[PATCH] net: fix segmentation of forwarding fraglist GRO

[Paolo Abeni]

On 12/17/25 4:55 AM, 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 <jibin.zhang at mediatek.com>

This looks like a fix, it should target the 'net' tree and include a
suitable Fixes tag.

> ---
>  net/ipv4/tcp_offload.c | 3 ++-
>  net/ipv4/udp_offload.c | 3 ++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
> index fdda18b1abda..162a384a15bb 100644
> --- a/net/ipv4/tcp_offload.c
> +++ b/net/ipv4/tcp_offload.c
> @@ -104,7 +104,8 @@ static struct sk_buff *tcp4_gso_segment(struct sk_buff *skb,
>  	if (!pskb_may_pull(skb, sizeof(struct tcphdr)))
>  		return ERR_PTR(-EINVAL);
>  
> -	if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) {
> +	if ((skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) && skb_has_frag_list(skb) &&
> +	    (skb->protocol == skb_shinfo(skb)->frag_list->protocol)) {
>  		struct tcphdr *th = tcp_hdr(skb);
>  
>  		if (skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size)
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index 19d0b5b09ffa..704fb32d10d7 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -512,7 +512,8 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
>  		return NULL;
>  	}
>  
> -	if (skb_shinfo(gso_skb)->gso_type & SKB_GSO_FRAGLIST) {
> +	if ((skb_shinfo(gso_skb)->gso_type & SKB_GSO_FRAGLIST) && skb_has_frag_list(gso_skb) &&
> +	    (gso_skb->protocol == skb_shinfo(gso_skb)->frag_list->protocol)) {
>  		 /* Detect modified geometry and pass those to skb_segment. */
>  		if (skb_pagelen(gso_skb) - sizeof(*uh) == skb_shinfo(gso_skb)->gso_size)
>  			return __udp_gso_segment_list(gso_skb, features, is_ipv6);

I guess checks should be needed for ipv6.

Also it looks like this skips the CSUM_PARTIAL preparation, and possibly
break csum offload.

Additionally I don't like the ever increasing stack of hacks needed to
let GSO_FRAGLIST operate in the most diverse setups, the simpler fix
would be disabling such aggregation.

/P