Merge branch 'udp-gso-fix-__udp_gso_segment-after-gso_partial-udp-length-change'

Gal Pressman says:

====================
udp: gso: Fix __udp_gso_segment() after GSO_PARTIAL UDP length change

This series fixes two issues introduced by commit b10b446ce7ad ("udp:
gso: Use single MSS length in UDP header for GSO_PARTIAL"), which
switched __udp_gso_segment() to write the single MSS length into the UDP
header for GSO_PARTIAL skbs.

Patch 1 (from Alice) fixes the UDP checksum adjustment in
__udp_gso_segment().
The patch adjusts the checksum by the correct delta, and since msslen
and newlen become equivalent before the loop, drops one of the two
variables to simplify the code.

Patch 2 handles the case where the last segment of a GSO_PARTIAL skb is
itself a GSO skb. This happens when the original packet size is an exact
multiple of MSS, so the post-loop segment is not a remainder skb but a
full GSO chunk and must also carry the single MSS length in its UDP
header.
====================

Link: https://patch.msgid.link/20260518062250.3019914-1-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 8 insertions, 14 deletions

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index a0813d425b71..29651b1a0bc7 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -482,11 +482,11 @@ 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;
@@ -555,15 +555,6 @@ 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.
-	 */
-	if (skb_is_gso(segs))
-		mss *= skb_shinfo(segs)->gso_segs;
-
 	seg = segs;
 	uh = udp_hdr(seg);
 
@@ -586,7 +577,7 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 		if (!seg->next)
 			break;
 
-		uh->len = msslen;
+		uh->len = newlen;
 		uh->check = check;
 
 		if (seg->ip_summed == CHECKSUM_PARTIAL)
@@ -599,9 +590,12 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 		uh = udp_hdr(seg);
 	}
 
-	/* last packet can be partial gso_size, account for that in checksum */
-	newlen = htons(skb_tail_pointer(seg) - skb_transport_header(seg) +
-		       seg->data_len);
+	/* Unless skb fits perfectly as GSO_PARTIAL, the trailing
+	 * segment may not be full MSS, account for that in the checksum
+	 */
+	if (!skb_is_gso(seg))
+		newlen = htons(skb_tail_pointer(seg) -
+			       skb_transport_header(seg) + seg->data_len);
 	check = csum16_add(csum16_sub(uh->check, uh->len), newlen);
 
 	uh->len = newlen;