Re: ORC unwinder bad backtrace

[Josh Poimboeuf]

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