[PATCH net v2] net: leave O_NONBLOCK to the sendmsg and recvmsg syscalls

[Jens Axboe <axboe@kernel.dk>]

____sys_sendmsg() and ____sys_recvmsg() turn O_NONBLOCK on the socket
into MSG_DONTWAIT. That is right for the system calls, but io_uring uses
__sys_sendmsg_sock() and __sys_recvmsg_sock() for a blocking attempt
from io-wq as well, after the non-blocking attempts with MSG_DONTWAIT
have failed. On an O_NONBLOCK socket that attempt can't block either and
the request completes with -EAGAIN, unlike IORING_OP_SEND and RECV,
which call sock_sendmsg() and sock_recvmsg() directly and don't look at
O_NONBLOCK at all.

Move the O_NONBLOCK handling to ___sys_sendmsg() and ___sys_recvmsg(),
which the sendmsg, sendmmsg, recvmsg and recvmmsg system calls go
through. The io_uring helpers then pass the flags the caller asked for.

Cc: stable@vger.kernel.org
Fixes: 0fa03c624d8f ("io_uring: add support for sendmsg()")
Fixes: aa1fa28fc73e ("io_uring: add support for recvmsg()")
Link: https://lore.kernel.org/io-uring/a5589b66-9a20-4b1c-a0f1-1b5b31af3667@app.fastmail.com/
Reported-by: Lukáš Lalinský <lukas@lalinsky.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---

v2: add fixes, CC stable, add link, correct Lukáš, correct subject

 net/socket.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/net/socket.c b/net/socket.c
index 2e3b5737c481..08658a9b8065 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2697,8 +2697,6 @@ static int ____sys_sendmsg(struct socket *sock, struct msghdr *msg_sys,
 	flags &= ~MSG_INTERNAL_SENDMSG_FLAGS;
 	msg_sys->msg_flags = flags;
 
-	if (sock->file->f_flags & O_NONBLOCK)
-		msg_sys->msg_flags |= MSG_DONTWAIT;
 	/*
 	 * If this is sendmmsg() and current destination address is same as
 	 * previously succeeded address, omit asking LSM's decision.
@@ -2762,6 +2760,9 @@ static int ___sys_sendmsg(struct socket *sock, struct user_msghdr __user *msg,
 
 	msg_sys->msg_name = &address;
 
+	if (sock->file->f_flags & O_NONBLOCK)
+		flags |= MSG_DONTWAIT;
+
 	err = sendmsg_copy_msghdr(msg_sys, msg, flags, &iov);
 	if (err < 0)
 		return err;
@@ -2926,9 +2927,6 @@ static int ____sys_recvmsg(struct socket *sock, struct msghdr *msg_sys,
 	/* We assume all kernel code knows the size of sockaddr_storage */
 	msg_sys->msg_namelen = 0;
 
-	if (sock->file->f_flags & O_NONBLOCK)
-		flags |= MSG_DONTWAIT;
-
 	if (unlikely(nosec))
 		err = sock_recvmsg_nosec(sock, msg_sys, flags);
 	else
@@ -2970,6 +2968,9 @@ static int ___sys_recvmsg(struct socket *sock, struct user_msghdr __user *msg,
 	struct sockaddr __user *uaddr;
 	ssize_t err;
 
+	if (sock->file->f_flags & O_NONBLOCK)
+		flags |= MSG_DONTWAIT;
+
 	err = recvmsg_copy_msghdr(msg_sys, msg, flags, &uaddr, &iov);
 	if (err < 0)
 		return err;
-- 
2.56.0