[PATCH net] tcp: devmem: properly export MSG_CTRUNC to userspace

[Stanislav Fomichev]

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))". Do not mask MSG_CTRUNC
to keep conventional cmsg-truncated signals in place.

If there is a concern with breaking UAPI, I can document this case in
devmem.rst instead.

Fixes: 8f0b3cc9a4c1 ("tcp: RX path for devmem TCP")
Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxxx>
---
 net/ipv4/tcp.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 0d704bda6c41..bdc9ac648d83 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2441,7 +2441,6 @@ static int tcp_recvmsg_dmabuf(struct sock *sk, const struct sk_buff *skb,
 			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;
 				goto out;
@@ -2504,7 +2503,6 @@ static int tcp_recvmsg_dmabuf(struct sock *sk, const struct sk_buff *skb,
 					       sizeof(dmabuf_cmsg),
 					       &dmabuf_cmsg);
 				if (err || msg->msg_flags & MSG_CTRUNC) {
-					msg->msg_flags &= ~MSG_CTRUNC;
 					if (!err)
 						err = -ETOOSMALL;
 					goto out;
-- 
2.48.1