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Mon, 16 Mar 2026 01:38:44 -0700 (PDT) Received: from [192.168.88.32] ([216.128.11.95]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4854b5f6c24sm377477905e9.5.2026.03.16.01.38.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 16 Mar 2026 01:38:43 -0700 (PDT) Message-ID: Date: Mon, 16 Mar 2026 09:38:42 +0100 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net v3] net: gso: Forbid IPv6 TSO with extensions on devices with only IPV6_CSUM To: Willem de Bruijn , xietangxin , Jakub Ramaseuski , netdev@vger.kernel.org Cc: kuba@kernel.org, horms@kernel.org, edumazet@google.com, sdf@fomichev.me, ahmed.zaki@intel.com, aleksander.lobakin@intel.com, benoit.monin@gmx.fr, willemb@google.com, Tianhao Zhao , Michal Schmidt References: <20250814105119.1525687-1-jramaseu@redhat.com> <0414e7e2-9a1c-4d7c-a99d-b9039cf68f40@yeah.net> <49dac359-326a-4f3a-8c18-9897ea7be498@redhat.com> <9186ce75-4038-4467-b492-7a7821659842@redhat.com> Content-Language: en-US From: Paolo Abeni In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 3/14/26 5:19 PM, Willem de Bruijn wrote: > Paolo Abeni wrote: >> On 3/6/26 7:32 AM, xietangxin wrote: >>> On 3/5/2026 11:21 PM, Paolo Abeni wrote: >>>> On 3/5/26 3:57 PM, Willem de Bruijn wrote: >>>>> xietangxin wrote: >>>>>> 在 2025/8/14 18:51, Jakub Ramaseuski 写道: >>>>>>> When performing Generic Segmentation Offload (GSO) on an IPv6 packet that >>>>>>> contains extension headers, the kernel incorrectly requests checksum offload >>>>>>> if the egress device only advertises NETIF_F_IPV6_CSUM feature, which has >>>>>>> a strict contract: it supports checksum offload only for plain TCP or UDP >>>>>>> over IPv6 and explicitly does not support packets with extension headers. >>>>>>> The current GSO logic violates this contract by failing to disable the feature >>>>>>> for packets with extension headers, such as those used in GREoIPv6 tunnels. >>>>>>> >>>>>>> This violation results in the device being asked to perform an operation >>>>>>> it cannot support, leading to a `skb_warn_bad_offload` warning and a collapse >>>>>>> of network throughput. While device TSO/USO is correctly bypassed in favor >>>>>>> of software GSO for these packets, the GSO stack must be explicitly told not >>>>>>> to request checksum offload. >>>>>>> >>>>>>> Mask NETIF_F_IPV6_CSUM, NETIF_F_TSO6 and NETIF_F_GSO_UDP_L4 >>>>>>> in gso_features_check if the IPv6 header contains extension headers to compute >>>>>>> checksum in software. >>>>>>> >>>>>>> The exception is a BIG TCP extension, which, as stated in commit >>>>>>> 68e068cabd2c6c53 ("net: reenable NETIF_F_IPV6_CSUM offload for BIG TCP packets"): >>>>>>> "The feature is only enabled on devices that support BIG TCP TSO. >>>>>>> The header is only present for PF_PACKET taps like tcpdump, >>>>>>> and not transmitted by physical devices." >>>>>>> >>>>>>> kernel log output (truncated): >>>>>>> WARNING: CPU: 1 PID: 5273 at net/core/dev.c:3535 skb_warn_bad_offload+0x81/0x140 >>>>>>> ... >>>>>>> Call Trace: >>>>>>> >>>>>>> skb_checksum_help+0x12a/0x1f0 >>>>>>> validate_xmit_skb+0x1a3/0x2d0 >>>>>>> validate_xmit_skb_list+0x4f/0x80 >>>>>>> sch_direct_xmit+0x1a2/0x380 >>>>>>> __dev_xmit_skb+0x242/0x670 >>>>>>> __dev_queue_xmit+0x3fc/0x7f0 >>>>>>> ip6_finish_output2+0x25e/0x5d0 >>>>>>> ip6_finish_output+0x1fc/0x3f0 >>>>>>> ip6_tnl_xmit+0x608/0xc00 [ip6_tunnel] >>>>>>> ip6gre_tunnel_xmit+0x1c0/0x390 [ip6_gre] >>>>>>> dev_hard_start_xmit+0x63/0x1c0 >>>>>>> __dev_queue_xmit+0x6d0/0x7f0 >>>>>>> ip6_finish_output2+0x214/0x5d0 >>>>>>> ip6_finish_output+0x1fc/0x3f0 >>>>>>> ip6_xmit+0x2ca/0x6f0 >>>>>>> ip6_finish_output+0x1fc/0x3f0 >>>>>>> ip6_xmit+0x2ca/0x6f0 >>>>>>> inet6_csk_xmit+0xeb/0x150 >>>>>>> __tcp_transmit_skb+0x555/0xa80 >>>>>>> tcp_write_xmit+0x32a/0xe90 >>>>>>> tcp_sendmsg_locked+0x437/0x1110 >>>>>>> tcp_sendmsg+0x2f/0x50 >>>>>>> ... >>>>>>> skb linear: 00000000: e4 3d 1a 7d ec 30 e4 3d 1a 7e 5d 90 86 dd 60 0e >>>>>>> skb linear: 00000010: 00 0a 1b 34 3c 40 20 11 00 00 00 00 00 00 00 00 >>>>>>> skb linear: 00000020: 00 00 00 00 00 12 20 11 00 00 00 00 00 00 00 00 >>>>>>> skb linear: 00000030: 00 00 00 00 00 11 2f 00 04 01 04 01 01 00 00 00 >>>>>>> skb linear: 00000040: 86 dd 60 0e 00 0a 1b 00 06 40 20 23 00 00 00 00 >>>>>>> skb linear: 00000050: 00 00 00 00 00 00 00 00 00 12 20 23 00 00 00 00 >>>>>>> skb linear: 00000060: 00 00 00 00 00 00 00 00 00 11 bf 96 14 51 13 f9 >>>>>>> skb linear: 00000070: ae 27 a0 a8 2b e3 80 18 00 40 5b 6f 00 00 01 01 >>>>>>> skb linear: 00000080: 08 0a 42 d4 50 d5 4b 70 f8 1a >>>>>>> >>>>>>> Fixes: 04c20a9356f283da ("net: skip offload for NETIF_F_IPV6_CSUM if ipv6 header contains extension") >>>>>>> Reported-by: Tianhao Zhao >>>>>>> Suggested-by: Michal Schmidt >>>>>>> Suggested-by: Willem de Bruijn >>>>>>> Signed-off-by: Jakub Ramaseuski >>>>>>> --- >>>>>>> --- >>>>>>> net/core/dev.c | 12 ++++++++++++ >>>>>>> 1 file changed, 12 insertions(+) >>>>>>> >>>>>>> diff --git a/net/core/dev.c b/net/core/dev.c >>>>>>> index b28ce68830b2b..1d8a4d1da911e 100644 >>>>>>> --- a/net/core/dev.c >>>>>>> +++ b/net/core/dev.c >>>>>>> @@ -3778,6 +3778,18 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb, >>>>>>> if (!(iph->frag_off & htons(IP_DF))) >>>>>>> features &= ~NETIF_F_TSO_MANGLEID; >>>>>>> } >>>>>>> + >>>>>>> + /* NETIF_F_IPV6_CSUM does not support IPv6 extension headers, >>>>>>> + * so neither does TSO that depends on it. >>>>>>> + */ >>>>>>> + if (features & NETIF_F_IPV6_CSUM && >>>>>>> + (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 || >>>>>>> + (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 && >>>>>>> + vlan_get_protocol(skb) == htons(ETH_P_IPV6))) && >>>>>>> + skb_transport_header_was_set(skb) && >>>>>>> + skb_network_header_len(skb) != sizeof(struct ipv6hdr) && >>>>>>> + !ipv6_has_hopopt_jumbo(skb)) >>>>>>> + features &= ~(NETIF_F_IPV6_CSUM | NETIF_F_TSO6 | NETIF_F_GSO_UDP_L4); >>>>>>> >>>>>>> return features; >>>>>>> } >>>>>> question about this patch affecting tunneled IPv6-in-IPv4 packets >>>>>> >>>>>> In our environment with a hinic NIC, we use VXLAN tunnels where >>>>>> the outer header is IPv4 and the inner is IPv6. After this commit, >>>>>> large packets no longer use hardware TSO and fall back to software segmentation. >>>>>> >>>>>> In the VXLAN IPv6-in-IPv4 case, `skb_shinfo(skb)->gso_type` includes >>>>>> `SKB_GSO_TCPV6` (inner is IPv6 TCP), but the network header points to the outer >>>>>> IPv4 header. Thus `skb_network_header_len(skb)` returns the IPv4 header length >>>>>> (usually 20), which is not equal to `sizeof(struct ipv6hdr)` (40). This causes >>>>>> the condition to trigger and clears `NETIF_F_TSO6`, even though the inner IPv6 >>>>>> packet has no extension headers and the device is capable of handling TSO for >>>>>> such packets. >>>>>> >>>>>> Is it the intended behavior to disable TSO for all tunneled IPv6-in-IPv4 packets >>>>>> when the NIC lacks NETIF_F_HW_CSUM, even if the inner IPv6 header has no extensions? >>>>>> >>>>>> Any feedback or guidance would be greatly appreciated. >>>>> >>>>> That is definitely unintended. >>>>> >>>>> Thanks for the clear analysis. >>>>> >>>>> I was about to write a refinement that might catch this case, >>>>> something like >>>>> >>>>> @@ -3819,8 +3819,10 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb, >>>>> (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 || >>>>> (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 && >>>>> vlan_get_protocol(skb) == htons(ETH_P_IPV6))) && >>>>> - skb_transport_header_was_set(skb) && >>>>> - skb_network_header_len(skb) != sizeof(struct ipv6hdr)) >>>>> + ((!skb->encapsulation && >>>>> + skb_transport_header_was_set(skb) && >>>>> + skb_network_header_len(skb) != sizeof(struct ipv6hdr)) || >>>>> + (skb_inner_network_header_len(skb) != sizeof(struct ipv6hdr)))) >>>>> features &= ~(NETIF_F_IPV6_CSUM | NETIF_F_TSO6 | NETIF_F_GSO_UDP_L4); >>>>> >>>>> But, how are these VXLAN IPv6-in-IPv4 packets having >>>>> vlan_get_protocol(skb) == htons(ETH_P_IPV6)? >>>>> >>>>> Shouldn't that be the protocol of the outer headr, so ETH_P_IP, and >>>>> thus this branch not reached at all? (Which itself would leave a false >>>>> positive as now an inner network header with extensions would not be >>>>> caught..) >>>> >>>> Also the tunnel could have ENCAP_TYPE_IPPROTO, and likely we need to >>>> disable csum even in that case? Possibly something alike the following >>>> could work? >>>> >>>> Side note, I *think* that replacing SKB_GSO_UDP_L4 with separate >>>> SKB_GSO_UDPV4_L4 SKB_GSO_UDPV6_L4 would remove a bit of complexity in >>>> serveral places, but I'm not sure how much invasive would be such a change. >>>> >>>> --- >>>> diff --git a/net/core/dev.c b/net/core/dev.c >>>> index 4af4cf2d63a4..f9824dfef376 100644 >>>> --- a/net/core/dev.c >>>> +++ b/net/core/dev.c >>>> @@ -3769,6 +3769,22 @@ static netdev_features_t >>>> dflt_features_check(struct sk_buff *skb, >>>> return vlan_features_check(skb, features); >>>> } >>>> >>>> +static bool skb_gso_has_extension_hdr(const struct sk_buff *skb) >>>> +{ >>>> + if (!skb->encapsulation) >>>> + return ((skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 || >>>> + (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 && >>>> + vlan_get_protocol(skb) == htons(ETH_P_IPV6))) && >>>> + skb_transport_header_was_set(skb) && >>>> + skb_network_header_len(skb) != sizeof(struct ipv6hdr)); >>>> + >>>> + return (skb->inner_protocol_type == ENCAP_TYPE_IPPROTO || >>>> + ((skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 || >>>> + (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 && >>>> + inner_ip_hdr(skb)->version == 6)) && >>>> + skb_inner_network_header_len(skb) != sizeof(struct ipv6hdr))); >>>> +} >>>> + >>>> static netdev_features_t gso_features_check(const struct sk_buff *skb, >>>> struct net_device *dev, >>>> netdev_features_t features) >>>> @@ -3815,12 +3831,7 @@ static netdev_features_t gso_features_check(const >>>> struct sk_buff *skb, >>>> /* NETIF_F_IPV6_CSUM does not support IPv6 extension headers, >>>> * so neither does TSO that depends on it. >>>> */ >>>> - if (features & NETIF_F_IPV6_CSUM && >>>> - (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 || >>>> - (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 && >>>> - vlan_get_protocol(skb) == htons(ETH_P_IPV6))) && >>>> - skb_transport_header_was_set(skb) && >>>> - skb_network_header_len(skb) != sizeof(struct ipv6hdr)) >>>> + if (features & NETIF_F_IPV6_CSUM && skb_gso_has_extension_hdr(skb)) >>>> features &= ~(NETIF_F_IPV6_CSUM | NETIF_F_TSO6 | NETIF_F_GSO_UDP_L4); >>>> >>>> return features; >>>> >>> Hi Paolo, Willem, >>> >>> Thank you both for the insightful analysis and the proposed fix. >>> >>> I have backported and tested Paolo's patch in our environment with hinic NIC. >>> We focused on the VXLAN (IPv6-in-IPv4) scenario and the Native IPv6 scenario : >>> >>> Scenario | IPv6 Ext-Headers | Result | Behavior >>> -----------------------|------------------|--------|--------------- >>> VXLAN (IPv6-in-IPv4) | No | PASS | HW TSO enabled >>> VXLAN (IPv6-in-IPv4) | Yes | PASS | SW GSO fallback >>> Native IPv6 | No | PASS | HW TSO enabled >>> Native IPv6 | Yes | PASS | SW GSO fallback >>> >>> Thanks again for the help! >> Please, if you will and can, take it over to cook it in a formal patch. > > Otherwise I can. > > The check is also needed for tunnels that set ENCAP_TYPE_IPPROTO, such > as sit. That condition can be removed as far as I can tell? I was thinking about i.e. sctp over UDP tunnel where we have: and no inner IP{v6} header. Possibly it's better to replace: skb->inner_protocol_type == ENCAP_TYPE_IPPROTO with: !skb_inner_network_header_was_set() (forcing segmentation if the the inner network header was not set). /P