linux-ext4 - Re: [PATCH] ext4: Ensure Inode flags consistency are checked in build time [V2]

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Message-ID: <20121206190534.GC31853@andromeda.usersys.redhat.com>
Date:	Thu, 6 Dec 2012 17:05:34 -0200
From:	Carlos Maiolino <cmaiolino@...hat.com>
To:	linux-ext4@...r.kernel.org
Cc:	sergiodj@...hat.com
Subject: Re: [PATCH] ext4: Ensure Inode flags consistency are checked in
 build time [V2]

CC'ing Sergio as I didn't when the patch was sent

On Mon, Dec 03, 2012 at 03:32:56PM -0200, Carlos Maiolino wrote:
> 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
> 
> CC'ing Sergio who helped me to achieve this conclusion, in case there is
> something else we need to discuss.
> 
> Signed-off-by: Carlos Maiolino <cmaiolino@...hat.com>
> ---
>  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++) {
> -- 
> 1.7.11.7
> 
> --
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-- 
Carlos
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