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

On 04/18/2017 05:04 AM, Corinna Vinschen wrote:
On Apr 17 03:39, Daniel Santos wrote:

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.

What about using RtlSecureZeroMemory instead?


Well that's surprising. Yes, it does solve the problem and I presume would be more portable. :) It even inlines the "memset", but uses a single byte rep stos. Technically, I think that a double-word stos could be used in this case, but I doubt that matters much.


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