[PATCH net-next v9 01/16] net: Declare MSG_SPLICE_PAGES internal sendmsg() flag

[David Howells <dhowells@redhat.com>]

Declare MSG_SPLICE_PAGES, an internal sendmsg() flag, that hints to a
network protocol that it should splice pages from the source iterator
rather than copying the data if it can.  This flag is added to a list that
is cleared by sendmsg syscalls on entry.

This is intended as a replacement for the ->sendpage() op, allowing a way
to splice in several multipage folios in one go.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: io-uring@vger.kernel.org
cc: netdev@vger.kernel.org
---

Notes:
    ver #7)
     - In ____sys_sendmsg(), clear internal flags before setting msg_flags.
     - Clear internal flags in uring io_send{,_zc}().

 include/linux/socket.h | 3 +++
 io_uring/net.c         | 2 ++
 net/socket.c           | 2 ++
 3 files changed, 7 insertions(+)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index 13c3a237b9c9..bd1cc3238851 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -327,6 +327,7 @@ struct ucred {
 					  */
 
 #define MSG_ZEROCOPY	0x4000000	/* Use user data in kernel path */
+#define MSG_SPLICE_PAGES 0x8000000	/* Splice the pages from the iterator in sendmsg() */
 #define MSG_FASTOPEN	0x20000000	/* Send data in TCP SYN */
 #define MSG_CMSG_CLOEXEC 0x40000000	/* Set close_on_exec for file
 					   descriptor received through
@@ -337,6 +338,8 @@ struct ucred {
 #define MSG_CMSG_COMPAT	0		/* We never have 32 bit fixups */
 #endif
 
+/* Flags to be cleared on entry by sendmsg and sendmmsg syscalls */
+#define MSG_INTERNAL_SENDMSG_FLAGS (MSG_SPLICE_PAGES)
 
 /* Setsockoptions(2) level. Thanks to BSD these must match IPPROTO_xxx */
 #define SOL_IP		0
diff --git a/io_uring/net.c b/io_uring/net.c
index 89e839013837..f7cbb3c7a575 100644
--- a/io_uring/net.c
+++ b/io_uring/net.c
@@ -389,6 +389,7 @@ int io_send(struct io_kiocb *req, unsigned int issue_flags)
 	if (flags & MSG_WAITALL)
 		min_ret = iov_iter_count(&msg.msg_iter);
 
+	flags &= ~MSG_INTERNAL_SENDMSG_FLAGS;
 	msg.msg_flags = flags;
 	ret = sock_sendmsg(sock, &msg);
 	if (ret < min_ret) {
@@ -1136,6 +1137,7 @@ int io_send_zc(struct io_kiocb *req, unsigned int issue_flags)
 		msg_flags |= MSG_DONTWAIT;
 	if (msg_flags & MSG_WAITALL)
 		min_ret = iov_iter_count(&msg.msg_iter);
+	msg_flags &= ~MSG_INTERNAL_SENDMSG_FLAGS;
 
 	msg.msg_flags = msg_flags;
 	msg.msg_ubuf = &io_notif_to_data(zc->notif)->uarg;
diff --git a/net/socket.c b/net/socket.c
index b7e01d0fe082..3df96e9ba4e2 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2138,6 +2138,7 @@ int __sys_sendto(int fd, void __user *buff, size_t len, unsigned int flags,
 		msg.msg_name = (struct sockaddr *)&address;
 		msg.msg_namelen = addr_len;
 	}
+	flags &= ~MSG_INTERNAL_SENDMSG_FLAGS;
 	if (sock->file->f_flags & O_NONBLOCK)
 		flags |= MSG_DONTWAIT;
 	msg.msg_flags = flags;
@@ -2483,6 +2484,7 @@ static int ____sys_sendmsg(struct socket *sock, struct msghdr *msg_sys,
 		msg_sys->msg_control = ctl_buf;
 		msg_sys->msg_control_is_user = false;
 	}
+	flags &= ~MSG_INTERNAL_SENDMSG_FLAGS;
 	msg_sys->msg_flags = flags;
 
 	if (sock->file->f_flags & O_NONBLOCK)