Use explicit --netns option rather than multiplexing with PID

When attaching to an existing namespace, pasta can take a PID or the name
or path of a network namespace as a non-option parameter.  We disambiguate
based on what the parameter looks like.  Make this more explicit by using
a --netns option for explicitly giving the path or name, and treating a
non-option argument always as a PID.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[sbrivio: Fix typo in man page]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

conf.c

@@ -490,34 +490,16 @@ out:
 }
 
 /**
- * conf_ns_opt() - Parse non-option argument to namespace paths
+ * conf_netns() - Parse --netns option
- * @userns:	buffer of size PATH_MAX, initially contains --userns
- *		argument (may be empty), updated with userns path
  * @netns:	buffer of size PATH_MAX, updated with netns path
- * @arg:	PID, path or name of network namespace
+ * @arg:	--netns argument
  *
  * Return: 0 on success, negative error code otherwise
  */
-static int conf_ns_opt(char *userns, char *netns, const char *arg)
+static int conf_netns(char *netns, const char *arg)
 {
-	char *endptr;
-	long pidval;
 	int ret;
 
-	pidval = strtol(arg, &endptr, 10);
-	if (!*endptr) {
-		/* Looks like a pid */
-		if (pidval < 0 || pidval > INT_MAX) {
-			err("Invalid PID %s", arg);
-			return -EINVAL;
-		}
-
-		snprintf(netns, PATH_MAX, "/proc/%ld/ns/net", pidval);
-		if (!*userns)
-			snprintf(userns, PATH_MAX, "/proc/%ld/ns/user", pidval);
-		return 0;
-	}
-
 	if (!strchr(arg, '/')) {
 		/* looks like a netns name */
 		ret = snprintf(netns, PATH_MAX, "%s/%s", NETNS_RUN_DIR, arg);
@@ -534,6 +516,43 @@ static int conf_ns_opt(char *userns, char *netns, const char *arg)
 	return 0;
 }
 
+/**
+ * conf_ns_pid() - Parse non-option argument as a PID
+ * @userns:	buffer of size PATH_MAX, initially contains --userns
+ *		argument (may be empty), updated with userns path
+ * @netns:	buffer of size PATH_MAX, initial contains --netns
+ *		argument (may be empty), updated with netns path
+ * @arg:	PID of network namespace
+ *
+ * Return: 0 on success, negative error code otherwise
+ */
+static int conf_ns_pid(char *userns, char *netns, const char *arg)
+{
+	char *endptr;
+	long pidval;
+
+	if (*netns) {
+		err("Both --netns and PID given");
+		return -EINVAL;
+	}
+
+	pidval = strtol(arg, &endptr, 10);
+	if (!*endptr) {
+		/* Looks like a pid */
+		if (pidval < 0 || pidval > INT_MAX) {
+			err("Invalid PID %s", arg);
+			return -EINVAL;
+		}
+
+		snprintf(netns, PATH_MAX, "/proc/%ld/ns/net", pidval);
+		if (!*userns)
+			snprintf(userns, PATH_MAX, "/proc/%ld/ns/user", pidval);
+		return 0;
+	}
+
+	return -EINVAL;
+}
+
 /**
  * conf_ns_check() - Check if we can enter configured namespaces
  * @arg:	Execution context
@@ -708,10 +727,13 @@ static unsigned int conf_ip6(unsigned int ifi,
 static void usage(const char *name)
 {
 	if (strstr(name, "pasta")) {
-		info("Usage: %s [OPTION]... [PID|PATH|NAME]", name);
+		info("Usage: %s [OPTION]... [COMMAND] [ARGS]...", name);
+		info("       %s [OPTION]... PID", name);
+		info("       %s [OPTION]... --netns [PATH|NAME]", name);
 		info("");
-		info("Without PID|PATH|NAME, run the default shell in a new");
+		info("Without PID or --netns, run the given command or a");
-		info("network and user namespace, and connect it via pasta.");
+		info("default shell in a new network and user namespace, and");
+		info("connect it via pasta.");
 	} else {
 		info("Usage: %s [OPTION]...", name);
 	}
@@ -858,6 +880,7 @@ pasta_opts:
 	info(   "    SPEC is as described above");
 	info(   "    default: auto");
 	info(   "  --userns NSPATH 	Target user namespace to join");
+	info(   "  --netns PATH|NAME	Target network namespace to join");
 	info(   "  --netns-only		Don't join existing user namespace");
 	info(   "    implied if PATH or NAME are given without --userns");
 	info(   "  --config-net		Configure tap interface in namespace");
@@ -1038,6 +1061,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		{"tcp-ns",	required_argument,	NULL,		'T' },
 		{"udp-ns",	required_argument,	NULL,		'U' },
 		{"userns",	required_argument,	NULL,		2 },
+		{"netns",	required_argument,	NULL,		3 },
 		{"netns-only",	no_argument,		&c->netns_only,	1 },
 		{"config-net",	no_argument,		&c->pasta_conf_ns, 1 },
 		{"ns-mac-addr",	required_argument,	NULL,		4 },
@@ -1091,6 +1115,16 @@ void conf(struct ctx *c, int argc, char **argv)
 				usage(argv[0]);
 			}
 			break;
+		case 3:
+			if (c->mode != MODE_PASTA) {
+				err("--netns is for pasta mode only");
+				usage(argv[0]);
+			}
+
+			ret = conf_netns(netns, optarg);
+			if (ret < 0)
+				usage(argv[0]);
+			break;
 		case 4:
 			if (c->mode != MODE_PASTA) {
 				err("--ns-mac-addr is for pasta mode only");
@@ -1465,11 +1499,12 @@ void conf(struct ctx *c, int argc, char **argv)
 	check_root(c);
 
 	if (c->mode == MODE_PASTA && optind + 1 == argc) {
-		ret = conf_ns_opt(userns, netns, argv[optind]);
+		ret = conf_ns_pid(userns, netns, argv[optind]);
 		if (ret < 0)
 			usage(argv[0]);
-	} else if (c->mode == MODE_PASTA && *userns && optind == argc) {
+	} else if (c->mode == MODE_PASTA && *userns
-		err("--userns requires PID, PATH or NAME");
+		   && !*netns && optind == argc) {
+		err("--userns requires --netns or PID");
 		usage(argv[0]);
 	} else if (optind != argc) {
 		usage(argv[0]);

passt.1

@@ -15,7 +15,10 @@
 [\fIOPTION\fR]...
 .br
 .B pasta
-[\fIOPTION\fR]... [\fIPID\fR|\fIPATH\fR|\fINAME\fR]
+[\fIOPTION\fR]... [\fIPID\fR]
+.br
+.B pasta
+[\fIOPTION\fR]... \fB--netns\fR [\fIPATH\fR|\fINAME\fR]
 
 .SH DESCRIPTION
 
@@ -59,7 +62,7 @@ or with the \fBqrap\fR(1) wrapper.
 equivalent functionality to network namespaces, as the one offered by
 \fBpasst\fR for virtual machines.
 
-If PID, PATH or NAME are given, \fBpasta\fR associates to an existing user and
+If PID or --netns are given, \fBpasta\fR associates to an existing user and
 network namespace. Otherwise, \fBpasta\fR creates a new user and network
 namespace, and spawns an interactive shell within this context. A \fItap\fR
 device within the network namespace is created to provide network connectivity.
@@ -445,7 +448,14 @@ Default is \fBauto\fR.
 Target user namespace to join, as a path. If PID is given, without this option,
 the user namespace will be the one of the corresponding process.
 
-This option requires PID, PATH or NAME to be specified.
+This option requires --netns or a PID to be specified.
+
+.TP
+.BR \-\-netns " " \fIspec
+Target network namespace to join, as a path or a name.  A name is treated as
+with \fBip-netns(8)\fR as equivalent to a path in \fI/run/netns\fR.
+
+This option can't be specified with a PID.
 
 .TP
 .BR \-\-netns-only