From: Yan Zhai <yan@cloudflare.com>
Date: Fri, 31 Jan 2025 08:31:39 +0000 (-0800)
Subject: udp: gso: do not drop small packets when PMTU reduces
X-Git-Tag: v5.10.235~308
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=41e185f541563f134414183e7c2763801de21a7e;p=thirdparty%2Fkernel%2Fstable.git

udp: gso: do not drop small packets when PMTU reduces

[ Upstream commit 235174b2bed88501fda689c113c55737f99332d8 ]

Commit 4094871db1d6 ("udp: only do GSO if # of segs > 1") avoided GSO
for small packets. But the kernel currently dismisses GSO requests only
after checking MTU/PMTU on gso_size. This means any packets, regardless
of their payload sizes, could be dropped when PMTU becomes smaller than
requested gso_size. We encountered this issue in production and it
caused a reliability problem that new QUIC connection cannot be
established before PMTU cache expired, while non GSO sockets still
worked fine at the same time.

Ideally, do not check any GSO related constraints when payload size is
smaller than requested gso_size, and return EMSGSIZE instead of EINVAL
on MTU/PMTU check failure to be more specific on the error cause.

Fixes: 4094871db1d6 ("udp: only do GSO if # of segs > 1")
Signed-off-by: Yan Zhai <yan@cloudflare.com>
Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 6ad25dc9710c1..d9583a5b8f354 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -923,9 +923,9 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
 		const int hlen = skb_network_header_len(skb) +
 				 sizeof(struct udphdr);
 
-		if (hlen + cork->gso_size > cork->fragsize) {
+		if (hlen + min_t(int, datalen, cork->gso_size) > cork->fragsize) {
 			kfree_skb(skb);
-			return -EINVAL;
+			return -EMSGSIZE;
 		}
 		if (datalen > cork->gso_size * UDP_MAX_SEGMENTS) {
 			kfree_skb(skb);
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 203a6d64d7e99..a23780434edd3 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1210,9 +1210,9 @@ static int udp_v6_send_skb(struct sk_buff *skb, struct flowi6 *fl6,
 		const int hlen = skb_network_header_len(skb) +
 				 sizeof(struct udphdr);
 
-		if (hlen + cork->gso_size > cork->fragsize) {
+		if (hlen + min_t(int, datalen, cork->gso_size) > cork->fragsize) {
 			kfree_skb(skb);
-			return -EINVAL;
+			return -EMSGSIZE;
 		}
 		if (datalen > cork->gso_size * UDP_MAX_SEGMENTS) {
 			kfree_skb(skb);
diff --git a/tools/testing/selftests/net/udpgso.c b/tools/testing/selftests/net/udpgso.c
index 7badaf215de28..0e137182a4f40 100644
--- a/tools/testing/selftests/net/udpgso.c
+++ b/tools/testing/selftests/net/udpgso.c
@@ -94,6 +94,19 @@ struct testcase testcases_v4[] = {
 		.gso_len = CONST_MSS_V4,
 		.r_num_mss = 1,
 	},
+	{
+		/* datalen <= MSS < gso_len: will fall back to no GSO */
+		.tlen = CONST_MSS_V4,
+		.gso_len = CONST_MSS_V4 + 1,
+		.r_num_mss = 0,
+		.r_len_last = CONST_MSS_V4,
+	},
+	{
+		/* MSS < datalen < gso_len: fail */
+		.tlen = CONST_MSS_V4 + 1,
+		.gso_len = CONST_MSS_V4 + 2,
+		.tfail = true,
+	},
 	{
 		/* send a single MSS + 1B */
 		.tlen = CONST_MSS_V4 + 1,
@@ -197,6 +210,19 @@ struct testcase testcases_v6[] = {
 		.gso_len = CONST_MSS_V6,
 		.r_num_mss = 1,
 	},
+	{
+		/* datalen <= MSS < gso_len: will fall back to no GSO */
+		.tlen = CONST_MSS_V6,
+		.gso_len = CONST_MSS_V6 + 1,
+		.r_num_mss = 0,
+		.r_len_last = CONST_MSS_V6,
+	},
+	{
+		/* MSS < datalen < gso_len: fail */
+		.tlen = CONST_MSS_V6 + 1,
+		.gso_len = CONST_MSS_V6 + 2,
+		.tfail = true
+	},
 	{
 		/* send a single MSS + 1B */
 		.tlen = CONST_MSS_V6 + 1,