[RFC][PR 67336][PING^6] Verify pointers during stack unwind

[Florian Weimer]

* Yuri Gribov:

> I've rebased the previous patch to trunk per Andrew's suggestion.
> Original patch description/motivation/questions are in
> https://gcc.gnu.org/ml/gcc-patches/2017-06/msg01869.html

What about the remaining TODOs?

> +  if (__builtin_expect (msync ((void *)page, len, MS_ASYNC), 0))
> +    return 0;

Is there any reason not to probe using mincore?  It won't trigger
writeback.