fix param_sysfs_builtin name length check

Commit faf8c714 caused a regression:
parameter names longer than MAX_KBUILD_MODNAME will now be rejected,
although we just need to keep the module name part that short.  This patch
restores the old behaviour while still avoiding that memchr is called with
its length parameter larger than the total string length.
Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Cc: Dave Young <hidave.darkstar@gmail.com>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/params.c

@@ -592,19 +592,16 @@ static void __init param_sysfs_builtin(void)
 
 	for (i=0; i < __stop___param - __start___param; i++) {
 		char *dot;
-		size_t kplen;
+		size_t max_name_len;
 
 		kp = &__start___param[i];
-		kplen = strlen(kp->name);
+		max_name_len =
+			min_t(size_t, MAX_KBUILD_MODNAME, strlen(kp->name));
 
-		/* We do not handle args without periods. */
-		if (kplen > MAX_KBUILD_MODNAME) {
-			DEBUGP("kernel parameter name is too long: %s\n", kp->name);
-			continue;
-		}
-		dot = memchr(kp->name, '.', kplen);
+		dot = memchr(kp->name, '.', max_name_len);
 		if (!dot) {
-			DEBUGP("couldn't find period in %s\n", kp->name);
+			DEBUGP("couldn't find period in first %d characters "
+			       "of %s\n", MAX_KBUILD_MODNAME, kp->name);
 			continue;
 		}
 		name_len = dot - kp->name;