linux-ext4 - Re: [PATCH] ext4: check s_chksum_driver when looking for bg csum presence

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Date:	Tue, 14 Oct 2014 01:23:21 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>,
	Theodore Ts'o <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: check s_chksum_driver when looking for bg csum presence

"Darrick J. Wong" <darrick.wong@...cle.com> writes:

> Convert the ext4_has_group_desc_csum predicate to look for a checksum
> driver instead of the metadata_csum flag and change the bg checksum
> calculation function to look for GDT_CSUM before taking the crc16
> path.
>
> Without this patch, if we mount with ^uninit_bg,^metadata_csum and
> later metadata_csum gets turned on by accident, the block group
> checksum functions will incorrectly assume that checksumming is
> enabled (metadata_csum) but that crc16 should be used
> (!s_chksum_driver).  This is totally wrong, so fix the predicate
> and the checksum formula selection.
>
> (Granted, if the metadata_csum feature bit gets enabled on a live FS
> then something underhanded is going on, but we could at least avoid
> writing garbage into the on-disk fields.)
>
Agree. feel free to add 
Ack-by: Dmitry Monakhov <dmonakhov@...nvz.org>
> Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> Cc: Dmitry Monakhov <dmonakhov@...nvz.org>
> ---
>  fs/ext4/ext4.h  |    4 ++--
>  fs/ext4/super.c |    4 ++++
>  2 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index fb6aadf..db38ca0f 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -2367,8 +2367,8 @@ extern int ext4_register_li_request(struct super_block *sb,
>  static inline int ext4_has_group_desc_csum(struct super_block *sb)
>  {
>  	return EXT4_HAS_RO_COMPAT_FEATURE(sb,
> -					  EXT4_FEATURE_RO_COMPAT_GDT_CSUM |
> -					  EXT4_FEATURE_RO_COMPAT_METADATA_CSUM);
> +					  EXT4_FEATURE_RO_COMPAT_GDT_CSUM) ||
> +	       (EXT4_SB(sb)->s_chksum_driver != NULL);
>  }
>  
>  static inline int ext4_has_metadata_csum(struct super_block *sb)
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 6ae47d4..de38a96 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -2032,6 +2032,10 @@ static __le16 ext4_group_desc_csum(struct ext4_sb_info *sbi, __u32 block_group,
>  	}
>  
>  	/* old crc16 code */
> +	if (!(sbi->s_es->s_feature_ro_compat &
> +	      cpu_to_le32(EXT4_FEATURE_RO_COMPAT_GDT_CSUM)))
> +		return 0;
> +
>  	offset = offsetof(struct ext4_group_desc, bg_checksum);
>  
>  	crc = crc16(~0, sbi->s_es->s_uuid, sizeof(sbi->s_es->s_uuid));

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