net: Mount ns' sysfs before dump

Some information about network devices may hide in sysfs, thus
it's required to have one at hands while dumping the netns.

Create the detached mount for that.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

net.c

@@ -6,17 +6,21 @@
 #include <net/if_arp.h>
 #include <sys/wait.h>
 #include <sched.h>
+#include <sys/mount.h>
+
 #include "syscall-types.h"
 #include "namespaces.h"
 #include "net.h"
 #include "libnetlink.h"
 #include "crtools.h"
 #include "sk-inet.h"
+#include "util-pie.h"
 
 #include "protobuf.h"
 #include "protobuf/netdev.pb-c.h"
 
 static int ns_fd = -1;
+static int ns_sysfs_fd = -1;
 
 void show_netdevices(int fd)
 {
@@ -356,11 +360,39 @@ static inline int restore_route(int pid)
 	return restore_ip_dump(CR_FD_ROUTE, pid, "route");
 }
 
+static int mount_ns_sysfs(void)
+{
+	char sys_mount[] = "crtools-sys.XXXXXX";
+
+	BUG_ON(ns_sysfs_fd != -1);
+
+	if (mkdtemp(sys_mount) == NULL) {
+		pr_perror("mkdtemp failed %s", sys_mount);
+		return -1;
+	}
+
+	/*
+	 * The setns() is called, so we're in proper context,
+	 * no need in pulling the mountpoint from parasite.
+	 */
+	pr_info("Mount ns' sysfs in %s\n", sys_mount);
+	if (mount("sysfs", sys_mount, "sysfs", MS_MGC_VAL, NULL)) {
+		pr_perror("mount failed");
+		rmdir(sys_mount);
+		return -1;
+	}
+
+	ns_sysfs_fd = open_detach_mount(sys_mount);
+	return ns_sysfs_fd >= 0 ? 0 : -1;
+}
+
 int dump_net_ns(int pid, struct cr_fdset *fds)
 {
 	int ret;
 
 	ret = switch_ns(pid, &net_ns_desc, NULL);
+	if (!ret)
+		ret = mount_ns_sysfs();
 	if (!ret)
 		ret = dump_links(fds);
 	if (!ret)
@@ -368,6 +400,9 @@ int dump_net_ns(int pid, struct cr_fdset *fds)
 	if (!ret)
 		ret = dump_route(fds);
 
+	close(ns_sysfs_fd);
+	ns_sysfs_fd = -1;
+
 	return ret;
 }