Re: [RFC PATCH 1/1] ovl: Use fsid as unique identifier for trusted origin

[Anand Jain <anajain.sg@gmail.com>]

On 21/10/25 05:43, Dave Chinner wrote:
> On Wed, Oct 15, 2025 at 07:46:34AM +0800, Anand Jain wrote:
>> On 14-Oct-25 12:39 PM, Christoph Hellwig wrote:
>>> On Mon, Oct 13, 2025 at 10:57:07PM -0300, André Almeida wrote:
>>>> Some filesystem have non-persistent UUIDs, that can change
>>>> between mounting, even if the filesystem is not modified. To
>>>> prevent false-positives when mounting overlayfs with index
>>>> enabled, use the fsid reported from statfs that is persistent
>>>> across mounts.
>>>
>>> Please fix btrfs to not change uuids, as that completely defeats
>>> the point of uuids.
>>
>> We needed cloned device mount support for an A/B testing use case,
>> but changing the on-disk UUID defeats the purpose.
>>
>> Right now, ext4 and Btrfs can mount identical devices, but XFS
>> can't.
> 
> Absolutely not true.
> 
> XFS has been able to mount filesystems with duplicate UUIDs on Linux
> for almost 25 years. The "-o nouuid" mount option (introduced in
> 2001) to bypass the duplicate uuid checks done at mount time.
> 

Damn, I completely missed the nouuid XFS option. My bad!!
> XFS tracks all mounted filesystem UUIDs largely to prevent multiple
> mounts of the same filesystem due to multipath storage presenting it
> via multiple different block devices.
 > > The nouuid mount option was added back when enterprise storage
> arrays started supporting hardware level thinp and LUN
> clone/snapshot functionality. Adding "-o nouuid" allowed cloned LUNs
> to be mounted for for backup/recovery purposes whilst the main
> filesystem was still mounted and in active use.
> 

Agree. Also, in some SAN error situations, the same device may
disappear and reappear with a new maj:min.

>> How about extending this to the common
>> VFS layer and adding a parameter to tell apart a cloned
>> device from the same device accessed through multiple
>> paths?
> 
> Perhaps we should lift the XFS UUID tracking code to the VFS
> and intercept "-o nouuid" at the VFS to allow duplicates only when
> that mount option is set?
> 
> -Dave.

It looks like XFS (with -o nouuid) and ext4 allow duplicate
FSIDs to pass into VFS. We may be able to extend this to Btrfs,
though there could be conflicts with fanotify? I'm not sure yet.
we still need -o nouuid, to alert admins to handle cases where a
device reappears with a new devt. I'm digging more.

Thanks, Anand