Re: [PATCH 1/2] powerpc/64: Fix checksum folding in csum_tcpudp_nofold and ip_fast_csum_nofold

On Tue, Nov 08, 2016 at 06:23:30PM +1100, Michael Ellerman wrote:

Paul Mackerras [off-list ref] writes:

quoted

These functions compute an IP checksum by computing a 64-bit sum and
folding it to 32 bits (the "nofold" in their names refers to folding
down to 16 bits).  However, doing (u32) (s + (s >> 32)) is not
sufficient to fold a 64-bit sum to 32 bits correctly.  The addition
can produce a carry out from bit 31, which needs to be added in to
the sum to produce the correct result.

To fix this, we copy the from64to32() function from lib/checksum.c
and use that.

This seems to have been broken since ~forever. Do we just not hit that
case very often, or do we just incorrectly report checksum failures?

I think there would be about a 1 in a billion chance of hitting it by
chance, though you could probably construct a test case that would hit
it every time.  If you did hit it in real life it would result in a
packet being dropped and presumably retransmitted, and I expect that
the IP header of the retransmitted packet would be sufficiently
different (i.e. different id field or something) that it wouldn't hit
the bug a second time.

Should it go to stable?

Probably... though nobody has actually noticed a problem in real life
and pinned it down to this.

Paul.