init: allow CONFIG_INIT_FALLBACK=n to disable defaults if init= fails

If a user puts init=/whatever on the command line and /whatever can't be
run, then the kernel will try a few default options before giving up.  If
init=/whatever came from a bootloader prompt, then this is unexpected but
probably harmless.  On the other hand, if it comes from a script (e.g.  a
tool like virtme or perhaps a future kselftest script), then the fallbacks
are likely to exist, but they'll do the wrong thing.  For example, they
might unexpectedly invoke systemd.

This adds a config option CONFIG_INIT_FALLBACK.  If unset, then a failure
to run the specified init= process be fatal.

The tentative plan is to remove CONFIG_INIT_FALLBACK for 3.20.

[[email protected]: coding-style fixes]
Signed-off-by: Andy Lutomirski <[email protected]>
Cc: Rob Landley <[email protected]>
Cc: Chuck Ebbert <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Frank Rowand <[email protected]>
Cc: Josh Triplett <[email protected]>
Acked-by: Rusty Russell <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

init/Kconfig

@@ -1280,6 +1280,22 @@ source "usr/Kconfig"
 
 endif
 
+config INIT_FALLBACK
+	bool "Fall back to defaults if init= parameter is bad"
+	default y
+	help
+	  If enabled, the kernel will try the default init binaries if an
+	  explicit request from the init= parameter fails.
+
+	  This can have unexpected effects.  For example, booting
+	  with init=/sbin/kiosk_app will run /sbin/init or even /bin/sh
+	  if /sbin/kiosk_app cannot be executed.
+
+	  The default value of Y is consistent with historical behavior.
+	  Selecting N is likely to be more appropriate for most uses,
+	  especially on kiosks and on kernels that are intended to be
+	  run under the control of a script.
+
 config CC_OPTIMIZE_FOR_SIZE
 	bool "Optimize for size"
 	help

init/main.c

@@ -952,8 +952,13 @@ static int __ref kernel_init(void *unused)
 		ret = run_init_process(execute_command);
 		if (!ret)
 			return 0;
+#ifndef CONFIG_INIT_FALLBACK
+		panic("Requested init %s failed (error %d).",
+		      execute_command, ret);
+#else
 		pr_err("Failed to execute %s (error %d).  Attempting defaults...\n",
-			execute_command, ret);
+		       execute_command, ret);
+#endif
 	}
 	if (!try_to_run_init_process("/sbin/init") ||
 	    !try_to_run_init_process("/etc/init") ||