Allow pasta to take a command to execute

When not given an existing PID or network namspace to attach to, pasta
spawns a shell.  Most commands which can spawn a shell in an altered
environment can also run other commands in that same environment, which can
be useful in automation.

Allow pasta to do the same thing; it can be given an arbitrary command to
run in the network and user namespace which pasta creates.  If neither a
command nor an existing PID or netns to attach to is given, continue to
spawn a default shell, as before.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

conf.c

@@ -550,7 +550,8 @@ static int conf_ns_pid(char *userns, char *netns, const char *arg)
 		return 0;
 	}
 
-	return -EINVAL;
+	/* Not a PID, later code will treat as a command */
+	return 0;
 }
 
 /**
@@ -1498,14 +1499,18 @@ void conf(struct ctx *c, int argc, char **argv)
 
 	check_root(c);
 
-	if (c->mode == MODE_PASTA && optind + 1 == argc) {
-		ret = conf_ns_pid(userns, netns, argv[optind]);
-		if (ret < 0)
+	if (c->mode == MODE_PASTA) {
+		if (*netns && optind != argc) {
+			err("Both --netns and PID or command given");
 			usage(argv[0]);
-	} else if (c->mode == MODE_PASTA && *userns
-		   && !*netns && optind == argc) {
-		err("--userns requires --netns or PID");
-		usage(argv[0]);
+		} else if (optind + 1 == argc) {
+			ret = conf_ns_pid(userns, netns, argv[optind]);
+			if (ret < 0)
+				usage(argv[0]);
+		} else if (*userns && !*netns && optind == argc) {
+			err("--userns requires --netns or PID");
+			usage(argv[0]);
+		}
 	} else if (optind != argc) {
 		usage(argv[0]);
 	}
@@ -1519,7 +1524,11 @@ void conf(struct ctx *c, int argc, char **argv)
 			if (ret < 0)
 				usage(argv[0]);
 		} else {
-			pasta_start_ns(c);
+			if (*userns) {
+				err("Both --userns and command given");
+				usage(argv[0]);
+			}
+			pasta_start_ns(c, argc - optind, argv + optind);
 		}
 	}
 

passt.1

@@ -15,7 +15,10 @@
 [\fIOPTION\fR]...
 .br
 .B pasta
-[\fIOPTION\fR]... [\fIPID\fR]
+[\fIOPTION\fR]... [\fICOMMAND\fR [\fIARG\fR]...]
+.br
+.B pasta
+[\fIOPTION\fR]... \fIPID\fR
 .br
 .B pasta
 [\fIOPTION\fR]... \fB--netns\fR [\fIPATH\fR|\fINAME\fR]
@@ -62,10 +65,11 @@ or with the \fBqrap\fR(1) wrapper.
 equivalent functionality to network namespaces, as the one offered by
 \fBpasst\fR for virtual machines.
 
-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.
+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 the given command or a default shell
+within this context. A \fItap\fR device within the network namespace
+is created to provide network connectivity.
 
 For local TCP and UDP traffic only, \fBpasta\fR also implements a bypass path
 directly mapping Layer-4 sockets between \fIinit\fR and target namespaces,

pasta.c

@@ -108,6 +108,7 @@ netns:
 struct pasta_setup_ns_arg {
 	struct ctx *c;
 	int euid;
+	char **argv;
 };
 
 /**
@@ -119,7 +120,6 @@ struct pasta_setup_ns_arg {
 static int pasta_setup_ns(void *arg)
 {
 	struct pasta_setup_ns_arg *a = (struct pasta_setup_ns_arg *)arg;
-	char *shell;
 
 	if (!a->c->netns_only) {
 		char buf[BUFSIZ];
@@ -139,29 +139,42 @@ static int pasta_setup_ns(void *arg)
 	FWRITE("/proc/sys/net/ipv4/ping_group_range", "0 0",
 	       "Cannot set ping_group_range, ICMP requests might fail");
 
-	shell = getenv("SHELL") ? getenv("SHELL") : "/bin/sh";
-	if (strstr(shell, "/bash"))
-		execve(shell, ((char *[]) { shell, "-l", NULL }), environ);
-	else
-		execve(shell, ((char *[]) { shell, NULL }), environ);
+	execvp(a->argv[0], a->argv);
 
-	perror("execve");
+	perror("execvp");
 	exit(EXIT_FAILURE);
 }
 
 /**
- * pasta_start_ns() - Fork shell in new namespace if target ns is not given
+ * pasta_start_ns() - Fork command in new namespace if target ns is not given
  * @c:		Execution context
+ * @argc:	Number of arguments for spawned command
+ * @argv:	Command to spawn and arguments
  */
-void pasta_start_ns(struct ctx *c)
+void pasta_start_ns(struct ctx *c, int argc, char *argv[])
 {
-	struct pasta_setup_ns_arg arg = { .c = c, .euid = geteuid() };
+	struct pasta_setup_ns_arg arg = {
+		.c = c,
+		.euid = geteuid(),
+		.argv = argv,
+	};
+	char *shell = getenv("SHELL") ? getenv("SHELL") : "/bin/sh";
+	char *sh_argv[] = { shell, NULL };
+	char *bash_argv[] = { shell, "-l", NULL };
 	char ns_fn_stack[NS_FN_STACK_SIZE];
 
 	c->foreground = 1;
 	if (!c->debug)
 		c->quiet = 1;
 
+	if (argc == 0) {
+		if (strstr(shell, "/bash")) {
+			arg.argv = bash_argv;
+		} else {
+			arg.argv = sh_argv;
+		}
+	}
+
 	pasta_child_pid = clone(pasta_setup_ns,
 				ns_fn_stack + sizeof(ns_fn_stack) / 2,
 				(c->netns_only ? 0 : CLONE_NEWNET) |

pasta.h

@@ -6,7 +6,7 @@
 #ifndef PASTA_H
 #define PASTA_H
 
-void pasta_start_ns(struct ctx *c);
+void pasta_start_ns(struct ctx *c, int argc, char *argv[]);
 void pasta_ns_conf(struct ctx *c);
 void pasta_child_handler(int signal);
 int pasta_netns_quit_init(struct ctx *c);