Re: [Libunwind-devel] [RFC][patch] Fix crash in _ULx86_64_tdep_trace when sigaltstack is too far away

[Paul Pluzhnikov]

On Fri, Nov 11, 2011 at 1:41 PM, Lassi Tuura <address@hidden> wrote:

> Yes, you are right, I noticed the same. But we probably shouldn't store
> the difference to the original stack but to the ucontext_t which should
> be in nearby offset, on the signal stack.

In fact, AFAICT for Linux ucontext_t is the very last thing copied onto
the stack before invoking the signal handler. That is, when signal handler
start running, the stack layout is:

  ... higher addresses ...
         ucontext
  CFA->
         __restore_rt (== pretcode in rt_sigframe from
                       linux-2.6/arch/x86/include/asm/sigframe.h)
  SP ->
        ... sighandler runs on this stack.

  ... lower addresses ...

This makes it very convenient to find ucontext from the CFA.

Attached patch re-tested on Linux/x86_64, no new failures.

Thanks,

P.S. test-setjmp is failing for me (before or after the patch).
When I enable assertions (to confirm my new assertions are correct), I see:

  lt-test-setjmp: ../../src/dwarf/Gparser.c:754: apply_reg_state: \
    Assertion `rs->reg[17].where == DWARF_WHERE_EXPR' failed.

which likely explains that failure.

-- 
Paul Pluzhnikov

libunwind-crash-in-fasttrace-20111114.txt
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