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ext4: ensure Inode flags consistency are checked at build time

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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>
This commit is contained in:
Carlos Maiolino 2012-12-10 16:30:45 -05:00 committed by Theodore Ts'o
parent 939da10844
commit 9a4c801947
2 changed files with 13 additions and 15 deletions
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27
fs/ext4/ext4.h
View file

@ -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);

1
fs/ext4/super.c
View file

@ -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++) {