tcp_recvmsg_dmabuf can export the following errors:
- EFAULT when linear copy fails
- ETOOSMALL when cmsg put fails
- ENODEV if one of the frags is readable
- ENOMEM on xarray failures

But they are all ignored and replaced by EFAULT in the caller
(tcp_recvmsg_locked). Expose real error to the userspace to
add more transparency on what specifically fails.

In non-devmem case (skb_copy_datagram_msg) doing `if (!copied)
copied=-EFAULT` is ok because skb_copy_datagram_msg can return only EFAULT.

Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
--
v2: s/err <= 0/err < 0/ since we never return 0 (Jakub)
---
 net/ipv4/tcp.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 588932c3cf1d..9c576dc9a1f7 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2818,9 +2818,9 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
 
 				err = tcp_recvmsg_dmabuf(sk, skb, offset, msg,
 							 used);
-				if (err <= 0) {
+				if (err < 0) {
 					if (!copied)
-						copied = -EFAULT;
+						copied = err;
 
 					break;
 				}
-- 
2.51.0