From b10b446ce7ad912559e3af523aaa9f5f34a18c2e Mon Sep 17 00:00:00 2001 From: Gal Pressman Date: Sun, 25 Jan 2026 14:16:47 +0200 Subject: [PATCH] udp: gso: Use single MSS length in UDP header for GSO_PARTIAL In GSO_PARTIAL segmentation, set the UDP length field to the single segment size (gso_size + UDP header) instead of the large MSS size. This provides hardware with a template length value for final segmentation, similar to how tunnel GSO_PARTIAL handles outer headers in UDP tunnels. This will remove the need to manually adjust the UDP header length in the drivers, as can be seen in subsequent patches. This was suggested by Alex in 2018: https://lore.kernel.org/netdev/CAKgT0UcdnUWgr3KQ=RnLKigokkiUuYefmL-ePpDvJOBNpKScFA@mail.gmail.com/ Reviewed-by: Dragos Tatulea Signed-off-by: Gal Pressman Reviewed-by: Willem de Bruijn Link: https://patch.msgid.link/20260125121649.778086-2-gal@nvidia.com Signed-off-by: Jakub Kicinski --- net/ipv4/udp_offload.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c index 19d0b5b09ffae..89e0b48b60ae3 100644 --- a/net/ipv4/udp_offload.c +++ b/net/ipv4/udp_offload.c @@ -483,11 +483,11 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb, struct sock *sk = gso_skb->sk; unsigned int sum_truesize = 0; struct sk_buff *segs, *seg; + __be16 newlen, msslen; struct udphdr *uh; unsigned int mss; bool copy_dtor; __sum16 check; - __be16 newlen; int ret = 0; mss = skb_shinfo(gso_skb)->gso_size; @@ -555,6 +555,8 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb, return segs; } + msslen = htons(sizeof(*uh) + mss); + /* GSO partial and frag_list segmentation only requires splitting * the frame into an MSS multiple and possibly a remainder, both * cases return a GSO skb. So update the mss now. @@ -584,7 +586,7 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb, if (!seg->next) break; - uh->len = newlen; + uh->len = msslen; uh->check = check; if (seg->ip_summed == CHECKSUM_PARTIAL) -- 2.47.3