[openssl-dev] [openssl.org #4094] Nonsensical pointer comparison in PACKET_buf_init

[Kurt Roeckx via RT]

On Fri, Oct 16, 2015 at 06:50:36PM +0000, Kurt Roeckx via RT wrote:
> On Fri, Oct 16, 2015 at 04:50:59PM +0000, Matt Caswell via RT wrote:
> > In a well-behaved program there is no undefined behaviour. The "buf +
> > len < buf" check will always evaluate to false, so in that sense is
> > useless but it *is* well defined.
> 
> The defined behaviour for the "buf + len" part is as far as I know
> that you're that the pointer should point inside the allocated
> object or 1 byte after it.  So as long as "len" is in the valid
> range, the "buf + len" part should be well defined.  The test with
> -1 is clearly undefined.
> 
> As far as I know in the comparison pointers they should point
> to the same object.  But the check seems to imply that they might
> not point to the same object or that buf is not the base of the
> object.  But since len is unsigned only the option that they don't
> point to the same object seems to be left.
> 
> So it's unclear to me if this is defined behaviour or not.

So thinking about this some more, it seems to be a check for
undefined behaviour that probably itself is still defined.

That probably also means the compiler can assume that it's always
false and eliminate the check, it's probably just not smart enough
yet.


Kurt