From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org (ozlabs.org [IPv6:2401:3900:2:1::2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3tD3Kj0ZFkzDvNW for ; Wed, 9 Nov 2016 09:06:01 +1100 (AEDT) Date: Wed, 9 Nov 2016 09:05:57 +1100 From: Paul Mackerras To: Michael Ellerman Cc: linuxppc-dev@ozlabs.org Subject: Re: [PATCH 1/2] powerpc/64: Fix checksum folding in csum_tcpudp_nofold and ip_fast_csum_nofold Message-ID: <20161108220557.GA28587@fergus.ozlabs.ibm.com> References: <20161103051055.GA8368@fergus.ozlabs.ibm.com> <878tsul9jh.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <878tsul9jh.fsf@concordia.ellerman.id.au> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, Nov 08, 2016 at 06:23:30PM +1100, Michael Ellerman wrote: > Paul Mackerras writes: > > > 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.