Subject: Re: [PATCH v3] net: fix segmentation of forwarding fraglist GRO

From: Willem de Bruijn <[email-masked]>
Date: Sun, 25 Jan 2026 17:00:54 -0500

Jibin Zhang wrote:
> This patch enhances GSO segment checks by verifying the presence
> of frag_list and protocol consistency, addressing low throughput
> issues on IPv4 servers when used as hotspots
> 
> Specifically, it fixes a bug in GSO segmentation when forwarding
> GRO packets with frag_list. The function skb_segment_list cannot
> correctly process GRO skbs converted by XLAT, because XLAT only
> converts the header of the head skb. As a result, skbs in the
> frag_list may remain unconverted, leading to protocol
> inconsistencies and reduced throughput.
> 
> To resolve this, the patch uses skb_segment to handle forwarded
> packets converted by XLAT, ensuring that all fragments are
> properly converted and segmented.
> 
> Signed-off-by: Jibin Zhang <[email-masked]>
> ---
> v3: Apply the same fix to tcp6_gso_segment(), as suggested.
> 
> v2: To apply the added condition to a narrower scop
> 
>   In this version, the condition (skb_has_frag_list(gso_skb) &&
> (gso_skb->protocol == skb_shinfo(gso_skb)->frag_list->protocol))
> is moved into inner 'if' statement to a narrower scope.
> 
>   Send out the patch again for further discussion because:
> 
> 1. This issue has a significant impact and has occurred in many
> countries and regions.
> 2. Currently, modifying BPF is not a good option, because BPF code
> cannot access the header of skb on the fraglist, and the required
> changes would affect a wide range of code.
> 3. Directly disabling GRO aggregation for XLAT flows is also not a
> good solution, as this change would disable GRO even when forwarding
> is not needed, and it would also require cooperation from all device
> drivers.
> 
> [2]: https://patchwork.kernel.org/patch/14375646
> 
> [1]: https://patchwork.kernel.org/patch/14350844

[PATCH net] and please include a Fixes tag and CC: stable.

> 
> ---
>  net/ipv4/tcp_offload.c   | 4 +++-
>  net/ipv4/udp_offload.c   | 4 +++-
>  net/ipv6/tcpv6_offload.c | 4 +++-
>  3 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
> index fdda18b1abda..6c2c10f37f87 100644
> --- a/net/ipv4/tcp_offload.c
> +++ b/net/ipv4/tcp_offload.c
> @@ -107,7 +107,9 @@ static struct sk_buff *tcp4_gso_segment(struct sk_buff *skb,
>  	if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) {
>  		struct tcphdr *th = tcp_hdr(skb);
>  
> -		if (skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size)
> +		if ((skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size) &&
> +		    skb_has_frag_list(skb) &&

Not all skbs with frag_list are SKB_GSO_FRAGLIST skbs.
Let's limit to those, which was the intent when skb_segment_list was
introduced.

Could XLAT set SKB_GSO_DODGY when modifying headers for GSO packets?
Not sure which exact code is being referred to. All such BPF helpers
in net/core/filter.c do. skb_segment has a further sanityf check for
odd frag_list geometry, but conditional on SKB_GSO_DODGY.

See also https://lore.kernel.org/netdev/[email-masked]/

But in general: it is always safe to downgrade from skb_segment_list
to skb_segment. And fine to err on the side of caution esp. for any
packets that were modified along the way, so Ack on the general idea.

> +		    (skb->protocol == skb_shinfo(skb)->frag_list->protocol))
>  			return __tcp4_gso_segment_list(skb, features);
>  
>  		skb->ip_summed = CHECKSUM_NONE;
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index 19d0b5b09ffa..2a99f011793f 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -514,7 +514,9 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
>  
>  	if (skb_shinfo(gso_skb)->gso_type & SKB_GSO_FRAGLIST) {
>  		 /* Detect modified geometry and pass those to skb_segment. */
> -		if (skb_pagelen(gso_skb) - sizeof(*uh) == skb_shinfo(gso_skb)->gso_size)
> +		if ((skb_pagelen(gso_skb) - sizeof(*uh) == skb_shinfo(gso_skb)->gso_size) &&
> +		    skb_has_frag_list(gso_skb) &&
> +		    (gso_skb->protocol == skb_shinfo(gso_skb)->frag_list->protocol))
>  			return __udp_gso_segment_list(gso_skb, features, is_ipv6);
>  
>  		ret = __skb_linearize(gso_skb);
> diff --git a/net/ipv6/tcpv6_offload.c b/net/ipv6/tcpv6_offload.c
> index effeba58630b..3c7fd0362475 100644
> --- a/net/ipv6/tcpv6_offload.c
> +++ b/net/ipv6/tcpv6_offload.c
> @@ -170,7 +170,9 @@ static struct sk_buff *tcp6_gso_segment(struct sk_buff *skb,
>  	if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) {
>  		struct tcphdr *th = tcp_hdr(skb);
>  
> -		if (skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size)
> +		if ((skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size) &&
> +		    skb_has_frag_list(skb) &&
> +		    (skb->protocol == skb_shinfo(skb)->frag_list->protocol))
>  			return __tcp6_gso_segment_list(skb, features);
>  
>  		skb->ip_summed = CHECKSUM_NONE;
> -- 
> 2.45.2
>