Re: [PATCH net] udp: Fix UDP length on last GSO_PARTIAL segment

["Alice Mikityanska" <alice.kernel@fastmail.im>]

On Wed, May 13, 2026, at 09:43, Gal Pressman wrote:
> Following the cited commit, __udp_gso_segment() writes single MSS length
> in the UDP header.
> The cited patch doesn't account for the fact that the last segment could
> be a GSO skb by itself. This could happen when the size of the packet is
> a multiple of MSS, hence the first segment is also the last one (there
> is no need for a remainder skb).
>
> When the post-loop segment is a GSO skb, assign the single MSS length in
> the UDP header.
>
> Fixes: b10b446ce7ad ("udp: gso: Use single MSS length in UDP header for 
> GSO_PARTIAL")
> Reported-by: Matthew Schwartz <matthew.schwartz@linux.dev>
> Closes: 
> https://lore.kernel.org/all/6c3fb15e-711d-4b8d-b152-e03d9b05293f@linux.dev/
> Tested-by: Matthew Schwartz <matthew.schwartz@linux.dev>
> Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
> Signed-off-by: Gal Pressman <gal@nvidia.com>
> ---
>  net/ipv4/udp_offload.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index a0813d425b71..71df45f9488a 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -604,7 +604,7 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
>  		       seg->data_len);
>  	check = csum16_add(csum16_sub(uh->check, uh->len), newlen);
> 
> -	uh->len = newlen;
> +	uh->len = skb_is_gso(seg) ? msslen : newlen;
>  	uh->check = check;

This is going to have the same checksum bug as your first commit, which
I'm fixing in [1]. You should use the right value of either msslen or
newlen when modifying check a couple of lines above.

That said, maybe you can base your patch on top of my checksum fix? For
the last packet, it will then be:

if (!skb_is_gso(seg))
        newlen = /* the new value */;
/* keep newlen as is otherwise: my newlen is your msslen */
check = csum16_add(csum16_sub(uh->check, uh->len), newlen);
uh->len = newlen;
uh->check = check;

[1]: https://lore.kernel.org/netdev/20260512165648.386518-3-alice.kernel@fastmail.im/

> 
>  	if (seg->ip_summed == CHECKSUM_PARTIAL)
> -- 
> 2.52.0