netdev - Re: [GIT] Networking

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Message-ID: <CALCETrUijiLwtYRwc-UHPkuhh+PoMh0scwffewuJ+7wRBR=Jfg@mail.gmail.com>
Date:	Mon, 2 Nov 2015 17:58:29 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Andy Lutomirski <luto@...nel.org>,
	David Miller <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Network Development <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [GIT] Networking

On Mon, Nov 2, 2015 at 5:54 PM, Linus Torvalds
<torvalds@...ux-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
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