Re: _FORTIFY_SOURCE for std::vector

[Florian Weimer <fweimer at redhat dot com>]

On 06/04/2012 09:07 PM, Marc Glisse wrote:
> On Mon, 4 Jun 2012, Florian Weimer wrote:
> > void
> > write(std::vector<float>& blob, unsigned n, float v1, float v2, float
> > v3, float v4)
> > {
> > blob[n] = v1;
> > blob[n + 1] = v2;
> > blob[n + 2] = v3;
> > blob[n + 3] = v4;
> > }
>
> Would be great if it ended up testing only n and n+3.

True.

> __attribute__((__noreturn__)) is not quite strong enough to allow this
> optimization, it would require something like
> __attribute__((__crashing__)) to let the compiler know that if the
> function is called, you don't care what happens to blob. And possibly
> the use of a signed n.

Interesting point, I had not realized that before.  Ada has a special 
rule for failures of language-defined checks, and they might give enough 
wiggle room to leave behind a partially updated vector in such situations.

But even without that, you could clone the if sequence, that is,

  if (blob.size() - n >= 4)
    {
      blob[n] = v1;
      blob[n + 1] = v2;
      blob[n + 2] = v3;
      blob[n + 3] = v4;
    }
  else
    {
       ... // individual checks
    }

Obviously, this has quite a bit of an impact on code size.

--
Florian Weimer / Red Hat Product Security Team