netdev - [PATCH net] udp: Pass 2 bytes of data with UDP

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Message-Id: <20230131174601.203127-1-jakub@cloudflare.com>
Date:   Tue, 31 Jan 2023 18:46:01 +0100
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, kernel-team@...udflare.com
Subject: [PATCH net] udp: Pass 2 bytes of data with UDP_GRO cmsg to user-space

While UDP_GRO cmsg interface lacks documentation, the selftests added in
commit 3327a9c46352 ("selftests: add functionals test for UDP GRO") suggest
that the user-space should allocate CMSG_SPACE for an u16 value and
interpret the returned bytes as such:

static int recv_msg(int fd, char *buf, int len, int *gso_size)
{
	char control[CMSG_SPACE(sizeof(uint16_t))] = {0};
	...
			if (cmsg->cmsg_level == SOL_UDP
			    && cmsg->cmsg_type == UDP_GRO) {
				gsosizeptr = (uint16_t *) CMSG_DATA(cmsg);
				*gso_size = *gsosizeptr;
				break;
			}
	...
}

Today user-space will receive 4 bytes of data with an UDP_GRO cmsg, because
the kernel packs an int into the cmsg data, as we can confirm with strace:

  recvmsg(8, {msg_name=...,
              msg_iov=[{iov_base="\0\0..."..., iov_len=96000}],
              msg_iovlen=1,
              msg_control=[{cmsg_len=20,         <-- sizeof(cmsghdr) + 4
                            cmsg_level=SOL_UDP,
                            cmsg_type=0x68}],    <-- UDP_GRO
                            msg_controllen=24,
                            msg_flags=0}, 0) = 11200

This means that either UDP_GRO selftests are broken on big endian, or this
is a programming error. Assume the latter and pass only the needed 2 bytes
of data with the cmsg.

Fixing it like that has an added advantage that the cmsg becomes compatible
with what is expected by UDP_SEGMENT cmsg. It becomes possible to reuse the
cmsg when GSO packets are received on one socket and sent out of another.

Fixes: bcd1665e3569 ("udp: add support for UDP_GRO cmsg")
Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
---
 include/linux/udp.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/udp.h b/include/linux/udp.h
index a2892e151644..44bb8d699248 100644
--- a/include/linux/udp.h
+++ b/include/linux/udp.h
@@ -125,7 +125,7 @@ static inline bool udp_get_no_check6_rx(struct sock *sk)
 static inline void udp_cmsg_recv(struct msghdr *msg, struct sock *sk,
 				 struct sk_buff *skb)
 {
-	int gso_size;
+	__u16 gso_size;
 
 	if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) {
 		gso_size = skb_shinfo(skb)->gso_size;
-- 
2.39.1