On Fri, Dec 02, 2016 at 09:31:02PM +0100, Rasmus Villemoes wrote:
> The expression info->free+size is technically undefined behaviour in
> exactly the case we want to test for. Moreover, the compiler is likely
> to translate the expression to
>
> (unsigned long)info->free + size > (unsigned long)info->end
>
> where there's at least a theoretical chance that the LHS could wrap
> around 0, giving a false negative.
>
> This might as well be written using pointer subtraction avoiding these
> issues.
> [...]
>
> - if (!info->slab_count || info->free + size > info->end) {
> + if (!info->slab_count || size > info->end - info->free) {
Yeah, I agree the correct way to write this is to compare the sizes
directly. That is how overflow checks _must_ be written. This one is
less likely to overflow, but even computing the value more than one past
the end of the array is technically undefined.
-Peff