From: Luis Henriques <luis.henriques@linux.dev>
To: Zhang Yi <yi.zhang@huaweicloud.com>
Cc: "Luis Henriques (SUSE)" <luis.henriques@linux.dev>,
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>
Subject: Re: [PATCH] ext4: fix infinite loop when replaying fast_commit
Date: Tue, 14 May 2024 14:04:10 +0100 [thread overview]
Message-ID: <87o798a6k5.fsf@brahms.olymp> (raw)
In-Reply-To: <2ee78957-b0a6-f346-5957-c4b2ebcea4ce@huaweicloud.com> (Zhang Yi's message of "Sat, 11 May 2024 14:24:17 +0800")
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?
Cheers,
--
Luis
>
> 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);
>>
>>
next prev parent reply other threads:[~2024-05-14 13:04 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-10 11:52 [PATCH] ext4: fix infinite loop when replaying fast_commit Luis Henriques (SUSE)
2024-05-11 6:24 ` Zhang Yi
2024-05-14 13:04 ` Luis Henriques [this message]
2024-05-15 4:59 ` Zhang Yi
2024-05-15 8:28 ` Luis Henriques
2024-05-15 8:52 ` Zhang Yi
2024-05-15 9:13 ` Luis Henriques
2024-05-15 12:24 ` Zhang Yi
2024-05-12 16:44 ` Markus Elfring
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