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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Andrew Lunn , David Ahern , Ido Schimmel , Trond Myklebust , Anna Schumaker , Chuck Lever , Jeff Layton , NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , Mina Almasry , Kaiyuan Zhang , Willem de Bruijn , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH net v2] net: skbuff: make skb_copy_and_csum_bits() report unreadable frags Date: Wed, 14 Oct 2026 17:27:18 +0000 Message-ID: <179199883858343.2.17884063473154560573@toxicpanda.com> In-Reply-To: References: <20261013-b4-skb-segment-nosg-unreadable-v2-1-6c28d8ffa51e@toxicpanda.com> <20261014135154.05de968e@pumpkin> <179198668564919.2.10323410751655182155@toxicpanda.com> <20261014173042.0a74a25d@pumpkin> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit On Wed, Oct 14, 2026 at 12:52 PM Josef Bacik wrote: > ...ERR_PTR would be fine too? I'm arguing for explicit return values, > not "0 means failure". But I don't care enough to argue, I'm > respinning. Thanks, I respun it with the sum starting from ~0, and it changes what goes on the wire, so I was wrong above about ~0 not changing the folded result. csum_add(~0, 0) stays ~0, and csum_fold(~0) is 0x0000 where csum_fold(0) is 0xffff. No other 32 bit value folds to 0xffff, so there's no non-zero value that can stand in for a zero sum. ICMPv4 folds the raw sum with no pseudo-header, so an echo request with id 0, seq 0 and an all-zero payload gets a reply with a checksum of 0x0000 instead of 0xffff, and the sender drops it. I checked that on loopback: the reply has 0xffff with v2 and 0x0000 with the ~0 seed. So 0 can't mean failure here without changing checksums, and v3 will keep the explicit error return from v2. Thanks, Josef