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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , David Ahern , Ido Schimmel , Shuah Khan , linux-kernel@vger.kernel.org, netdev@vger.kernel.org, linux-kselftest@vger.kernel.org, bpf@vger.kernel.org, David Lebrun , Daniel Borkmann , Mathieu Xhonneux , David Lebrun , Stefano Salsano , Andrea Mayer Subject: Re: [PATCH net 2/6] seg6: update skb checksum when ipv6_srh_rcv() decrements the Hop Limit Message-ID: References: <20261031225033.64-1-andrea.mayer@uniroma2.it> <20261031225033.64-3-andrea.mayer@uniroma2.it> X-Mailing-List: bpf@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261031225033.64-3-andrea.mayer@uniroma2.it> On Sat, Oct 31, 2026 at 11:50:29PM +0100, Andrea Mayer wrote: > When the next segment is a local address, ipv6_srh_rcv() processes it > directly and also decrements the Hop Limit. With CHECKSUM_COMPLETE, > skb->csum is not updated for this change. If the packet is then delivered > on the same node and skb->csum was computed in software, for instance by > conntrack, UDP can drop the packet as a checksum error. With skb->csum > computed by the device, TCP can instead log a "hw csum failure", as if the > device had computed a wrong skb->csum. > > Update skb->csum for the Hop Limit decrement with update_csum_diff4(). > > Depends on patch "seg6: fix skb checksum update helpers". > > Found by manual code inspection, and reproduced in a VM without kernel > changes or error injection. A following commit adds a selftest that > reproduces the problem with a tc ct action. With this patch, the selftest > passes for the path with local SRH segments. Also tested against the srv6 > selftests, and with a ConnectX-6 Lx NIC (mlx5) that provides > CHECKSUM_COMPLETE. > > Fixes: 1ababeba4a21 ("ipv6: implement dataplane support for rthdr type 4 (Segment Routing Header)") > Signed-off-by: Andrea Mayer > --- > net/ipv6/exthdrs.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c > index 96c8402a27a5..9d3ba310065a 100644 > --- a/net/ipv6/exthdrs.c > +++ b/net/ipv6/exthdrs.c > @@ -460,6 +460,10 @@ static int ipv6_srh_rcv(struct sk_buff *skb, struct inet6_dev *idev) > } > > if (skb_dst_dev(skb)->flags & IFF_LOOPBACK) { > + /* payload_len, nexthdr and hop_limit */ > + __be32 *hlim_word = (__be32 *)ipv6_hdr(skb) + 1; > + __be32 old; > + > if (ipv6_hdr(skb)->hop_limit <= 1) { > __IP6_INC_STATS(net, idev, IPSTATS_MIB_INHDRERRORS); > icmpv6_send(skb, ICMPV6_TIME_EXCEED, > @@ -467,8 +471,12 @@ static int ipv6_srh_rcv(struct sk_buff *skb, struct inet6_dev *idev) > kfree_skb_reason(skb, SKB_DROP_REASON_IP_TTL_EXCEEDED); > return -1; > } > + old = *hlim_word; > ipv6_hdr(skb)->hop_limit--; > > + if (skb->ip_summed == CHECKSUM_COMPLETE) > + update_csum_diff4(skb, old, *hlim_word); > + > skb_pull(skb, skb_transport_offset(skb)); > goto looped_back; > } > -- > 2.43.0 > Reviewed-by: Hangbin Liu