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

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Message-ID: <87bk57phel.fsf@brahms.olymp>
Date: Wed, 15 May 2024 10:13:06 +0100
From: Luis Henriques <luis.henriques@...ux.dev>
To: Zhang Yi <yi.zhang@...weicloud.com>
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 Wed 15 May 2024 04:52:54 PM +08, Zhang Yi wrote;

> 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.

Oh, talking about testing, I forgot to mention that I see the same
behaviour with generic/311.  I.e. this test also enters an infinite loop,
but fixed with this patch.

Cheers,
-- 
Luis