netdev - [PATCH net-next v2] net: devmem: expose tcp_recvmsg

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Message-ID: <20250910162429.4127997-1-sdf@fomichev.me>
Date: Wed, 10 Sep 2025 09:24:29 -0700
From: Stanislav Fomichev <sdf@...ichev.me>
To: netdev@...r.kernel.org
Cc: davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	ncardwell@...gle.com,
	kuniyu@...gle.com,
	dsahern@...nel.org,
	horms@...nel.org,
	linux-kernel@...r.kernel.org,
	Mina Almasry <almasrymina@...gle.com>
Subject: [PATCH net-next v2] net: devmem: expose tcp_recvmsg_locked errors

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@...nel.org>
Reviewed-by: Mina Almasry <almasrymina@...gle.com>
Reviewed-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Stanislav Fomichev <sdf@...ichev.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