[PATCH iproute2-next v2] netns: add subcommand to attach an existing network namespace

[Matteo Croce <mcroce@redhat.com>]

ip tracks namespaces with dummy files in /var/run/netns/, but can't see
namespaces created with other tools.
Creating the dummy file and bind mounting the correct procfs entry will
make ip aware of that namespace.
Add an ip netns subcommand to automate this task.

Signed-off-by: Matteo Croce <mcroce@redhat.com>
---
 ip/ipnetns.c        | 62 ++++++++++++++++++++++++++++++++++-----------
 man/man8/ip-netns.8 | 10 ++++++++
 2 files changed, 57 insertions(+), 15 deletions(-)

diff --git a/ip/ipnetns.c b/ip/ipnetns.c
index 03879b49..430d8844 100644
--- a/ip/ipnetns.c
+++ b/ip/ipnetns.c
@@ -28,6 +28,7 @@ static int usage(void)
 {
 	fprintf(stderr, "Usage: ip netns list\n");
 	fprintf(stderr, "       ip netns add NAME\n");
+	fprintf(stderr, "       ip netns attach NAME PID\n");
 	fprintf(stderr, "       ip netns set NAME NETNSID\n");
 	fprintf(stderr, "       ip [-all] netns delete [NAME]\n");
 	fprintf(stderr, "       ip netns identify [PID]\n");
@@ -632,24 +633,40 @@ static int create_netns_dir(void)
 	return 0;
 }
 
-static int netns_add(int argc, char **argv)
+static int netns_add(int argc, char **argv, bool create)
 {
 	/* This function creates a new network namespace and
 	 * a new mount namespace and bind them into a well known
 	 * location in the filesystem based on the name provided.
 	 *
+	 * If create is true, a new namespace will be created,
+	 * otherwise an existing one will be attached to the file.
+	 *
 	 * The mount namespace is created so that any necessary
 	 * userspace tweaks like remounting /sys, or bind mounting
-	 * a new /etc/resolv.conf can be shared between uers.
+	 * a new /etc/resolv.conf can be shared between users.
 	 */
-	char netns_path[PATH_MAX];
+	char netns_path[PATH_MAX], proc_path[PATH_MAX];
 	const char *name;
+	pid_t pid;
 	int fd;
 	int made_netns_run_dir_mount = 0;
 
-	if (argc < 1) {
-		fprintf(stderr, "No netns name specified\n");
-		return -1;
+	if (create) {
+		if (argc < 1) {
+			fprintf(stderr, "No netns name specified\n");
+			return -1;
+		}
+	} else {
+		if (argc < 2) {
+			fprintf(stderr, "No netns name and PID specified\n");
+			return -1;
+		}
+
+		if (get_s32(&pid, argv[1], 0) || !pid) {
+			fprintf(stderr, "Invalid PID: %s\n", argv[1]);
+			return -1;
+		}
 	}
 	name = argv[0];
 
@@ -689,21 +706,33 @@ static int netns_add(int argc, char **argv)
 		return -1;
 	}
 	close(fd);
-	if (unshare(CLONE_NEWNET) < 0) {
-		fprintf(stderr, "Failed to create a new network namespace \"%s\": %s\n",
-			name, strerror(errno));
-		goto out_delete;
+
+	if (create) {
+		if (unshare(CLONE_NEWNET) < 0) {
+			fprintf(stderr, "Failed to create a new network namespace \"%s\": %s\n",
+				name, strerror(errno));
+			goto out_delete;
+		}
+
+		strcpy(proc_path, "/proc/self/ns/net");
+	} else {
+		snprintf(proc_path, sizeof(proc_path), "/proc/%d/ns/net", pid);
 	}
 
 	/* Bind the netns last so I can watch for it */
-	if (mount("/proc/self/ns/net", netns_path, "none", MS_BIND, NULL) < 0) {
-		fprintf(stderr, "Bind /proc/self/ns/net -> %s failed: %s\n",
-			netns_path, strerror(errno));
+	if (mount(proc_path, netns_path, "none", MS_BIND, NULL) < 0) {
+		fprintf(stderr, "Bind %s -> %s failed: %s\n",
+			proc_path, netns_path, strerror(errno));
 		goto out_delete;
 	}
 	return 0;
 out_delete:
-	netns_delete(argc, argv);
+	if (create) {
+		netns_delete(argc, argv);
+	} else if (unlink(netns_path) < 0) {
+		fprintf(stderr, "Cannot remove namespace file \"%s\": %s\n",
+			netns_path, strerror(errno));
+	}
 	return -1;
 }
 
@@ -846,7 +875,7 @@ int do_netns(int argc, char **argv)
 		return usage();
 
 	if (matches(*argv, "add") == 0)
-		return netns_add(argc-1, argv+1);
+		return netns_add(argc-1, argv+1, true);
 
 	if (matches(*argv, "set") == 0)
 		return netns_set(argc-1, argv+1);
@@ -866,6 +895,9 @@ int do_netns(int argc, char **argv)
 	if (matches(*argv, "monitor") == 0)
 		return netns_monitor(argc-1, argv+1);
 
+	if (matches(*argv, "attach") == 0)
+		return netns_add(argc-1, argv+1, false);
+
 	fprintf(stderr, "Command \"%s\" is unknown, try \"ip netns help\".\n", *argv);
 	exit(-1);
 }
diff --git a/man/man8/ip-netns.8 b/man/man8/ip-netns.8
index d539f18b..39a10e76 100644
--- a/man/man8/ip-netns.8
+++ b/man/man8/ip-netns.8
@@ -19,6 +19,10 @@ ip-netns \- process network namespace management
 .B ip netns add
 .I NETNSNAME
 
+.ti -8
+.B ip netns attach
+.I NETNSNAME PID
+
 .ti -8
 .B ip [-all] netns del
 .RI "[ " NETNSNAME " ]"
@@ -89,6 +93,12 @@ This command displays all of the network namespaces in /var/run/netns
 If NAME is available in /var/run/netns/ this command creates a new
 network namespace and assigns NAME.
 
+.TP
+.B ip netns attach NAME PID - create a new named network namespace
+.sp
+If NAME is available in /var/run/netns/ this command attaches the network
+namespace of the process PID to NAME as if it were created with ip netns.
+
 .TP
 .B ip [-all] netns delete [ NAME ] - delete the name of a network namespace(s)
 .sp
-- 
2.20.1