libbb: fix '--help' handling in FEATURE_SH_NOFORK=y

Most BusyBox applets respond to the '--help' option by printing
a usage message.  This is normally handled by busybox_main() so
applet main routines don't have support for '--help'.

In standalone shell mode with FEATURE_SH_NOFORK enabled nofork
applets are invoked directly, bypassing busybox_main().  This
results in inconsistent handling of '--help':

- applets which call getopt() report "unrecognized option '--help'"
  and print help anyway;

- realpath says "--help: No such file or directory" and doesn't
  print help;

- usleep says "invalid number '--help'" and doesn't print help.

Avoid inconsistency by checking for '--help' in run_nofork_applet().

Bug found by Ron Yorston.

function                                             old     new   delta
show_usage_if_dash_dash_help                           -      70     +70
run_nofork_applet                                    347     362     +15
run_applet_no_and_exit                               432     365     -67
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/1 up/down: 85/-67)             Total: 18 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/libbb.h

@@ -1219,6 +1219,7 @@ void run_noexec_applet_and_exit(int a, const char *name, char **argv) NORETURN F
 int find_applet_by_name(const char *name) FAST_FUNC;
 void run_applet_no_and_exit(int a, const char *name, char **argv) NORETURN FAST_FUNC;
 #endif
+void show_usage_if_dash_dash_help(int applet_no, char **argv) FAST_FUNC;
 #if defined(__linux__)
 void set_task_comm(const char *comm) FAST_FUNC;
 #else

libbb/appletlib.c

@@ -905,16 +905,8 @@ int busybox_main(int argc UNUSED_PARAM, char **argv)
 # endif
 
 # if NUM_APPLETS > 0
-void FAST_FUNC run_applet_no_and_exit(int applet_no, const char *name, char **argv)
+void FAST_FUNC show_usage_if_dash_dash_help(int applet_no, char **argv)
 {
-	int argc = string_array_len(argv);
-
-	/*
-	 * We do not use argv[0]: do not want to repeat massaging of
-	 * "-/sbin/halt" -> "halt", for example.
-	 */
-	applet_name = name;
-
 	/* Special case. POSIX says "test --help"
 	 * should be no different from e.g. "test --foo".
 	 * Thus for "test", we skip --help check.
@@ -931,15 +923,32 @@ void FAST_FUNC run_applet_no_and_exit(int applet_no, const char *name, char **ar
 	 && applet_no != APPLET_NO_false
 #  endif
 	) {
-		if (argc == 2 && strcmp(argv[1], "--help") == 0) {
+		if (argv[1] && !argv[2] && strcmp(argv[1], "--help") == 0) {
 			/* Make "foo --help" exit with 0: */
 			xfunc_error_retval = 0;
 			bb_show_usage();
 		}
 	}
+}
+
+void FAST_FUNC run_applet_no_and_exit(int applet_no, const char *name, char **argv)
+{
+	int argc;
+
+	/*
+	 * We do not use argv[0]: do not want to repeat massaging of
+	 * "-/sbin/halt" -> "halt", for example.
+	 */
+	applet_name = name;
+
+	show_usage_if_dash_dash_help(applet_no, argv);
+
 	if (ENABLE_FEATURE_SUID)
 		check_suid(applet_no);
+
+	argc = string_array_len(argv);
 	xfunc_error_retval = applet_main[applet_no](argc, argv);
+
 	/* Note: applet_main() may also not return (die on a xfunc or such) */
 	xfunc_die();
 }

libbb/vfork_daemon_rexec.c

@@ -109,8 +109,13 @@ int FAST_FUNC run_nofork_applet(int applet_no, char **argv)
 		char *tmp_argv[argc+1];
 		memcpy(tmp_argv, argv, (argc+1) * sizeof(tmp_argv[0]));
 		applet_name = tmp_argv[0];
+
+		/* longjmp's (instead of returning) if --help is seen */
+		show_usage_if_dash_dash_help(applet_no, argv);
+
 		/* Finally we can call NOFORK applet's main() */
 		rc = applet_main[applet_no](argc, tmp_argv);
+
 		/* Important for shells: `which CMD` was failing */
 		fflush_all();
 	} else {