12:29 a.m.
Hi Tao Liu,
<...snip...>
> For now it seems to work fine on ppc64, I will go through the code
> changes once. By the way, can you post a rebased patch series for x86 ?
I noticed there is some problem with the ppc64 change thread context
support. Currently for x86_64, we use "set <pid>" cmd to switch the
current task context to be the task <pid>'s, then we use cmd "gdb
bt"
to view the stack unwinding of task <pid>, by which we can view
arbitrary task's stack unwinding.
However for ppc64 code, there is no "get arbitrary tasks register"
code. E.g. in ppc64_get_cpu_reg():
task = get_active_task(cpu);
tc = task_to_context(task);
So no matter how we set any pid, the registers are always the active
tasks' register, instead of <pid> tasks' register.
Hmm. True, I will check that, true, gdb as of now will get active tasks
registers only, as this feature was meant for that, since getting gdb to
assist for the crashing process was the priority.
Setting context to arbitrary pid ('set <pid>') and backtrace of any
arbitrary pid works on ppc64, so there should be a way to get those
registers, i will go through the code once to check this.
But that arbitrary backtrace will be a very good plus.
It would be simple to get this fixed, however the more challenging
part, which is difficult for me, is how to get the inactive tasks
registers, So we can do similar things as in
x86_64.c:x86_64_get_stack_frame() of
https://github.com/liutgnu/crash-dev?
I see there is a structure as task_struct -> thread_struct -> struct
pt_regs *regs. But sometimes the regs will give NULL value.
True, the registers are NULL atleast in case of the swapper processes.
I will update on my findings.
Thanks,
Aditya Gupta
>
> Thanks,
> Tao Liu
>
>
>
>
>
>
>
> >
> > Thanks,
> > Aditya Gupta
> >
> > >
> > > Thanks,
> > > Tao Liu
> > >
> > >
> > > >
> > > > Thanks,
> > > > Kazu
> > > >
> > > > On 2024/01/05 16:30, Aditya Gupta 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.
> > > > >
> > > > > Note: This doesn't support live debugging on ppc64, since
registers are not
> > > > > available to be read
> > > > >
> > > > > Implications on Architectures:
> > > > > ====================================
> > > > >
> > > > > No architecture other than PPC64 has been affected, other than in
case of
> > > > > 'frame' command
> > > > >
> > > > > As mentioned in patch #2, since frame will not be prohibited, so
it will print:
> > > > >
> > > > > crash> frame
> > > > > #0 <unavailable> in ?? ()
> > > > >
> > > > > Instead of before prohibited message:
> > > > >
> > > > > crash> frame
> > > > > crash: prohibited gdb command: frame
> > > > >
> > > > > 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, it will be able to give all this
information.
> > > > >
> > > > > Testing:
> > > > > ========
> > > > >
> > > > > Git tree with this patch series applied:
> > > > > https://github.com/adi-g15-ibm/crash/tree/stack-unwind-v6
> > > > >
> > > > > To test various gdb passthroughs:
> > > > >
> > > > > (crash) set
> > > > > (crash) set gdb on
> > > > > gdb> thread
> > > > > gdb> bt
> > > > > gdb> info threads
> > > > > gdb> info threads
> > > > > gdb> info locals
> > > > > gdb> info variables irq_rover_lock
> > > > > gdb> info args
> > > > > gdb> thread 2
> > > > > gdb> set gdb off
> > > > > (crash) set
> > > > > (crash) set -c 6
> > > > > (crash) gdb thread
> > > > > (crash) bt
> > > > > (crash) gdb bt
> > > > > (crash) frame
> > > > > (crash) up
> > > > > (crash) down
> > > > > (crash) info locals
> > > > >
> > > > > Known Issues:
> > > > > =============
> > > > >
> > > > > 1. 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 merged upstream:
> > > > >
> > > > > This can also cause some 'invalid kernel virtual
address' errors during gdb
> > > > > unwinding the stack registers
> > > > >
> > > > > Commit:
https://github.com/torvalds/linux/commit/b684c09f09e7a6af3794d4233ef78581...
> > > > >
> > > > > 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.
> > > > >
> > > > > Changelog:
> > > > > ==========
> > > > >
> > > > > V6:
> > > > > + changes in patch #5: fix bug introduced in v5 that caused
initial gdb thread
> > > > > to be thread 1
> > > > >
> > > > > V5:
> > > > > + changes in patch #1: made ppc64_get_cpu_reg static, and remove
unreachable
> > > > > code
> > > > > + changes in patch #3: fixed typo 'ppc64_renum' instead
of 'ppc64_regnum',
> > > > > remove unneeded if condition
> > > > > + changes in patch #5: implement refresh regcache on per thread,
instead of all
> > > > > threads at once
> > > > >
> > > > > V4:
> > > > > + fix segmentation fault in live debugging (change in patch #1)
> > > > > + mention live debugging not supported in cover letter and patch
#1
> > > > > + fixed some checkpatch warnings (change in patch #5)
> > > > >
> > > > > 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(-)
> > > > >
> > >
> >
>