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SubjectRe: ORC unwinder bad backtrace
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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