Hi Aditya,
On Thu, Feb 22, 2024 at 1:23 PM Aditya Gupta <[email protected]> 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-v9
I doubt the v9 patch will not work without my x86's trial patchset. I
see your repo, you directly applied my "ppc64 arbitrary task stack
unwind support" patch onto yours. However the patch has some
dependency on my x86 trial patchset.
E.g. I added a new member "bool need_free" for defs.h:struct bt_info
in the "x86 unwind support" patch of mine, however you didn't pick the
one. And in ppc64.c:ppc64_get_stack_frame() and
ppc64.c:ppc64_get_cpu_reg(), the member will be used as:
if (bt_info.need_free) {
FREEBUF(pt_regs);
bt_info.need_free = FALSE;
}
So I guess (not tried yet) that your patchset v9 will not work.
Currently I'm still struggling with some failing cases of x86_64
unwinding. So I didn't arrange my patchsets, along with the patch
commit log well, since they are all "trial" patches.
I agree the patch "ppc64 arbitrary task stack unwind support" is
better to go with the ppc patch series. But I suggest we make some
modifications for it:
1) I'm OK with it being a stand alone patch, or merging the code
changes of this one into your previous patches, but I prefer the
latter one :)
2) If you'd like to go with a stand alone patch, could you please
rewrite a commit log and title for this one?
Thanks,
Tao Liu
>
> 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) gdb up
> (crash) gdb 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/b684c09f09e7a6af3794d4233ef785819e72db79
>
> 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:
> ==========
>
> V9:
> + minor change in patch #5: sync gdb context on a 'set' and 'set -p'
> + add taoliu's patch for using current context, and fixes in ppc64_get_cpu_reg
>
> V8:
> + use get_active_task instead of depending on CURRENT_CONTEXT in
> ppc64_get_cpu_reg
> + rebase to upstream/master (5977936c0a91)
>
> V7:
> + move changes in gdb-10.2.patch to the end (minor change in patch #3,4,5)
> + fix a memory leak in ppc64_get_cpu_reg (minor change in patch #1)
> + use ascii diagram in patch #1 description
>
> 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(-)
>
> --
> 2.41.0
>
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