, Tao and Aditya.
Is anyone interested in implementing it on other arches(eg. S390x, ...)
if possible?
Thanks.
Lianbo
Some notes:
1. First thing to try can be to implement 'machdep->get_cpu_reg' for the
architectures to give register values for asked cpu.
2. Need to check the register numbers for each register, eg. PC (Program
Counter) register is regno=64 in ppc64. If no documentation is there for this
mapping, then can consider just printing register numbers and names in
the loop in 'crash_target::fetch_registers'.
3. I will recommend applying atleast patch #2, #3, #4, #5. They are architecture
independent and will make the job easier, they take care of synchronising the
crash and gdb contexts (which is required so that gdb can get correct
registers for it's current thread), and also fixes a problematic bug in
gdb_interface which took my days to debug.
Only patch #1 is ppc specific.
4. Debugging gdb with gdb can be an headache, so I will recommend adding these
to your ~/.gdbinit to ignore some continuous interrupts:
set history save on
set print pretty on
set pagination off
set confirm off
# ignore this signal while debugging gdb via gdb
handle SIGTTOU nostop
handle SIGTTOU ignore
handle SIGTTOU noprint
5. Putting a breakpoint at 'get_current_frame' can be helpful :)
Thanks a lot for the detailed info! I will keep your suggestions in mind.
Thanks,
Tao Liu
Thanks,
Aditya Gupta
Thanks,
Tao Liu
Changelog:
==========
V3:
+ default gdb thread will be the crashing thread, instead of being
thread '0'
+ synchronise crash cpu and gdb thread context
+ fix bug in gdb_interface, that replaced gdb's output stream, losing
output in some cases, such as info threads and extra output in info
variables
+ fix 'info threads'
RFC V2:
- removed patch implementing 'frame', 'up', 'down' in crash
- updated the cover letter by removing the mention of those commands other
than the respective gdb passthrough
Aditya Gupta (5):
ppc64: correct gdb passthroughs by implementing machdep->get_cpu_reg
remove 'frame' from prohibited commands list
synchronise cpu context changes between crash/gdb
fix gdb_interface: restore gdb's output streams at end of
gdb_interface
fix 'info threads' command
crash_target.c | 44 ++++++++++++++++
defs.h | 130 +++++++++++++++++++++++++++++++++++++++++++++++-
gdb-10.2.patch | 110 +++++++++++++++++++++++++++++++++++++++-
gdb_interface.c | 2 +-
kernel.c | 47 +++++++++++++++--
ppc64.c | 95 +++++++++++++++++++++++++++++++++--
task.c | 14 ++++++
tools.c | 2 +-
8 files changed, 434 insertions(+), 10 deletions(-)
--
2.41.0
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