[Bug sanitizer/109446] Possible destination array overflow without diagnosis in memcpy

["marxin at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109446

--- Comment #9 from Martin Liška <marxin at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #8)
> (In reply to Jakub Jelinek from comment #7)
> > (In reply to Richard Biener from comment #6)
> > > not sure if we should prevent all of those transforms.  But the question is
> > > why ASAN doesn't instrument the generated aggregate copy?  Maybe because
> > > in C/C++ you cannot write an aggregate array copy?
> > 
> > We do instrument those.  But only instrument them by checking the first and
> > last byte
> > of the copy, not all bytes in between (because that would be for inline
> > checking too large - we'd need to emit inline a loop over those bytes).
> 
> OK, that's lack of an appropriate API then?  But still the first and last
> byte should be sufficient to detect the problem (but maybe I'm missing
> something here).

No, because the last byte is out of redzone:

=>0x7ffff5300000: f1 f1 f1 f1 00 00 00[f3]f3 f3 f3 f3 00 00 00 00

the 'f3' redzone is covering 5*8 bytes after the data type only.