lib/socket: add nl_socket_set_fd() function

This is based on the patch by sagil@infinidat.com, but heavily modified.

Add a function nl_socket_set_fd(), I renamed it from nl_connect_fd().

Now nl_connect() and nl_socket_set_fd() are implemented independently as
they share little code. But they have similar functionality:
to initialize a libnl socket and set it's file descriptor.

A user who wants libnl to setup the socket can continue to use nl_connect().
A user with special requirements should setup the socket entirely. That includes
calling socket() (with or without SOCK_CLOEXEC), bind(), setting buffer size.

For the same reason I dropped nl_create_fd(). It didn't do much more then
calling socket() -- which the user can do directly.

https://github.com/thom311/libnl/pull/68

Signed-off-by: Thomas Haller <thaller@redhat.com>

include/netlink/socket.h

@@ -60,6 +60,7 @@ extern void		nl_socket_disable_auto_ack(struct nl_sock *);
 extern void		nl_socket_enable_auto_ack(struct nl_sock *);
 
 extern int		nl_socket_get_fd(const struct nl_sock *);
+extern int              nl_socket_set_fd(struct nl_sock *sk, int protocol, int fd);
 extern int		nl_socket_set_nonblocking(const struct nl_sock *);
 extern void		nl_socket_enable_msg_peek(struct nl_sock *);
 extern void		nl_socket_disable_msg_peek(struct nl_sock *);

lib/nl.c

@@ -86,8 +86,13 @@
  *       This capability is indicated by
  *       `%NL_CAPABILITY_NL_CONNECT_RETRY_GENERATE_PORT_ON_ADDRINUSE`.
  *
+ * @note nl_connect() creates and sets the file descriptor. You can setup the file
+ *       descriptor yourself by creating and binding it, and then calling
+ *       nl_socket_set_fd(). The result will be the same.
+ *
  * @see nl_socket_alloc()
  * @see nl_close()
+ * @see nl_socket_set_fd()
  *
  * @return 0 on success or a negative error code.
  *
@@ -105,8 +110,8 @@ int nl_connect(struct nl_sock *sk, int protocol)
 	flags |= SOCK_CLOEXEC;
 #endif
 
-        if (sk->s_fd != -1)
-                return -NLE_BAD_SOCK;
+	if (sk->s_fd != -1)
+		return -NLE_BAD_SOCK;
 
 	sk->s_fd = socket(AF_NETLINK, SOCK_RAW | flags, protocol);
 	if (sk->s_fd < 0) {

lib/socket.c

@@ -572,6 +572,68 @@ int nl_socket_get_fd(const struct nl_sock *sk)
 	return sk->s_fd;
 }
 
+/**
+ * Set the socket file descriptor externally which initializes the
+ * socket similar to nl_connect().
+ *
+ * @arg sk         Netlink socket (required)
+ * @arg protocol   Netlink protocol to use (required)
+ * @arg fd         Socket file descriptor to use (required)
+ *
+ * Set the socket file descriptor. @fd must be valid and bind'ed.
+ *
+ * This is an alternative to nl_connect(). nl_connect() creates, binds and
+ * sets the socket. With this function you can set the socket to an externally
+ * created file descriptor.
+ *
+ * @see nl_connect()
+ *
+ * @return 0 on success or a negative error code. On error, @fd is not closed but
+ * possibly unusable.
+ *
+ * @retval -NLE_BAD_SOCK Netlink socket is already connected
+ * @retval -NLE_INVAL Socket is not connected
+ */
+int nl_socket_set_fd(struct nl_sock *sk, int protocol, int fd)
+{
+	int err = 0;
+	socklen_t addrlen;
+	char buf[64];
+	struct sockaddr_nl local = { 0 };
+
+	if (sk->s_fd != -1)
+		return -NLE_BAD_SOCK;
+	if (fd < 0)
+		return -NLE_INVAL;
+
+	addrlen = sizeof(local);
+	err = getsockname(fd, (struct sockaddr *) &local,
+	                  &addrlen);
+	if (err < 0) {
+		NL_DBG(4, "nl_socket_set_fd(%p): getsockname() failed with %d (%s)\n",
+		       sk, errno, strerror_r(errno, buf, sizeof(buf)));
+		return -nl_syserr2nlerr(errno);
+	}
+
+	if (addrlen != sizeof(local))
+		return -NLE_NOADDR;
+
+	if (local.nl_family != AF_NETLINK)
+		return -NLE_AF_NOSUPPORT;
+
+	if (sk->s_local.nl_pid != local.nl_pid) {
+		/* the port id differs. The socket is using a port id not managed by
+		 * libnl, hence reset the local port id to release a possibly generated
+		 * port. */
+		nl_socket_set_local_port (sk, local.nl_pid);
+	}
+	sk->s_local = local;
+	sk->s_fd = fd;
+	sk->s_proto = protocol;
+
+	return 0;
+}
+
 /**
  * Set file descriptor of socket to non-blocking state
  * @arg sk		Netlink socket.

libnl-3.sym

@@ -328,4 +328,5 @@ libnl_3_2_26 {
 global:
 	nl_cache_pickup_checkdup;
 	nl_pickup_keep_syserr;
+	nl_socket_set_fd;
 } libnl_3;