linux-ext4 - Re: [PATCH] ext4: fix infinite loop when replaying fast

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Date: Sat, 11 May 2024 14:24:17 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: "Luis Henriques (SUSE)" <luis.henriques@...ux.dev>
Cc: linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
 Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger@...ger.ca>,
 Harshad Shirwadkar <harshadshirwadkar@...il.com>
Subject: Re: [PATCH] ext4: fix infinite loop when replaying fast_commit

On 2024/5/10 19:52, Luis Henriques (SUSE) wrote:
> When doing fast_commit replay an infinite loop may occur due to an
> uninitialized extent_status struct.  ext4_ext_determine_insert_hole() does
> not detect the replay and calls ext4_es_find_extent_range(), which will
> return immediately without initializing the 'es' variable.
> 
> Because 'es' contains garbage, an integer overflow may happen causing an
> infinite loop in this function, easily reproducible using fstest generic/039.
> 
> This commit fixes this issue by detecting the replay in function
> ext4_ext_determine_insert_hole().  It also adds initialization code to the
> error path in function ext4_es_find_extent_range().
> 
> Thanks to Zhang Yi, for figuring out the real problem!
> 
> Fixes: 8016e29f4362 ("ext4: fast commit recovery path")
> Signed-off-by: Luis Henriques (SUSE) <luis.henriques@...ux.dev>
> ---
> Hi!
> 
> Two comments:
> 1) The change in ext4_ext_map_blocks() could probably use the min_not_zero
>    macro instead.  I decided not to do so simply because I wasn't sure if
>    that would be safe, but I'm fine changing that if you think it is.
> 
> 2) I thought about returning 'EXT_MAX_BLOCKS' instead of '0' in
>    ext4_lblk_t ext4_ext_determine_insert_hole(), which would then avoid
>    the extra change to ext4_ext_map_blocks().  '0' sounds like the right
>    value to return, but I'm also OK using 'EXT_MAX_BLOCKS' instead.
> 
> And again thanks to Zhang Yi for pointing me the *real* problem!
> 
>  fs/ext4/extents.c        | 6 +++++-
>  fs/ext4/extents_status.c | 5 ++++-
>  2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index e57054bdc5fd..b5bfcb6c18a0 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -4052,6 +4052,9 @@ static ext4_lblk_t ext4_ext_determine_insert_hole(struct inode *inode,
>  	ext4_lblk_t hole_start, len;
>  	struct extent_status es;
>  
> +	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
> +		return 0;
> +

Sorry, I think it's may not correct. When replaying the jouranl, although
we don't use the extent statue tree, we still need to query the accurate
hole length, e.g. please see skip_hole(). If you do this, the hole length
becomes incorrect, right?

Thanks,
Yi.

>  	hole_start = lblk;
>  	len = ext4_ext_find_hole(inode, path, &hole_start);
>  again:
> @@ -4226,7 +4229,8 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
>  		len = ext4_ext_determine_insert_hole(inode, path, map->m_lblk);
>  
>  		map->m_pblk = 0;
> -		map->m_len = min_t(unsigned int, map->m_len, len);
> +		if (len > 0)
> +			map->m_len = min_t(unsigned int, map->m_len, len);
>  		goto out;
>  	}
>  
> diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
> index 4a00e2f019d9..acb9616ca119 100644
> --- a/fs/ext4/extents_status.c
> +++ b/fs/ext4/extents_status.c
> @@ -310,8 +310,11 @@ void ext4_es_find_extent_range(struct inode *inode,
>  			       ext4_lblk_t lblk, ext4_lblk_t end,
>  			       struct extent_status *es)
>  {
> -	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
> +	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY) {
> +		/* Initialize extent to zero */
> +		es->es_lblk = es->es_len = es->es_pblk = 0;
>  		return;
> +	}
>  
>  	trace_ext4_es_find_extent_range_enter(inode, lblk);
>  
>