Messages in this thread | | | Date | Tue, 1 May 2018 09:28:00 -0500 | From | Josh Poimboeuf <> | Subject | Re: ORC unwinder bad backtrace |
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On Wed, Apr 18, 2018 at 10:45:48AM -0500, Josh Poimboeuf wrote: > On Wed, Apr 18, 2018 at 06:54:38AM -0700, Matthew Wilcox wrote: > > f81061192 <pte_clear.constprop.18>: > > ... > > ffffffff810611bf: 90 nop > > ffffffff810611c0 <perf_trace_x86_exceptions>: > > > > I suspect an off-by-one error; you don't really mean to point to the > > byte before perf_trace_x86_exception, you mean to point to byte 0 of > > perf_trace_x86_exception. > > > > I'm going to archive up this compilation in case there's anything useful > > I can extract for you from it later. > > Thanks for reporting this. So there are really two issues: > > 1) The question marks mean the ORC unwinder got confused (and had to > fall back to the crude "just print all text addresses on the stack"). > This is the real issue. > > 2) As you found, what should be "perf_trace_x86_exceptions+0x0" is > actually printed as "pte_clear.constprop.18+0x2e". I don't think > this is fixable, because this is printed by the oops fallback code > which just blindly prints out all the text addresses it finds on the > stack when the unwinder fails. It can't know whether the address was > a call return address (the usual case) or was something else (in this > case I suspect it's just a function pointer which just happens to be > on the stack), so it assumes the former, and prints it accordingly. > This isn't fixable per se -- but it will be "fixed" when we fix #1, > which will give a deterministic stack trace instead of using the dumb > fallback code. > > Is it possible for you to copy the vmlinux somewhere? That would be the > easiest option for debugging. > > Otherwise I may ask for some specifics for you to gather from it. > > Is it recreatable? Once I come up with a fix, it would be helpful to > test with the same scenario. > > Also has the root cause of the stack recursion been found? It looks > like the perf_trace_x86_exceptions() tracepoint code is doing something > bad.
Matthew, ping?
-- Josh
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