[RFC bpf-next v7 1/6] net: socket: add update_socket_protocol hook

[Geliang Tang <geliang.tang@suse.com>]

Add a hook named update_socket_protocol in __sys_socket(), for bpf progs
to attach to and update socket protocol. One user case is to force
legacy TCP apps to create and use MPTCP sockets instead of TCP ones.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
---
 net/socket.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/net/socket.c b/net/socket.c
index 2b0e54b2405c..6837a517ac9f 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1644,11 +1644,37 @@ struct file *__sys_socket_file(int family, int type, int protocol)
 	return sock_alloc_file(sock, flags, NULL);
 }
 
+/*
+ * A hook for bpf progs to attach to and update socket protocol.
+ *
+ * A static noinline declaration here could cause the compiler to optimize away
+ * the function. A global noinline declaration will keep the definition, but may
+ * optimize away the callsite. Therefore, __weak is needed to ensure that the
+ * call is still emitted, by telling the compiler that we don't know what the
+ * function might eventually be.
+ *
+ * __diag_* below are needed to dismiss the missing prototype warning.
+ */
+__diag_push();
+__diag_ignore_all("-Wmissing-prototypes",
+		  "kfuncs which will be used in BPF programs");
+
+__weak noinline int update_socket_protocol(int family, int type, int protocol)
+{
+	return protocol;
+}
+
+__diag_pop();
+
 int __sys_socket(int family, int type, int protocol)
 {
 	struct socket *sock;
 	int flags;
 
+	protocol = update_socket_protocol(family, type, protocol);
+	if (protocol < 0 || protocol >= IPPROTO_MAX)
+		return -EINVAL;
+
 	sock = __sys_socket_create(family, type, protocol);
 	if (IS_ERR(sock))
 		return PTR_ERR(sock);
-- 
2.35.3