[PATCH net-next v2 02/12] udp: gso: Simplify handling length in GSO_PARTIAL

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

From: Alice Mikityanska <alice@isovalent.com>

Taking further the idea of commit b10b446ce7ad ("udp: gso: Use single
MSS length in UDP header for GSO_PARTIAL"), simplify the implementation
and fix the checksum (apparently ignored by hardware anyway).

The mentioned commit started using msslen for uh->len, but still uses
newlen to adjust uh->check. If the formula for check is fixed, newlen is
assigned but never used before the loop, and newlen is overwritten after
the loop. This makes msslen not really necessary, as we can reuse
newlen, if we don't adjust mss before. The adjustment of mss can be
simply dropped, because mss is not used anywhere else below.

This brings us back to one variable, drops an unneeded arithmetic for
mss, and fixes the UDP checksum.

Signed-off-by: Alice Mikityanska <alice@isovalent.com>
Cc: Gal Pressman <gal@nvidia.com>
---
 net/ipv4/udp_offload.c | 13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 6b1654c1ad4a..e831234326c4 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;
@@ -556,15 +556,6 @@ 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);
 
@@ -587,7 +578,7 @@ 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)
-- 
2.52.0