Re: [PATCH] strace: Fix crash caused over-optimization

[Daniel Santos <daniel dot santos at pobox dot com>]

On 04/16/2017 05:21 AM, Jon Turney wrote:
> On 15/04/2017 23:27, Daniel Santos wrote:
> > Recent versions of gcc are optimizing away the TLS buffer allocated in
> > main, so we need to tell gcc that it's really used.
> > ---
> >  winsup/utils/strace.cc | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/winsup/utils/strace.cc b/winsup/utils/strace.cc
> > index beab67b90..1e581b4a4 100644
> > --- a/winsup/utils/strace.cc
> > +++ b/winsup/utils/strace.cc
> > @@ -1192,6 +1192,8 @@ main (int argc, char **argv)
> >    char buf[CYGTLS_PADSIZE];
> >
> >    memset (buf, 0, sizeof (buf));
> > +  /* Prevent buf from being optimized away.  */
> > +  __asm__ __volatile__("" :: "m" (buf));
>
> wouldn't adding volatile to the definition of buf be a better way to 
> write this?

I actually did try that, although I had guessed it wouldn't (and 
shouldn't) work.  I believe that the reason is that rather the accesses 
are volatile or not, gcc can see nothing else using it and memset can be 
a treated as a compiler built-in (per the C spec, maybe C89?), so it can 
presume the outcome.  If there's a cleaner way to do this, I would 
really love to learn that.  __attribute__ ((used)) only works on 
variables with static storage.

Now I suspect that I may have found a little bug in gcc, because if I 
call memset by casting it directly as a volatile function pointer, it is 
still optimized away, and it should not:

  ((void *(*volatile)(void *, int, size_t))memset) (buf, 0, sizeof (buf));

And most interestingly, if I first assign a local volatile function 
pointer to the address, then gcc properly does *not* optimize it away:

  void *(*volatile vol_memset)(void *, int, size_t) = memset;
  vol_memset (buf, 0, sizeof (buf));

I'm actually really glad for your response and that I played with this 
because I need to make sure that this problem doesn't exist in gcc7.  I 
have changes going into gcc8 shortly and this could mask problems from 
my test program where I cast functions as volatile w/o assigning using a 
local.

Daniel