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

Hi Tao, On Thu, Feb 22, 2024 at 04:57:23PM +0800, Tao Liu wrote: > Hi Aditya, > > On Thu, Feb 22, 2024 at 1:23 PM Aditya Gupta <adityag(a)linux.ibm.com&gt; 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-v9 > > I doubt the v9 patch will not work without my x86's trial patchset. I > see your repo, you directly applied my "ppc64 arbitrary task stack > unwind support" patch onto yours. However the patch has some > dependency on my x86 trial patchset. Sorry, and thanks for the quick mail. > > E.g. I added a new member "bool need_free" for defs.h:struct bt_info > in the "x86 unwind support" patch of mine, however you didn't pick the > one. And in ppc64.c:ppc64_get_stack_frame() and > ppc64.c:ppc64_get_cpu_reg(), the member will be used as: > > if (bt_info.need_free) { > FREEBUF(pt_regs); > bt_info.need_free = FALSE; > } > > So I guess (not tried yet) that your patchset v9 will not work. True, it will not work. Just checked, I had already applied your patches also and then that patch, where I tested this series. My mistake, please skip testing this one. > > Currently I'm still struggling with some failing cases of x86_64 > unwinding. So I didn't arrange my patchsets, along with the patch > commit log well, since they are all "trial" patches. > > I agree the patch "ppc64 arbitrary task stack unwind support" is > better to go with the ppc patch series. But I suggest we make some > modifications for it: > > 1) I'm OK with it being a stand alone patch, or merging the code > changes of this one into your previous patches, but I prefer the > latter one :) > > 2) If you'd like to go with a stand alone patch, could you please > rewrite a commit log and title for this one? Sure, I am okay with either. Let's go with the latter one you suggested, in that case I will add the line adding 'need_free' in 'bt_info' structure also, to make that work. By the way, shouldn't it be 'FREEBUF(bt_info.stackbuf)' instead of 'FREEBUF(pt_regs)' ? Also, should I add you as a co-author of the patch ?

Thanks, Aditya Gupta > > Thanks, > Tao Liu > > > > > > 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 > > >