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 3f9c4b74fdc0c7246faa62eec09feb2b0b60a4d3..88da11922d677d927f8c4b78ec554113f22ae17a 100644 (file)
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -939,9 +939,9 @@ 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(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 f55d08d2096ae9d7a8b152c7d63bac0a2f469699..4b063aa37e389963ddda8f07da2a3222012063cd 100644 (file)
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1256,9 +1256,9 @@ 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(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 b02080d09fbc057e2e7e136a1de345e51ae52042..d0fba50bd6ef085440de8ce800b0fc6216c5c6aa 100644 (file)
--- 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,