[Crash-utility] Re: [PATCH v10 0/5] Improve stack unwind on ppc64

On Tue, Mar 12, 2024 at 02:46:17PM +0800, Lianbo Jiang wrote: Hi Lianbo, Thanks for the ack. - Aditya Gupta > > > Thanks > > Lianbo > > On 3/4/24 17:59, 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. > > > > 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-v10 > > > > 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/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: > > ========== > > > > V10: > > + introduce 'bt_info.need_free' to prevent memory leak in get_cpu_reg > > > > 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 | 57 ++++++++++++++++++++ > > defs.h | 131 ++++++++++++++++++++++++++++++++++++++++++++- > > gdb-10.2.patch | 137 ++++++++++++++++++++++++++++++++++++++++++++++++ > > gdb_interface.c | 2 +- > > kernel.c | 49 +++++++++++++++-- > > ppc64.c | 131 ++++++++++++++++++++++++++++++++++++++++++--- > > task.c | 32 ++++++++--- > > tools.c | 10 ++-- > > 8 files changed, 522 insertions(+), 27 deletions(-) > > >