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.
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,
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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>
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