sockets: Do not call modprobe to make sure diag modules are there

There was a problem with diag modules, that were auto-loaded on dump and
put some data into uevent sockets thus causing the dump to fail on that
socket. The solution that time was to call modprobe in advance to make
sure the diag modules are there.

This turned out to be not nice solution as modprobe may fail if the module
is missing and the requred functionality is built-in.

To handle this for diag stuff it is mush faster and more reliable to ping
the kernel with simple diag request and check that it reqched the required
module.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

sockets.c

@@ -31,6 +31,22 @@
 #define SO_GET_FILTER	SO_ATTACH_FILTER
 #endif
 
+struct sock_diag_greq {
+	u8	family;
+	u8	protocol;
+};
+
+struct sock_diag_req {
+	struct nlmsghdr hdr;
+	union {
+		struct unix_diag_req	u;
+		struct inet_diag_req_v2	i;
+		struct packet_diag_req	p;
+		struct netlink_diag_req n;
+		struct sock_diag_greq	g;
+	} r;
+};
+
 enum socket_cl_bits
 {
 	NETLINK_CL_BIT,
@@ -96,39 +112,93 @@ bool socket_test_collect_bit(unsigned int family, unsigned int proto)
 	return test_bit(nr, socket_cl_bits) != 0;
 }
 
+static int probe_recv_one(struct nlmsghdr *h, void *arg)
+{
+	pr_err("PROBE RECEIVED\n");
+	return -1;
+}
+
+static int probe_err(int err, void *arg)
+{
+	int expected_err = *(int *)arg;
+
+	if (err == expected_err)
+		return 0;
+
+	pr_err("Diag module missing (%d)\n", err);
+	return err;
+}
+
+static inline void probe_diag(int nl, struct sock_diag_req *req, int expected_err)
+{
+	do_rtnl_req(nl, req, req->hdr.nlmsg_len, probe_recv_one, probe_err, &expected_err);
+}
+
 void preload_socket_modules()
 {
+	int nl;
+	struct sock_diag_req req;
+
 	/*
 	 * If the task to dump (e.g. an LXC container) has any netlink
 	 * KOBJECT_UEVENT socket open and the _diag modules aren't
 	 * loaded is dumped, criu will freeze the task and then the
 	 * kernel will send it messages on the socket, and then we will
 	 * fail to dump because the socket has pending data. The Real
-	 * Solution is to dump this pending data, but we just modprobe
-	 * things beforehand for now so that the first dump doesn't
-	 * fail.
-	 *
-	 * We ignore failure since these could be compiled directly
-	 * in, instead of being kernel modules.
+	 * Solution is to dump this pending data, but we just make sure
+	 * modules are there beforehand for now so that the first dump
+	 * doesn't fail.
 	 */
-	char *modules[] = {
-		"netlink_diag",
-		"af_packet_diag",
-		"udp_diag",
-		"tcp_diag",
-		"unix_diag",
-		NULL,
-	};
-	int i;
-	char *args[2] = {
-		"modprobe",
-		NULL
-	};
-
-	for (i = 0; modules[i]; i++) {
-		args[1] = modules[i];
-		cr_system(-1, -1, -1, args[0], args);
-	}
+
+	nl = socket(PF_NETLINK, SOCK_RAW, NETLINK_SOCK_DIAG);
+	if (nl < 0)
+		return;
+
+	pr_info("Probing sock diag modules\n");
+
+	memset(&req, 0, sizeof(req));
+	req.hdr.nlmsg_type	= SOCK_DIAG_BY_FAMILY;
+	req.hdr.nlmsg_seq	= CR_NLMSG_SEQ;
+
+	/*
+	 * Probe UNIX, netlink and packet diag-s by feeding
+	 * to the kernel request that is shorter than they
+	 * expect, byt still containing the family to make
+	 * sure the family handler is there. The family-level
+	 * diag module would report EINVAL in this case.
+	 */
+
+	req.hdr.nlmsg_len = sizeof(req.hdr) + sizeof(req.r.g);
+	req.hdr.nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST;
+
+	req.r.g.family = AF_UNIX;
+	probe_diag(nl, &req, -EINVAL);
+
+	req.r.g.family = AF_PACKET;
+	probe_diag(nl, &req, -EINVAL);
+
+	req.r.g.family = AF_NETLINK;
+	probe_diag(nl, &req, -EINVAL);
+
+	/*
+	 * TCP and UDP(LITE) diags do not support such trick, only
+	 * inet_diag module can be probed like that. For the protocol
+	 * level ones it's OK to request for exact non-existing socket
+	 * and check for ENOENT being reported back as error.
+	 */
+
+	req.hdr.nlmsg_len = sizeof(req.hdr) + sizeof(req.r.i);
+	req.hdr.nlmsg_flags = NLM_F_REQUEST;
+	req.r.i.sdiag_family = AF_INET;
+
+	req.r.i.sdiag_protocol = IPPROTO_TCP;
+	probe_diag(nl, &req, -ENOENT);
+
+	req.r.i.sdiag_protocol = IPPROTO_UDP; /* UDLITE is merged with UDP */
+	probe_diag(nl, &req, -ENOENT);
+
+	close(nl);
+	pr_info("Done probing\n");
 }
 
 static int dump_bound_dev(int sk, SkOptsEntry *soe)
@@ -523,16 +593,6 @@ static int inet_receive_one(struct nlmsghdr *h, void *arg)
 	return inet_collect_one(h, i->sdiag_family, type);
 }
 
-struct sock_diag_req {
-	struct nlmsghdr hdr;
-	union {
-		struct unix_diag_req	u;
-		struct inet_diag_req_v2	i;
-		struct packet_diag_req	p;
-		struct netlink_diag_req n;
-	} r;
-};
-
 static int do_collect_req(int nl, struct sock_diag_req *req, int size,
 		int (*receive_callback)(struct nlmsghdr *h, void *), void *arg)
 {