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

[Prithvi Tambewagh <activprithvi@gmail.com>]

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);

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?

Thank you,
Prithvi

>
>Thanks,
>Joseph
>
>> +		status = ocfs2_error(ac->ac_inode->i_sb,
>> +				     "Chain allocator dinode %llu has 0 chains\n",
>> +				     (unsigned long long)le64_to_cpu(fe->i_blkno));
>> +		goto bail;
>> +	}
>> +
>>  	victim = ocfs2_find_victim_chain(cl);
>>  	ac->ac_chain = victim;
>>
>>
>> base-commit: 939f15e640f193616691d3bcde0089760e75b0d3
>