[PATCH] Compile-time check re world-writeable module params

One of the mistakes a module_param() user can make is to supply default
value of module parameter as the last argument.  module_param() accepts
permissions instead.  If default value is, say, 3 (-------wx), parameter
becomes world-writeable.

So far, the only remedy was to apply grep(1) and read drivers submitted
to -mm. BTDT.

With this patch applied, compiler will finally do some job.

*) bounds checking on permissions
*) world-writeable bit checking on permissions
*) compile breakage if checks trigger

First version of this check (only "& 2" part) directly caught 4 out of 7
places during my last grep.

    Subject: Neverending module_param() bugs
    [X] drivers/acpi/sbs.c:101:module_param(capacity_mode, int, CAPACITY_UNIT);
    [X] drivers/acpi/sbs.c:102:module_param(update_mode, int, UPDATE_MODE);
    [ ] drivers/acpi/sbs.c:103:module_param(update_info_mode, int, UPDATE_INFO_MODE);
    [ ] drivers/acpi/sbs.c:104:module_param(update_time, int, UPDATE_TIME);
    [ ] drivers/acpi/sbs.c:105:module_param(update_time2, int, UPDATE_TIME2);
    [X] drivers/char/watchdog/sbc8360.c:203:module_param(timeout, int, 27);
    [X] drivers/media/video/tuner-simple.c:13:module_param(offset, int, 0666);
Signed-off-by: NAlexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

include/linux/moduleparam.h

@@ -63,6 +63,9 @@ struct kparam_array
    not there, read bits mean it's readable, write bits mean it's
    writable. */
 #define __module_param_call(prefix, name, set, get, arg, perm)		\
+	/* Default value instead of permissions? */			\
+	static int __param_perm_check_##name __attribute__((unused)) =	\
+	BUILD_BUG_ON_ZERO((perm) < 0 || (perm) > 0777 || ((perm) & 2));	\
 	static char __param_str_##name[] = prefix #name;		\
 	static struct kernel_param const __param_##name			\
 	__attribute_used__						\