[Crash-utility] Re: [PATCH v3 0/5] Improve stack unwind on ppc64

Hi Lianbo, On Wed, Dec 13, 2023 at 03:20:40PM +0800, Lianbo Jiang wrote: Thanks for trying it. Was it 'crash-utility' which itself crashed due to the patches ? That is wierd, it should not cause any segmentation fault with the patches, can you please share the steps to reproduce this ? I will fix it. The patches are not intended to apply to gdb as such, but to provide the feature to have backtrace in gdb mode inside crash-utility. But the message by gdb seems to say it couldn't read the dump file: I will try to cause a crash with upstream kernel and see if anything breaks. Will let you know. Thanks, Aditya Gupta > > Thanks. > > Lianbo > > On 12/4/23 22:59, 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/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. > > > > 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(-) > > >