Re: [PATCH] fs: ocfs2: fix kernel BUG in ocfs2_find_victim_chain

[Prithvi Tambewagh <activprithvi@gmail.com>]

On Mon, Dec 01, 2025 at 03:07:56PM +0800, Joseph Qi wrote:
>
>
>On 2025/12/1 14:24, Prithvi Tambewagh wrote:
>> On Mon, Dec 01, 2025 at 10:51:49AM +0800, Joseph Qi wrote:
>>>
>>>
>>> On 2025/11/30 18:46, Prithvi Tambewagh wrote:
>>>> syzbot reported a kernel BUG in ocfs2_find_victim_chain() because the
>>>> `cl_next_free_rec` field of the allocation chain list is 0, triggring the
>>>> BUG_ON(!cl->cl_next_free_rec) condition and panicking the kernel.
>>>>
>>>> To fix this, `cl_next_free_rec` is checked inside the caller of
>>>> ocfs2_find_victim_chain() i.e. ocfs2_claim_suballoc_bits() and if it is
>>>> equal to 0, ocfs2_error() is called, to log the corruption and force the
>>>> filesystem into read-only mode, to prevent further damage.
>>>>
>>>> Reported-by: syzbot+96d38c6e1655c1420a72@syzkaller.appspotmail.com
>>>> Closes: https://syzkaller.appspot.com/bug?extid=96d38c6e1655c1420a72
>>>> Tested-by: syzbot+96d38c6e1655c1420a72@syzkaller.appspotmail.com
>>>> Cc: stable@vger.kernel.org
>>>> Signed-off-by: Prithvi Tambewagh <activprithvi@gmail.com>
>>>> ---
>>>>  fs/ocfs2/suballoc.c | 7 +++++++
>>>>  1 file changed, 7 insertions(+)
>>>>
>>>> diff --git a/fs/ocfs2/suballoc.c b/fs/ocfs2/suballoc.c
>>>> index 6ac4dcd54588..84bb2d11c2aa 100644
>>>> --- a/fs/ocfs2/suballoc.c
>>>> +++ b/fs/ocfs2/suballoc.c
>>>> @@ -1993,6 +1993,13 @@ static int ocfs2_claim_suballoc_bits(struct ocfs2_alloc_context *ac,
>>>>
>>>>      cl = (struct ocfs2_chain_list *) &fe->id2.i_chain;
>>>>
>>>
>>> This blank line can be eliminated.
>>>
>>>> +    if (le16_to_cpu(cl->cl_next_free_rec) == 0) {
>>>
>>> Better to add the upper limit check as well. e.g.
>>>
>>> !le16_to_cpu(cl->cl_next_free_rec) ||
>>> le16_to_cpu(cl->cl_next_free_rec) > le16_to_cpu(cl->cl_count)
>>
>> Hello Joseph,
>>
>> I went through the code in fs/ocfs2/suballoc.c, like this function
>> static inline u16 ocfs2_find_smallest_chain(struct ocfs2_chain_list *cl)
>> {
>>     u16 curr, best;
>>
>>     best = curr = 0;
>>     while (curr < le16_to_cpu(cl->cl_count)) {
>>         if (le32_to_cpu(cl->cl_recs[best].c_total) >
>>             le32_to_cpu(cl->cl_recs[curr].c_total))
>>             best = curr;
>>         curr++;
>>     }
>>     return best;
>> }
>>
>> and in function ocfs2_block_group_alloc() these lines
>> if (le16_to_cpu(cl->cl_next_free_rec) < le16_to_cpu(cl->cl_count))
>>     le16_add_cpu(&cl->cl_next_free_rec, 1);
>>
>After this, cl_next_free_rec may equal to cl_count.
>
>
>> and observed that according to the architecture of ocfs2, the chain list is in the form of 0-indexed array. In that case, the change you suggested for upper limit, could be re-written as
>> le16_to_cpu(cl->cl_next_free_rec) >= le16_to_cpu(cl->cl_count)
>>
>> since value of cl->cl_next_free_rec greater than or equal to cl->cl_count will indicate that there are no available chains. Can you please review this?
>>
>Yes, it's full. But 'cl_next_free_rec == cl_count' is a designed behavior, see mkfs or fsck.

I get it. We are trying to catch a state of disk corruption, so your 
suggestion

le16_to_cpu(cl->cl_next_free_rec) > le16_to_cpu(cl->cl_count)

fits best here. Thanks...I will make v2 for the patch.

Best Reards,
Prithvi

>
>Joseph
>