Re: Segfault in _Unwind_* code called from pthread_exit

From: Andreas Tobler <>
Date: Sat, 21 Oct 2017 22:02:38 +0200
On 26.08.17 20:40, Konstantin Belousov wrote:
> On Sat, Aug 26, 2017 at 08:28:13PM +0200, Tijl Coosemans wrote:
>> On Sat, 26 Aug 2017 02:44:42 +0300 Konstantin Belousov <> wrote:
>>> How does llvm unwinder detects that the return address is a garbage ?
>> It just stops unwinding when it can't find frame information (stored in
>> .eh_frame sections).  GCC unwinder doesn't give up yet and checks if the
>> return address points to the signal trampoline (which means the current
>> frame is that of a signal handler).  It has built-in knowledge of how to
>> unwind to the signal trampoline frame.
> So llvm just gives up on signal frames ?
>> A noreturn attribute isn't enough.  You can still unwind such functions.
>> They are allowed to throw exceptions for example.
> Ok.
>> I did consider using
>> a CFI directive (see patch below) and it works, but it's architecture
>> specific and it's inserted after the function prologue so there's still
>> a window of a few instructions where a stack unwinder will try to use
>> the return address.
>> Index: lib/libthr/thread/thr_create.c
>> ===================================================================
>> --- lib/libthr/thread/thr_create.c      (revision 322802)
>> +++ lib/libthr/thread/thr_create.c      (working copy)
>> _at__at_ -251,6 +251,7 _at__at_ create_stack(struct pthread_attr *pattr)
>>   static void
>>   thread_start(struct pthread *curthread)
>>   {
>> +       __asm(".cfi_undefined %rip");
>>          sigset_t set;
>>          if (curthread->attr.suspend == THR_CREATE_SUSPENDED)
> I like this approach much more than the previous patch.  What can be
> done is to provide asm trampoline which calls thread_start().  There you
> can add the .cfi_undefined right at the entry.
> It is somewhat more work than just setting the return address on the
> kernel-constructed pseudo stack frame, but I believe this is ultimately
> correct way.  You still can do it only on some arches, if you do not
> have incentive to code asm for all of them.
> Also crt1 probably should get the same treatment, despite we already set
> %rbp to zero AFAIR.

Did some commit result out of this discussion or is this subject still 
under investigation?

Curious because I got this gcc PR:

Received on Sat Oct 21 2017 - 18:02:50 UTC

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