linux-kernel - Re: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead of 0 as initial sum

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Date:   Tue, 21 Jul 2020 13:58:47 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead
 of 0 as initial sum

On Tue, Jul 21, 2020 at 1:55 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> This seems dangerous to me.
>
> Maybe some implementation depends on the fact that they actually do
> the csum 16 bits at a time, and never see an overflow in "int",
> because they keep folding things.
>
> You now break that assumption, and give it an initial value that the
> csum code itself would never generate, and wouldn't handle right.
>
> But I didn't check. Maybe we don't have anything that stupid in the kernel.

I take it back. The very first place I looked seemed to do exactly that.

See "do_csum()" in the kernel. It doesn't handle carry for any of the
usual cases, exactly because it knows it doesn't need to.

Ok, so do_csum() doesn't take that initial value, but it's very much
an example of the kind of algorithm I was thinking of: it does do
things 32 bits at a time and handles the carry bit in that inner loop,
but internally it knows that the val;ues are limited in other places,
and doesn't need to handle carry everywhere.

                Linus