On Mon, Dec 11, 2023 at 08:04:50PM +0800, Lianbo Jiang wrote:
> On 12/9/23 20:45, Aditya Gupta wrote:
>
> > Hi, just a ping. Any comments on the series ?
>
> Hi, Aditya
>
>
> Thank you for the update. I will have a look and do the tests this week. And
> give some feedback.
Sure. Thanks Lianbo.
- Aditya Gupta
>
> Thanks.
>
> Lianbo
>
> >
> > On Mon, Dec 04, 2023 at 08:29:36PM +0530, 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.
> > >
> > > 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-3
> > >
> > > To test various gdb passthroughs:
> > >
> > > gdb> set
> > > gdb> 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
> > > gdb> set
> > > gdb> set -c 6
> > > gdb> gdb thread
> > > gdb> bt
> > > gdb> gdb bt
> > > gdb> frame
> > > gdb> up
> > > gdb> down
> > > gdb> 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:
> > >
> > > 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.
> > >
> > > Additional Notes:
> > > =================
> > >
> > > Sorry, it took a long time to send this version. Tried fixing 'info
> > > threads' but wasn't able to. Gave it time again, and was able to fix it
> > > this time after multiple days of debugging.
> > >
> > > Some other things from last version review:
> > >
> > > * 'info rv' not working:
> > > It's not supported in gdb, instead we need to use 'info locals rv' or
> > > 'info variables rv'
> > >
> > > * 'info variables' command hangs... and prints nothing after hanging for long
> > > It likely hangs due to a lot of symbols being there, and it's trying to
> > > get all gdb's output and page it, so Control+C messes it up, but if we pass
> > > a regex filter to limit the output, eg. info variables rq, then it doesn't
> > > hang, and prints the variables/symbols.
> > > Even with gdb, ie. simply running 'gdb vmlinux vmcore' also hangs due
> > > to the lot of symbols
> > >
> > > * making crashing thread as default in gdb:
> > > This is implemented now, along with synchronising crash & gdb contexts, in
> > > patch #3
> > >
> > > * 'info threads' not working:
> > > This turned to be due to a bug in gdb_interface. I fixed 'info
> > > threads' in 2 patches, to simplify it, first for the gdb_interface,
> > > and another patch for setting the context correctly in crash
> > >
> > > * other info commands:
> > > I tested all the info commands, in crash along with this patch.
> > > Most of those that fail in crash are due to gdb itself not supporting
> > > them with vmcores, and other than that is the 'info pretty' command,
> > > which might not be needed in crash anyways
> > >
> > > * live debugging showing only one thread:
> > > I tried it with crash, crash shows only the current thread, ie.
> > > itself, so it does not have information of registers for the other
> > > CPUs. Similarly gdb does not support live kernel debugging (without
> > > connecting to a gdbstub/QEMU etc.).
> > > If you need I can make it show the current thread id correctly for
> > > the one thread, but I don't think it might help much with live
> > > debugging
> > >
> > > Hope, I set the context, thanks for the reviews, I replied and worked
> > > on your suggestions, but got stuck there due to 'info threads'
> > >
> > > 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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