linux-kernel - Re: [PATCH v3 2/3] ext4: fix corrupt backup group descriptors after online resize

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Date:   Mon, 21 Nov 2022 10:35:57 +0100
From:   Jan Kara <jack@...e.cz>
To:     Baokun Li <libaokun1@...wei.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, jack@...e.cz, ritesh.list@...il.com,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        yukuai3@...wei.com
Subject: Re: [PATCH v3 2/3] ext4: fix corrupt backup group descriptors after
 online resize

On Thu 17-11-22 12:03:40, Baokun Li wrote:
> In commit 9a8c5b0d0615 ("ext4: update the backup superblock's at the end
> of the online resize"), it is assumed that update_backups() only updates
> backup superblocks, so each b_data is treated as a backupsuper block to
> update its s_block_group_nr and s_checksum. However, update_backups()
> also updates the backup group descriptors, which causes the backup group
> descriptors to be corrupted.
> 
> The above commit fixes the problem of invalid checksum of the backup
> superblock. The root cause of this problem is that the checksum of
> ext4_update_super() is not set correctly. This problem has been fixed
> in the previous patch ("ext4: fix bad checksum after online resize").
> 
> However, we do need to set block_group_nr for the backup superblock in
> update_backups(). When a block is in a group that contains a backup
> superblock, and the block is the first block in the group, the block is
> definitely a superblock. We add a helper function that includes setting
> s_block_group_nr and updating checksum, and then call it only when the
> above conditions are met to prevent the backup group descriptors from
> being incorrectly modified.
> 
> Fixes: 9a8c5b0d0615 ("ext4: update the backup superblock's at the end of the online resize")
> Signed-off-by: Baokun Li <libaokun1@...wei.com>

Looks good to me now. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
> V2->V3:
>     Modify the scheme and retain s_block_group_nr.
> 
>  fs/ext4/resize.c | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> index cb99b410c9fa..66ceabbd83ad 100644
> --- a/fs/ext4/resize.c
> +++ b/fs/ext4/resize.c
> @@ -1110,6 +1110,16 @@ static int reserve_backup_gdb(handle_t *handle, struct inode *inode,
>  	return err;
>  }
>  
> +static inline void ext4_set_block_group_nr(struct super_block *sb, char *data,
> +					   ext4_group_t group)
> +{
> +	struct ext4_super_block *es = (struct ext4_super_block *) data;
> +
> +	es->s_block_group_nr = cpu_to_le16(group);
> +	if (ext4_has_metadata_csum(sb))
> +		es->s_checksum = ext4_superblock_csum(sb, es);
> +}
> +
>  /*
>   * Update the backup copies of the ext4 metadata.  These don't need to be part
>   * of the main resize transaction, because e2fsck will re-write them if there
> @@ -1158,7 +1168,8 @@ static void update_backups(struct super_block *sb, sector_t blk_off, char *data,
>  	while (group < sbi->s_groups_count) {
>  		struct buffer_head *bh;
>  		ext4_fsblk_t backup_block;
> -		struct ext4_super_block *es;
> +		int has_super = ext4_bg_has_super(sb, group);
> +		ext4_fsblk_t first_block = ext4_group_first_block_no(sb, group);
>  
>  		/* Out of journal space, and can't get more - abort - so sad */
>  		err = ext4_resize_ensure_credits_batch(handle, 1);
> @@ -1168,8 +1179,7 @@ static void update_backups(struct super_block *sb, sector_t blk_off, char *data,
>  		if (meta_bg == 0)
>  			backup_block = ((ext4_fsblk_t)group) * bpg + blk_off;
>  		else
> -			backup_block = (ext4_group_first_block_no(sb, group) +
> -					ext4_bg_has_super(sb, group));
> +			backup_block = first_block + has_super;
>  
>  		bh = sb_getblk(sb, backup_block);
>  		if (unlikely(!bh)) {
> @@ -1187,10 +1197,8 @@ static void update_backups(struct super_block *sb, sector_t blk_off, char *data,
>  		memcpy(bh->b_data, data, size);
>  		if (rest)
>  			memset(bh->b_data + size, 0, rest);
> -		es = (struct ext4_super_block *) bh->b_data;
> -		es->s_block_group_nr = cpu_to_le16(group);
> -		if (ext4_has_metadata_csum(sb))
> -			es->s_checksum = ext4_superblock_csum(sb, es);
> +		if (has_super && (backup_block == first_block))
> +			ext4_set_block_group_nr(sb, bh->b_data, group);
>  		set_buffer_uptodate(bh);
>  		unlock_buffer(bh);
>  		err = ext4_handle_dirty_metadata(handle, NULL, bh);
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR