Re: [syzbot] [fs?] WARNING in minix_unlink

[Suraj Sonawane <surajsonawane0215@gmail.com>]

On 11/25/24 01:40, Al Viro wrote:
> On Sun, Nov 24, 2024 at 07:47:01PM +0000, Al Viro wrote:
>> On Sun, Nov 24, 2024 at 11:41:01AM -0800, syzbot wrote:
>>> Hello,
>>>
>>> syzbot has tested the proposed patch but the reproducer is still triggering an issue:
>>> WARNING in minix_unlink
>>
>> Predictably, since the warning has nothing to do with marking an unchanged
>> buffer dirty...
>>
>> What happens there is that on a badly corrupt image we have an on-disk
>> inode with link count below the actual number of links.  And after
>> unlinks remove enough of those to drive the link count to 0, inode
>> is freed.  After that point, all remaining links are pointing to a freed
>> on-disk inode, which is discovered when they need to decrement of link
>> count that is already 0.  Which does deserve a warning, probably without
>> a stack trace.
>>
>> There's nothing the kernel can do about that, short of scanning the entire
>> filesystem at mount time and verifying that link counts are accurate...
> 
> Theoretically we could check if there's an associated dentry at the time of
> decrement-to-0 and refuse to do that decrement in such case, marking the
> in-core inode so that no extra dentries would be associated with it
> from that point on.  Not sure if that'd make for a good mitigation strategy,
> though - and it wouldn't help in case of extra links we hadn't seen by
> that point; they would become dangling pointers and reuse of on-disk inode
> would still be possible...

Thank you for the detailed explanation. I understand that the warning 
stems from corrupted filesystem metadata rather than the proposed patch. 
Thank you again for your guidance!