Re: [GIT] Networking

Andy Lutomirski
2015-11-03T01:58:51Z

On Mon, Nov 2, 2015 at 5:54 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> The biggest problem - and where the compiler could actually help us -
> tends to be multiplication overflows. We have several (not *many*, but
> certainly more than just a couple) cases where we simply check by
> dividing MAX_INT or something.
>
> See for example kmalloc_array(), which does
>
> if (size != 0 && n > SIZE_MAX / size)
> return NULL;
>
> exactly to avoid the overflow when it does the "n*size" allocation.
>
> So for multiplication, we really *could* use overflow logic. It's not
> horribly common, but it definitely happens.
>

Based in part on an old patch by Sasha, what if we relied on CSE:

if (mul_would_overflow(size, n))
return NULL;
do_something_with(size * n);

I haven't checked, but it would be sad if gcc couldn't optimize this
correctly if we use the builtins.

The downside is that I don't see off the top of my head how this could
be implemented using inline asm if we want a fast fallback when the
builtins aren't available.

--Andy