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.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Rob Landley <rob@landley.net>
Cc: Chuck Ebbert <cebbert.lkml@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Shuah Khan <shuah.kh@samsung.com>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 22 insertions, 1 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 7e9fbd48e2ab..9afb971497f4 100644
--- a/init/Kconfig
+++ b/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
diff --git a/init/main.c b/init/main.c
index d2e4ead4891f..ca380ec685de 100644
--- a/init/main.c
+++ b/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") ||