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 3tCglV4TH3zDvTG for ; Tue, 8 Nov 2016 18:23:34 +1100 (AEDT) From: Michael Ellerman To: Paul Mackerras , linuxppc-dev@ozlabs.org Subject: Re: [PATCH 1/2] powerpc/64: Fix checksum folding in csum_tcpudp_nofold and ip_fast_csum_nofold In-Reply-To: <20161103051055.GA8368@fergus.ozlabs.ibm.com> References: <20161103051055.GA8368@fergus.ozlabs.ibm.com> Date: Tue, 08 Nov 2016 18:23:30 +1100 Message-ID: <878tsul9jh.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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? Should it go to stable? cheers