ext4: ensure Inode flags consistency are checked at build time

Flags being used by atomic operations in inode flags (e.g.
ext4_test_inode_flag(), should be consistent with that actually stored
in inodes, i.e.: EXT4_XXX_FL.

It ensures that this consistency is checked at build-time, not at
run-time.

Currently, the flags consistency are being checked at run-time, but,
there is no real reason to not do a build-time check instead of a
run-time check. The code is comparing macro defined values with enum
type variables, where both are constants, so, there is no problem in
comparing constants at build-time.

enum variables are treated as constants by the C compiler, according
to the C99 specs (see www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf 
sec. 6.2.5, item 16), so, there is no real problem in comparing an
enumeration type at build time

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ext4.h

@@ -463,25 +463,22 @@ enum {
 	EXT4_INODE_RESERVED	= 31,	/* reserved for ext4 lib */
 };
 
-#define TEST_FLAG_VALUE(FLAG) (EXT4_##FLAG##_FL == (1 << EXT4_INODE_##FLAG))
-#define CHECK_FLAG_VALUE(FLAG) if (!TEST_FLAG_VALUE(FLAG)) { \
-	printk(KERN_EMERG "EXT4 flag fail: " #FLAG ": %d %d\n", \
-		EXT4_##FLAG##_FL, EXT4_INODE_##FLAG); BUG_ON(1); }
-
 /*
- * Since it's pretty easy to mix up bit numbers and hex values, and we
- * can't do a compile-time test for ENUM values, we use a run-time
- * test to make sure that EXT4_XXX_FL is consistent with respect to
- * EXT4_INODE_XXX.  If all is well the printk and BUG_ON will all drop
- * out so it won't cost any extra space in the compiled kernel image.
- * But it's important that these values are the same, since we are
- * using EXT4_INODE_XXX to test for the flag values, but EXT4_XX_FL
- * must be consistent with the values of FS_XXX_FL defined in
- * include/linux/fs.h and the on-disk values found in ext2, ext3, and
- * ext4 filesystems, and of course the values defined in e2fsprogs.
+ * Since it's pretty easy to mix up bit numbers and hex values, we use a
+ * build-time check to make sure that EXT4_XXX_FL is consistent with respect to
+ * EXT4_INODE_XXX. If all is well, the macros will be dropped, so, it won't cost
+ * any extra space in the compiled kernel image, otherwise, the build will fail.
+ * It's important that these values are the same, since we are using
+ * EXT4_INODE_XXX to test for flag values, but EXT4_XXX_FL must be consistent
+ * with the values of FS_XXX_FL defined in include/linux/fs.h and the on-disk
+ * values found in ext2, ext3 and ext4 filesystems, and of course the values
+ * defined in e2fsprogs.
  *
  * It's not paranoia if the Murphy's Law really *is* out to get you.  :-)
  */
+#define TEST_FLAG_VALUE(FLAG) (EXT4_##FLAG##_FL == (1 << EXT4_INODE_##FLAG))
+#define CHECK_FLAG_VALUE(FLAG) BUILD_BUG_ON(!TEST_FLAG_VALUE(FLAG))
+
 static inline void ext4_check_flag_values(void)
 {
 	CHECK_FLAG_VALUE(SECRM);

fs/ext4/super.c

@@ -5278,6 +5278,7 @@ static int __init ext4_init_fs(void)
 	ext4_li_info = NULL;
 	mutex_init(&ext4_li_mtx);
 
+	/* Build-time check for flags consistency */
 	ext4_check_flag_values();
 
 	for (i = 0; i < EXT4_WQ_HASH_SZ; i++) {