init: Implement 'exec' command.

(cherry-pick of d05ab3952ec0e38f33a0e80ce6b9eb45b0064ba4.)

Change-Id: Id6d9bb32e51a0ad090ed8240cc505dc45b57b35d

init/builtins.c

@@ -205,6 +205,68 @@ int do_exec(int nargs, char **args)
     return -1;
 }
 
+int do_execonce(int nargs, char **args)
+{
+    pid_t child;
+    int child_status = 0;
+    static int already_done;
+
+    if (already_done) {
+      return -1;
+    }
+    already_done = 1;
+    if (!(child = fork())) {
+        /*
+         * Child process.
+         */
+        zap_stdio();
+        char *exec_args[100];
+        int i;
+        int num_process_args = nargs;
+
+        memset(exec_args, 0, sizeof(exec_args));
+        if (num_process_args > ARRAY_SIZE(exec_args) - 1) {
+            ERROR("exec called with %d args, limit is %d", num_process_args,
+                  ARRAY_SIZE(exec_args) - 1);
+            _exit(1);
+        }
+        for (i = 1; i < num_process_args; i++)
+            exec_args[i - 1] = args[i];
+
+        if (execv(exec_args[0], exec_args) == -1) {
+            ERROR("Failed to execv '%s' (%s)", exec_args[0], strerror(errno));
+            _exit(1);
+        }
+        ERROR("Returned from execv()!");
+        _exit(1);
+    }
+
+    /*
+     * Parent process.
+     */
+    if (child == -1) {
+        ERROR("Fork failed\n");
+        return -1;
+    }
+
+    if (TEMP_FAILURE_RETRY(waitpid(child, &child_status, 0)) == -1) {
+        ERROR("waitpid(): failed (%s)\n", strerror(errno));
+        return -1;
+    }
+
+    if (WIFSIGNALED(child_status)) {
+        INFO("Child exited due to signal %d\n", WTERMSIG(child_status));
+        return -1;
+    } else if (WIFEXITED(child_status)) {
+        INFO("Child exited normally (exit code %d)\n", WEXITSTATUS(child_status));
+        return WEXITSTATUS(child_status);
+    }
+
+    ERROR("Abnormal child process exit\n");
+
+    return -1;
+}
+
 int do_export(int nargs, char **args)
 {
     return add_environment(args[1], args[2]);

init/init.c

@@ -126,7 +126,7 @@ int add_environment(const char *key, const char *val)
     return -1;
 }
 
-static void zap_stdio(void)
+void zap_stdio(void)
 {
     int fd;
     fd = open("/dev/null", O_RDWR);

init/init.h

@@ -149,5 +149,6 @@ void property_changed(const char *name, const char *value);
 extern struct selabel_handle *sehandle;
 extern struct selabel_handle *sehandle_prop;
 extern int selinux_reload_policy(void);
+void zap_stdio(void);
 
 #endif	/* _INIT_INIT_H */

init/init_parser.c

@@ -97,6 +97,7 @@ static int lookup_keyword(const char *s)
     case 'e':
         if (!strcmp(s, "nable")) return K_enable;
         if (!strcmp(s, "xec")) return K_exec;
+        if (!strcmp(s, "xeconce")) return K_execonce;
         if (!strcmp(s, "xport")) return K_export;
         break;
     case 'g':

init/keywords.h

@@ -8,6 +8,7 @@ int do_class_reset(int nargs, char **args);
 int do_domainname(int nargs, char **args);
 int do_enable(int nargs, char **args);
 int do_exec(int nargs, char **args);
+int do_execonce(int nargs, char **args);
 int do_export(int nargs, char **args);
 int do_hostname(int nargs, char **args);
 int do_ifup(int nargs, char **args);
@@ -59,6 +60,7 @@ enum {
     KEYWORD(domainname,  COMMAND, 1, do_domainname)
     KEYWORD(enable,      COMMAND, 1, do_enable)
     KEYWORD(exec,        COMMAND, 1, do_exec)
+    KEYWORD(execonce,    COMMAND, 1, do_execonce)
     KEYWORD(export,      COMMAND, 2, do_export)
     KEYWORD(group,       OPTION,  0, 0)
     KEYWORD(hostname,    COMMAND, 1, do_hostname)

init/readme.txt

@@ -136,10 +136,14 @@ Commands
 --------
 
 exec <path> [ <argument> ]*
+   This command is not implemented.
+
+execonce <path> [ <argument> ]*
    Fork and execute a program (<path>).  This will block until
-   the program completes execution.  It is best to avoid exec
-   as unlike the builtin commands, it runs the risk of getting
-   init "stuck". (??? maybe there should be a timeout?)
+   the program completes execution.  This command can be run at most
+   once during init's lifetime. Subsequent invocations are ignored.
+   It is best to avoid exec as unlike the builtin commands, it runs
+   the risk of getting init "stuck".
 
 export <name> <value>
    Set the environment variable <name> equal to <value> in the