Re: [PATCH] ext4: Ensure Inode flags consistency are checked in build time

["Lukáš Czerner" <lczerner@redhat.com>]

On Fri, 30 Nov 2012, Carlos Maiolino wrote:

> Date: Fri, 30 Nov 2012 14:11:26 -0200
> From: Carlos Maiolino <cmaiolino@redhat.com>
> To: linux-ext4@vger.kernel.org
> Subject: [PATCH] ext4: Ensure Inode flags consistency are checked in 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.
> 
> Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>

Hi Carlos,

I wonder why Dmitry when he wrote this noted that we can not do
compile time test on enum values. I have to admit I do not know
enough about that, but it seems to work just fine with your patch,
so can you give us some explanation in the commit description why
that is not true ?

Also if the values are not matching we get this error:

error: size of unnamed array is negative

which is not explain the problem at all, but I guess it's enough,
since it points to the variable which does not match.

Adding Dmitry to the cc.

Thanks!
-Lukas

> ---
>  fs/ext4/ext4.h  | 29 +++++++++++++----------------
>  fs/ext4/super.c |  1 +
>  2 files changed, 14 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 3c20de1..4ac0523 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -451,25 +451,22 @@ 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);
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 80928f7..e6f6f8b 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -5282,6 +5282,7 @@ 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++) {
>