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

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Message-ID: <5137ce36-c3b4-4a0a-83af-e00892feaf43@gmail.com>
Date: Wed, 15 Oct 2025 07:46:34 +0800
From: Anand Jain <anajain.sg@...il.com>
To: Christoph Hellwig <hch@...radead.org>,
 André Almeida <andrealmeid@...lia.com>
Cc: linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
 linux-unionfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 kernel-dev@...lia.com, Miklos Szeredi <miklos@...redi.hu>,
 Amir Goldstein <amir73il@...il.com>, Chris Mason <clm@...com>,
 David Sterba <dsterba@...e.com>, "Guilherme G . Piccoli"
 <gpiccoli@...lia.com>
Subject: Re: [RFC PATCH 1/1] ovl: Use fsid as unique identifier for trusted
 origin

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. 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? I haven't looked into the details yet, but I can
dig it further.