Re: [Valgrind-users] threads vs main and invalid stack writes

[John R. <jr...@bi...>]

>     alloca(4096);
>     __yell();

> 
> (gdb) monitor get_vbits 0xffeffeed0 256
> ________ ________ ________ ________ ________ ________ ________ ________
> ________ ________ ________ ________ ________ ________ ________ ________
> ________ ________ ________ ________ ________ ________ ________ ________
> ________ ________ ________ ________ ________ ________ ________ ________
> ________ ________ ________ ________ ffffffff ffffffff ffffffff ffffffff
> ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff
> ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff
> ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff
> Address 0xFFEFFEED0 len 256 has 144 bytes unaddressable
> 
> Any ideas? It seems to depend on:
> 
>  - Some (small) number of threads being spawned.
>  - A > page-sized alloca().
>  - Reasonably sized memset on top.
>  - It's always the main thread which suffers.

Thank you for the small, reproducible testcase!

It's a bug in valgrind, so please file a bug report, and include the testcase.
See the "Bug Reports" entry in the left column of the main page
http://www.valgrind.org/ .

Meanwhile the trivial workaround is to memset every result of alloca.
Oh, and "#include <string.h>"