tcp: devmem: don't write truncated dmabuf CMSGs to userspace

Currently, we report -ETOOSMALL (err) only on the first iteration
(!sent). When we get put_cmsg error after a bunch of successful
put_cmsg calls, we don't signal the error at all. This might be
confusing on the userspace side which will see truncated CMSGs
but no MSG_CTRUNC signal.

Consider the following case:
- sizeof(struct cmsghdr) = 16
- sizeof(struct dmabuf_cmsg) = 24
- total cmsg size (CMSG_LEN) = 40 (16+24)

When calling recvmsg with msg_controllen=60, the userspace
will receive two(!) dmabuf_cmsg(s), the first one will
be a valid one and the second one will be silently truncated. There is no
easy way to discover the truncation besides doing something like
"cm->cmsg_len != CMSG_LEN(sizeof(dmabuf_cmsg))".

Introduce new put_devmem_cmsg wrapper that reports an error instead
of doing the truncation. Mina suggests that it's the intended way
this API should work.

Note that we might now report MSG_CTRUNC when the users (incorrectly)
call us with msg_control == NULL.

Fixes: 8f0b3cc9a4c1 ("tcp: RX path for devmem TCP")
Reviewed-by: Mina Almasry <[email protected]>
Signed-off-by: Stanislav Fomichev <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

1 files changed, 10 insertions, 16 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 0d704bda6c41..d74281eca14f 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2438,14 +2438,12 @@ static int tcp_recvmsg_dmabuf(struct sock *sk, const struct sk_buff *skb,
 			 */
 			memset(&dmabuf_cmsg, 0, sizeof(dmabuf_cmsg));
 			dmabuf_cmsg.frag_size = copy;
-			err = put_cmsg(msg, SOL_SOCKET, SO_DEVMEM_LINEAR,
-				       sizeof(dmabuf_cmsg), &dmabuf_cmsg);
-			if (err || msg->msg_flags & MSG_CTRUNC) {
-				msg->msg_flags &= ~MSG_CTRUNC;
-				if (!err)
-					err = -ETOOSMALL;
+			err = put_cmsg_notrunc(msg, SOL_SOCKET,
+					       SO_DEVMEM_LINEAR,
+					       sizeof(dmabuf_cmsg),
+					       &dmabuf_cmsg);
+			if (err)
 				goto out;
-			}
 
 			sent += copy;
 
@@ -2499,16 +2497,12 @@ static int tcp_recvmsg_dmabuf(struct sock *sk, const struct sk_buff *skb,
 				offset += copy;
 				remaining_len -= copy;
 
-				err = put_cmsg(msg, SOL_SOCKET,
-					       SO_DEVMEM_DMABUF,
-					       sizeof(dmabuf_cmsg),
-					       &dmabuf_cmsg);
-				if (err || msg->msg_flags & MSG_CTRUNC) {
-					msg->msg_flags &= ~MSG_CTRUNC;
-					if (!err)
-						err = -ETOOSMALL;
+				err = put_cmsg_notrunc(msg, SOL_SOCKET,
+						       SO_DEVMEM_DMABUF,
+						       sizeof(dmabuf_cmsg),
+						       &dmabuf_cmsg);
+				if (err)
 					goto out;
-				}
 
 				atomic_long_inc(&niov->pp_ref_count);
 				tcp_xa_pool.netmems[tcp_xa_pool.idx++] = skb_frag_netmem(frag);