[Crash-utility] Re: [PATCH v9 0/5] Improve stack unwind on ppc64

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

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: ========== 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