ns: Add ability to save original ns and restoring it back while switcing

This will be required for parasite transport socket creation -- it will
have to be created in a net ns we're putting parasite in and then we'll
have to restore it back to original to go on dumping.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/namespaces.h

@@ -7,5 +7,6 @@ int dump_namespaces(struct pid *pid, unsigned int ns_flags);
 int prepare_namespace(int pid, unsigned long clone_flags);
 struct cr_options;
 int try_show_namespaces(int pid, struct cr_options *);
-int switch_ns(int pid, int type, char *ns);
+int switch_ns(int pid, int type, char *ns, int *rst);
+int restore_ns(int rst, int type);
 #endif

ipc_ns.c

@@ -456,7 +456,7 @@ int dump_ipc_ns(int ns_pid, const struct cr_fdset *fdset)
 {
 	int ret;
 
-	ret = switch_ns(ns_pid, CLONE_NEWIPC, "ipc");
+	ret = switch_ns(ns_pid, CLONE_NEWIPC, "ipc", NULL);
 	if (ret < 0)
 		return ret;
 

namespaces.c

@@ -9,7 +9,7 @@
 #include "mount.h"
 #include "namespaces.h"
 
-int switch_ns(int pid, int type, char *ns)
+int switch_ns(int pid, int type, char *ns, int *rst)
 {
 	char buf[32];
 	int nsfd;
@@ -19,15 +19,44 @@ int switch_ns(int pid, int type, char *ns)
 	nsfd = open(buf, O_RDONLY);
 	if (nsfd < 0) {
 		pr_perror("Can't open ipcns file");
-		goto out;
+		goto err_ns;
+	}
+
+	if (rst) {
+		snprintf(buf, sizeof(buf), "/proc/self/ns/%s", ns);
+		*rst = open(buf, O_RDONLY);
+		if (*rst < 0) {
+			pr_perror("Can't open ns file");
+			goto err_rst;
+		}
 	}
 
 	ret = setns(nsfd, type);
-	if (ret < 0)
+	if (ret < 0) {
 		pr_perror("Can't setns %d/%s", pid, ns);
+		goto err_set;
+	}
+
+	close(nsfd);
+	return 0;
 
+err_set:
+	if (rst)
+		close(*rst);
+err_rst:
 	close(nsfd);
-out:
+err_ns:
+	return -1;
+}
+
+int restore_ns(int rst, int type)
+{
+	int ret;
+
+	ret = setns(rst, type);
+	if (ret < 0)
+		pr_perror("Can't restore ns back");
+
 	return ret;
 }
 

uts_ns.c

@@ -18,7 +18,7 @@ int dump_uts_ns(int ns_pid, struct cr_fdset *fdset)
 	struct utsname ubuf;
 	UtsnsEntry ue = UTSNS_ENTRY__INIT;
 
-	ret = switch_ns(ns_pid, CLONE_NEWUTS, "uts");
+	ret = switch_ns(ns_pid, CLONE_NEWUTS, "uts", NULL);
 	if (ret < 0)
 		return ret;