Subject: Re: [PATCH net v3] net: gso: Forbid IPv6 TSO with extensions on devices with only IPV6_CSUM

From: Willem de Bruijn <[email-masked]>
Date: Sat, 14 Mar 2026 12:19:37 -0400

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:
> >>>>>  <TASK>
> >>>>>  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 <[email-masked]>
> >>>>> Suggested-by: Michal Schmidt <[email-masked]>
> >>>>> Suggested-by: Willem de Bruijn <[email-masked]>
> >>>>> Signed-off-by: Jakub Ramaseuski <[email-masked]>
> >>>>> ---
> >>>>> ---
> >>>>>  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?

Only, I still do not see how this condition can have triggered, as
vlan_get_protocol(skb) should be htons(ETH_P_IP).

I built a simple reproducer using vxlan over veth in virtme-ng, while
changing veth's NETIF_F_.._CSUM to reach this code. That indeed shows
correct ETH_P_IP.

Tangxin, can you show a stack trace when this condition hits? For
instance by adding a WARN_ON_ONCE(1) inside that branch, or by using
bpftrace:

sudo bpftrace -e 'kfunc:netif_skb_features { if (args->skb->encapsulation && args->skb->protocol == 0xDD86) { @[kstack] = count(); } }'