[Crash-utility] [RFC PATCH 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 Also, backtrace can be slightly different in gdb and crash (due to gdb being able to print inline frames also). so it can cause confusion of 'bt' working in crash context, but 'frame'/'up'/'down' working as 'gdb passthrough', showing different frames. This has been explained in patch #4. So to prevent confusion mentioned above, implement 'frame', 'up', 'down' as commands in default crash mode also, instead of working via gdb passthroughs which they do currently. So, now in default mode, 'bt','frame','up','down' will be consistent with each other. Implications on Architectures: ==================================== No architecture other than PPC64 has been affected, other than in case of 'frame', 'up', 'down' commands 1. frame: As mentioned in patch #2, that frame will not be prohibited, and will print: crash> frame #0 <unavailable> in ?? () Instead of before prohibited message: crash> frame crash: prohibited gdb command: frame 2. up/down: These commands will now be run as native crash commands by default instead of showing crash> up crash: ambiguous command: up (symbol and gdb command) crash> down crash: ambiguous command: down (symbol and gdb command) On PPC64, the default mode ("crash mode") will not have ANY OTHER changes, other than the 'frame', 'up', 'down' as mentioned above. 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 Testing: ======== Git tree with this patch series applied: https://github.ibm.com/adityag/crash (replace this link with github.com later) To test 'frame'/'up'/'down' in crash (implemented in patch #3): crash> bt crash> frame crash> up crash> down crash> up 4 To test 'bt'/'frame'/'up'/'down'/'info locals' gdb passthroughs: crash> set gdb on gdb> thread 3 # or any other thread number to change context in gdb gdb> bt gdb> frame gdb> up gdb> down gdb> info locals Known Issues: ============= 1. In gdb mode, 'info threads' might hang for few seconds, and print only 2 threads 2. 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 accepted upstream: Commit: https://github.com/linuxppc/linux/commit/b684c09f09e7a6af3794d4233ef78581... TODO: ===== 1. Introduce automatic thread selection in gdb mode, to select the crashing thread in gdb, eliminating the need to manually run "thread <id>" after switching to gdb mode. Aditya Gupta (5): add generic get_dumpfile_regs to read registers ppc64: fix gdb passthrough by implementing machdep->get_cpu_reg remove 'frame' from prohibited commands list implement 'frame', 'up', 'down' inside crash make cpu context change transparent to crash/gdb defs.h | 135 ++++++++++++++++++++++++++ gdb-10.2.patch | 28 ++++++ gdb_interface.c | 2 +- global_data.c | 3 + help.c | 34 +++++++ kernel.c | 183 +++++++++++++++++++++++++++++++++++ ppc64.c | 250 +++++++++++++++++++++++++++++++++++++++++++++++- task.c | 1 + tools.c | 12 ++- 9 files changed, 641 insertions(+), 7 deletions(-) -- 2.41.0