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

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Date: Wed, 15 May 2024 16:52:54 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: Luis Henriques <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 2024/5/15 16:28, Luis Henriques wrote:
> On Wed 15 May 2024 12:59:26 PM +08, Zhang Yi wrote;
> 
>> 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@...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?
>>>
>>> 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;
> 
> Thank you, Yi.  I'll send out v2 shortly.  Although, to be fair, the real
> patch author shouldn't be me. :-)
> 

Never mind, I just give a suggestion and also I didn't do a full test on
this change.

Thanks,
Yi.