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

[Linus Torvalds]

On Tue, Jul 21, 2020 at 1:55 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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