Messages in this thread | | | Date | Wed, 12 Apr 2023 15:01:53 +0800 | Subject | Re: [PATCH 1/2] x86: profiling: remove lock functions hack for !FRAME_POINTER | From | Chen Zhongjin <> |
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On 2023/4/11 3:34, Dave Hansen wrote: > On 4/9/23 19:22, Chen Zhongjin wrote: >> Syzbot has been reporting the problem of stack-out-of-bounds in >> profile_pc for a long time: >> https://syzkaller.appspot.com/bug?extid=84fe685c02cd112a2ac3 >> >> profile_pc tries to get pc if current regs is inside lock function. For >> !CONFIG_FRAME_POINTER it used a hack way to get the pc from stack, which >> is not work with ORC. It makes profile_pc read illeagal address, return >> wrong result, and frequently triggers KASAN. >> >> Since lock profiling can be handled with much better other tools, It's >> reasonable to remove lock functions hack for !FRAME_POINTER kernel. > OK, so let me make sure I understand what's going on: > > 1. This whole issue is limited to kernel/profile.c which is what drives > readprofile(8) and /proc/profile > 2. This is removing code that got added in 2006: > 0cb91a229364 ("[PATCH] i386: Account spinlocks to the caller during > profiling for !FP kernels") > 3. This was an OK hack back in the day, but it outright breaks today > in some situations. KASAN also didn't exist in 2006. Yes, and whether KASAN is enabled it can make problem. Some lock_function will save registers on stack (this may not happen in 2006). These registers can be recorded by profile and be read outside of kernel, which is risky theoretically. > 4. !CONFIG_FRAME_POINTER is probably even more rare today than it was in > 2006 No. !CONFIG_FRAME_POINTER is more common today because of UNWINDER_ORC. And that is why the bug is triggered more frequently. > 5. Lock function caller information is available at _least_ from perf, > maybe other places too?? (What "much better other tools" are there?)
Yes, it's basically about perf function graph.
> Given all that, this patch suggests that we can remove the stack peeking > hack. The downside is that /proc/profile users will see their profiles > pointing to the spinlock functions like they did in 2005. The upside is > that we won't get any more KASAN reports. > > If anyone complains, I assume we're just going to tell them to run 'perf > --call-graph' and to go away (which also probably didn't exist in 2006). > > If I got all that right, the end result seems sane to me. It would be > _nice_ if you could make a more coherent changelog out of that and > resend. Also, considering that your two "profile" issues are quite > independent, you can probably just resend the two patches separately. Thanks for review and I'll send another version to provide better details.
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