[PATCH] udp: gso: fix MTU check for small packets

[Yan Zhai <yan@cloudflare.com>]

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 on gso_size. This means any packets, regardless of
their payload sizes, would be dropped when MTU is smaller than requested
gso_size. Meanwhile, EINVAL would be returned in this case, making it
very misleading to debug.

Ideally, do not check any GSO related constraints when payload size is
smaller than requested gso_size, and return EMSGSIZE on MTU check
failure consistently for all packets to ease debugging.

Fixes: 4094871db1d6 ("udp: only do GSO if # of segs > 1")
Signed-off-by: Yan Zhai <yan@cloudflare.com>
---
 net/ipv4/udp.c                       | 18 ++++++++----------
 net/ipv6/udp.c                       | 18 ++++++++----------
 tools/testing/selftests/net/udpgso.c | 14 ++++++++++++++
 3 files changed, 30 insertions(+), 20 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index c472c9a57cf6..9aed1b4a871f 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1137,13 +1137,13 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
 	uh->len = htons(len);
 	uh->check = 0;
 
-	if (cork->gso_size) {
+	if (cork->gso_size && datalen > cork->gso_size) {
 		const int hlen = skb_network_header_len(skb) +
 				 sizeof(struct udphdr);
 
 		if (hlen + cork->gso_size > cork->fragsize) {
 			kfree_skb(skb);
-			return -EINVAL;
+			return -EMSGSIZE;
 		}
 		if (datalen > cork->gso_size * UDP_MAX_SEGMENTS) {
 			kfree_skb(skb);
@@ -1158,15 +1158,13 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
 			return -EIO;
 		}
 
-		if (datalen > cork->gso_size) {
-			skb_shinfo(skb)->gso_size = cork->gso_size;
-			skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
-			skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
-								 cork->gso_size);
+		skb_shinfo(skb)->gso_size = cork->gso_size;
+		skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
+		skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
+							 cork->gso_size);
 
-			/* Don't checksum the payload, skb will get segmented */
-			goto csum_partial;
-		}
+		/* Don't checksum the payload, skb will get segmented */
+		goto csum_partial;
 	}
 
 	if (is_udplite)  				 /*     UDP-Lite      */
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 6671daa67f4f..6cdc8ce4c6f9 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1385,13 +1385,13 @@ static int udp_v6_send_skb(struct sk_buff *skb, struct flowi6 *fl6,
 	uh->len = htons(len);
 	uh->check = 0;
 
-	if (cork->gso_size) {
+	if (cork->gso_size && datalen > cork->gso_size) {
 		const int hlen = skb_network_header_len(skb) +
 				 sizeof(struct udphdr);
 
 		if (hlen + cork->gso_size > cork->fragsize) {
 			kfree_skb(skb);
-			return -EINVAL;
+			return -EMSGSIZE;
 		}
 		if (datalen > cork->gso_size * UDP_MAX_SEGMENTS) {
 			kfree_skb(skb);
@@ -1406,15 +1406,13 @@ static int udp_v6_send_skb(struct sk_buff *skb, struct flowi6 *fl6,
 			return -EIO;
 		}
 
-		if (datalen > cork->gso_size) {
-			skb_shinfo(skb)->gso_size = cork->gso_size;
-			skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
-			skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
-								 cork->gso_size);
+		skb_shinfo(skb)->gso_size = cork->gso_size;
+		skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
+		skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
+							 cork->gso_size);
 
-			/* Don't checksum the payload, skb will get segmented */
-			goto csum_partial;
-		}
+		/* Don't checksum the payload, skb will get segmented */
+		goto csum_partial;
 	}
 
 	if (is_udplite)
diff --git a/tools/testing/selftests/net/udpgso.c b/tools/testing/selftests/net/udpgso.c
index 3f2fca02fec5..fb73f1c331fb 100644
--- a/tools/testing/selftests/net/udpgso.c
+++ b/tools/testing/selftests/net/udpgso.c
@@ -102,6 +102,13 @@ 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,
+	},
 	{
 		/* send a single MSS + 1B */
 		.tlen = CONST_MSS_V4 + 1,
@@ -205,6 +212,13 @@ 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,
+	},
 	{
 		/* send a single MSS + 1B */
 		.tlen = CONST_MSS_V6 + 1,
-- 
2.30.2