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Subject: Re: [PATCH] ext4: fix infinite loop when replaying fast_commit
To: Luis Henriques <luis.henriques@linux.dev>
Cc: linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
 Theodore Ts'o <tytso@mit.edu>, Andreas Dilger <adilger@dilger.ca>,
 Harshad Shirwadkar <harshadshirwadkar@gmail.com>
References: <20240510115252.11850-1-luis.henriques@linux.dev>
 <2ee78957-b0a6-f346-5957-c4b2ebcea4ce@huaweicloud.com>
 <87o798a6k5.fsf@brahms.olymp>
From: Zhang Yi <yi.zhang@huaweicloud.com>
Message-ID: <a49a72d2-98aa-1c87-fc3a-58cae0f90257@huaweicloud.com>
Date: Wed, 15 May 2024 12:59:26 +0800
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On 2024/5/14 21:04, Luis Henriques wrote:
> On Sat 11 May 2024 02:24:17 PM +08, Zhang Yi wrote;
> 
>> 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@linux.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?
> 
> Thank you for your review (and sorry for my delay replying).
> 
> So, I see three different options to follow your suggestion:
> 
> 1) Initialize 'es' immediately when declaring it in function
>    ext4_ext_determine_insert_hole():
> 
> 	es.es_lblk = es.es_len = es.es_pblk = 0;
> 
> 2) Initialize 'es' only in ext4_es_find_extent_range() when checking if an
>    fc replay is in progress (my patch was already doing something like
>    that):
> 
> 	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;
> 	}
> 
> 3) Remove the check for fc replay in function ext4_es_find_extent_range(),
>    which will then unconditionally call __es_find_extent_range().  This
>    will effectively also initialize the 'es' fields to '0' and, because
>    __es_tree_search() will return NULL (at least in generic/039 test!),
>    nothing else will be done.
> 
> Since all these 3 options seem to have the same result, I believe option
> 1) is probably the best as it initializes the structure shortly after it's
> declaration.  Would you agree?  Or did I misunderstood you?
> 

Both 1 and 2 are looks fine to me, but I would prefer to initialize it
unconditionally in ext4_es_find_extent_range().

@@ -310,6 +310,8 @@ void ext4_es_find_extent_range(struct inode *inode,
				ext4_lblk_t lblk, ext4_lblk_t end,
				struct extent_status *es)
 {
+	es->es_lblk = es->es_len = es->es_pblk = 0;
+
	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
		return;

Thanks,
Yi.