linux-kernel - Re: [RFC PATCH 0/1] ovl: brtfs' temp_fsid doesn't work with ovl index=on

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Message-ID: <20251014182414.GD13776@twin.jikos.cz>
Date: Tue, 14 Oct 2025 20:24:14 +0200
From: David Sterba <dsterba@...e.cz>
To: 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>, Anand Jain <anand.jain@...cle.com>,
	"Guilherme G . Piccoli" <gpiccoli@...lia.com>
Subject: Re: [RFC PATCH 0/1] ovl: brtfs' temp_fsid doesn't work with ovl
 index=on

On Mon, Oct 13, 2025 at 10:57:06PM -0300, André Almeida wrote:
> Hi everyone,
> 
> When using overlayfs with the mount option index=on, the first time a directory is
> used as upper dir, overlayfs stores in a xattr "overlay.origin" the UUID of the
> filesystem being used in the layers. If the upper dir is reused, overlayfs
> refuses to mount for a different filesystem, by comparing the UUID with what's
> stored at overlay.origin, and it fails with "failed to verify upper root origin"
> on dmesg. Remounting with the very same fs is supported and works fine.
> 
> However, btrfs mounts may have volatiles UUIDs. When mounting the exact same
> disk image with btrfs, a random UUID is assigned for the following disks each
> time they are mounted, stored at temp_fsid and used across the kernel as the
> disk UUID. `btrfs filesystem show` presents that. Calling statfs() however shows
> the original (and duplicated) UUID for all disks.
> 
> This feature doesn't work well with overlayfs with index=on, as when the image
> is mounted a second time, will get a different UUID and ovl will refuse to
> mount, breaking the user expectation that using the same image should work. A
> small script can be find in the end of this cover letter that illustrates this.
> 
> >From this, I can think of some options:
> 
> - Use statfs() internally to always get the fsid, that is persistent. The patch
> here illustrates that approach, but doesn't fully implement it.
> - Create a new sb op, called get_uuid() so the filesystem returns what's
> appropriated.
> - Have a workaround in ovl for btrfs.
> - Document this as unsupported, and userland needs to erase overlay.origin each
> time it wants to remount.
> - If ovl detects that temp_fsid and index are being used at the same time,
> refuses to mount.
> 
> I'm not sure which one would be better here, so I would like to hear some ideas
> on this.

I haven't looked deeper if there's a workable solution, but the feature
combination should be refused. I don't think this will affect many
users.