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

[David Ahern <dsahern@gmail.com>]

On 1/24/19 8:50 AM, Matteo Croce wrote:
> 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        | 84 +++++++++++++++++++++++++++++++++++++++++++++
>  man/man8/ip-netns.8 | 10 ++++++
>  2 files changed, 94 insertions(+)
> 
> diff --git a/ip/ipnetns.c b/ip/ipnetns.c
> index 03879b49..86b1a36b 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");
> @@ -811,6 +812,86 @@ static int netns_monitor(int argc, char **argv)
>  	return 0;
>  }
>  
> +static int netns_attach(int argc, char **argv)
> +{
> +	/* This function bind mounts an existing network namespace to a
> +	 * well known location in the filesystem based on the name provided.
> +	 * If everything succeeds, the result is the same as netns_add.
> +	 *
> +	 * 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 users.
> +	 */
> +	char netns_path[PATH_MAX], proc_path[PATH_MAX];
> +	const char *name;
> +	int fd;
> +	pid_t pid;
> +	int made_netns_run_dir_mount = 0;
> +
> +	if (argc < 2) {
> +		fprintf(stderr, "No netns name and PID specified\n");
> +		return -1;
> +	}
> +	name = argv[0];
> +
> +	if (get_s32(&pid, argv[1], 0) || !pid) {
> +		fprintf(stderr, "Invalid PID: %s\n", argv[1]);
> +		return -1;
> +	}
> +
> +	snprintf(netns_path, sizeof(netns_path), "%s/%s", NETNS_RUN_DIR, name);
> +
> +	if (create_netns_dir())
> +		return -1;
> +
> +	/* Make it possible for network namespace mounts to propagate between
> +	 * mount namespaces.  This makes it likely that a unmounting a network
> +	 * namespace file in one namespace will unmount the network namespace
> +	 * file in all namespaces allowing the network namespace to be freed
> +	 * sooner.
> +	 */
> +	while (mount("", NETNS_RUN_DIR, "none", MS_SHARED | MS_REC, NULL)) {
> +		/* Fail unless we need to make the mount point */
> +		if (errno != EINVAL || made_netns_run_dir_mount) {
> +			fprintf(stderr, "mount --make-shared %s failed: %s\n",
> +				NETNS_RUN_DIR, strerror(errno));
> +			return -1;
> +		}
> +
> +		/* Upgrade NETNS_RUN_DIR to a mount point */
> +		if (mount(NETNS_RUN_DIR, NETNS_RUN_DIR, "none", MS_BIND | MS_REC, NULL)) {
> +			fprintf(stderr, "mount --bind %s %s failed: %s\n",
> +				NETNS_RUN_DIR, NETNS_RUN_DIR, strerror(errno));
> +			return -1;
> +		}
> +		made_netns_run_dir_mount = 1;
> +	}
> +
> +	/* Create the filesystem state */
> +	fd = open(netns_path, O_RDONLY|O_CREAT|O_EXCL, 0);
> +	if (fd < 0) {
> +		fprintf(stderr, "Cannot create namespace file \"%s\": %s\n",
> +			netns_path, strerror(errno));
> +		return -1;
> +	}
> +	close(fd);
> +
> +	snprintf(proc_path, sizeof(proc_path), "/proc/%d/ns/net", pid);
> +
> +	/* Bind the netns last so I can watch for it */
> +	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:
> +	if (unlink(netns_path) < 0)
> +		fprintf(stderr, "Cannot remove namespace file \"%s\": %s\n",
> +			netns_path, strerror(errno));
> +	return -1;
> +}
> +

Rather than duplicate netns_add, refactor it for use by both attach and add.