From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.85.151]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C62A7D44 for ; Wed, 6 Dec 2023 03:11:03 -0800 (PST) Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with both STARTTLS and AUTH (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-4-2hKIkG_FPuSAvM53S8Wr_w-1; Wed, 06 Dec 2023 11:10:59 +0000 X-MC-Unique: 2hKIkG_FPuSAvM53S8Wr_w-1 Received: from AcuMS.Aculab.com (10.202.163.6) by AcuMS.aculab.com (10.202.163.6) with Microsoft SMTP Server (TLS) id 15.0.1497.48; Wed, 6 Dec 2023 11:10:45 +0000 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Wed, 6 Dec 2023 11:10:45 +0000 From: David Laight To: 'Al Viro' , "linux-arch@vger.kernel.org" CC: gus Gusenleitner Klaus , Al Viro , Thomas Gleixner , lkml , Ingo Molnar , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "x86@kernel.org" , "David S. Miller" , "dsahern@kernel.org" , "kuba@kernel.org" , Paolo Abeni , Eric Dumazet Subject: RE: [RFC][PATCHES v2] checksum stuff Thread-Topic: [RFC][PATCHES v2] checksum stuff Thread-Index: AQHaJyGyb9WP/snuGU2gdqpp5IxoMLCcGsYA Date: Wed, 6 Dec 2023 11:10:45 +0000 Message-ID: <602ab11ffa2c4cc49bb9ecae2f0540b0@AcuMS.aculab.com> References: <20231018154205.GT800259@ZenIV> <20231019050250.GV800259@ZenIV> <20231019061427.GW800259@ZenIV> <20231019063925.GX800259@ZenIV> <20231019080615.GY800259@ZenIV> <20231021071525.GA789610@ZenIV> <20231021222203.GA800259@ZenIV> <20231022194020.GA972254@ZenIV> <20231205022100.GB1674809@ZenIV> In-Reply-To: <20231205022100.GB1674809@ZenIV> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable From: Al Viro > Sent: 05 December 2023 02:21 >=20 > We need a way for csum_and_copy_{from,to}_user() to report faults. > The approach taken back in 2020 (avoid 0 as return value by starting > summing from ~0U, use 0 to report faults) had been broken; it does > yield the right value modulo 2^16-1, but the case when data is > entirely zero-filled is not handled right. It almost works, since > for most of the codepaths we have a non-zero value added in > and there 0 is not different from anything divisible by 0xffff. > However, there are cases (ICMPv4 replies, for example) where we > are not guaranteed that. >=20 > In other words, we really need to have those primitives return 0 > on filled-with-zeroes input. Do we? I've not seen any justification for this at all. IIRC the ICMPv4 reply code needs the checksum function return 0xffff for all-zero input. So the correct and simple fix is to initialise the sum to 0xffff in the checksum function. =09David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)