crtools: Namespaces support skeleton

New option -n to dump/restore namespaces.

Fork the namespaces dumping task and write a helper for switching a namespace.

Prepare the restorer code for restoring namespaces before root task.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>

namespaces.c

@@ -0,0 +1,91 @@
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/wait.h>
+#include <sys/types.h>
+#include "util.h"
+#include "crtools.h"
+#include "syscall.h"
+
+static int switch_ns(int pid, int type, char *ns)
+{
+	char buf[32];
+	int nsfd, ret;
+
+	snprintf(buf, sizeof(buf), "/proc/%d/ns/%s", pid, ns);
+	nsfd = open(buf, O_RDONLY);
+	if (nsfd < 0) {
+		pr_perror("Can't open ipcns file\n");
+		goto out;
+	}
+
+	ret = setns(nsfd, type);
+	if (ret < 0)
+		pr_perror("Can't setns %d/%s\n", pid, ns);
+
+	close(nsfd);
+out:
+	return ret;
+}
+
+static int do_dump_namespaces(int ns_pid)
+{
+	return 0;
+}
+
+int dump_namespaces(int ns_pid)
+{
+	int pid, ret, status;
+
+	/*
+	 * The setns syscall is cool, we can switch to the other
+	 * namespace and then return back to our initial one, but
+	 * for me it's much easier just to fork another task and
+	 * let it do the job, all the more so it can be done in
+	 * parallel with task dumping routine.
+	 *
+	 * However, the question how to dump sockets from the target
+	 * net namesapce with this is still open
+	 */
+
+	pr_info("Dumping %d's namespaces\n", ns_pid);
+
+	pid = fork();
+	if (pid < 0) {
+		pr_perror("Can't fork ns dumper\n");
+		return -1;
+	}
+
+	if (pid == 0) {
+		ret = do_dump_namespaces(ns_pid);
+		exit(ret);
+	}
+
+	ret = waitpid(pid, &status, 0);
+	if (ret != pid) {
+		pr_perror("Can't wait ns dumper\n");
+		return -1;
+	}
+
+	if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {
+		pr_err("Namespaces dumping finished with error %d\n", status);
+		return -1;
+	}
+
+	pr_info("Namespaces dump complete\n");
+	return 0;
+}
+
+int prepare_namespace(int pid, unsigned long clone_flags)
+{
+	int ret = 0;
+
+	pr_info("Restoring namespaces %d flags %lx\n",
+			pid, clone_flags);
+
+	return ret;
+}
+
+int try_show_namespaces(int ns_pid)
+{
+	return 0;
+}