net: socket: add __sys_bind() helper; remove in-kernel call to syscall

Using the net-internal helper __sys_bind() allows us to avoid the
internal calls to the sys_bind() syscall.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

include/linux/socket.h

@@ -364,5 +364,6 @@ extern int __sys_sendto(int fd, void __user *buff, size_t len,
 extern int __sys_accept4(int fd, struct sockaddr __user *upeer_sockaddr,
 			 int __user *upeer_addrlen, int flags);
 extern int __sys_socket(int family, int type, int protocol);
+extern int __sys_bind(int fd, struct sockaddr __user *umyaddr, int addrlen);
 
 #endif /* _LINUX_SOCKET_H */

net/compat.c

@@ -814,7 +814,7 @@ COMPAT_SYSCALL_DEFINE2(socketcall, int, call, u32 __user *, args)
 		ret = __sys_socket(a0, a1, a[2]);
 		break;
 	case SYS_BIND:
-		ret = sys_bind(a0, compat_ptr(a1), a[2]);
+		ret = __sys_bind(a0, compat_ptr(a1), a[2]);
 		break;
 	case SYS_CONNECT:
 		ret = sys_connect(a0, compat_ptr(a1), a[2]);

net/socket.c

@@ -1462,7 +1462,7 @@ SYSCALL_DEFINE4(socketpair, int, family, int, type, int, protocol,
  *	the protocol layer (having also checked the address is ok).
  */
 
-SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)
+int __sys_bind(int fd, struct sockaddr __user *umyaddr, int addrlen)
 {
 	struct socket *sock;
 	struct sockaddr_storage address;
@@ -1485,6 +1485,11 @@ SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)
 	return err;
 }
 
+SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)
+{
+	return __sys_bind(fd, umyaddr, addrlen);
+}
+
 /*
  *	Perform a listen. Basically, we allow the protocol to do anything
  *	necessary for a listen, and if that works, we mark the socket as
@@ -2471,7 +2476,7 @@ SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args)
 		err = __sys_socket(a0, a1, a[2]);
 		break;
 	case SYS_BIND:
-		err = sys_bind(a0, (struct sockaddr __user *)a1, a[2]);
+		err = __sys_bind(a0, (struct sockaddr __user *)a1, a[2]);
 		break;
 	case SYS_CONNECT:
 		err = sys_connect(a0, (struct sockaddr __user *)a1, a[2]);