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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <CAOQ4uxhrQQmK+tc+eOjm7Pz2u=S6_2cnneyo4mNjVgyA7RNooA@mail.gmail.com>
Date: Wed, 15 Oct 2025 13:09:26 +0200
From: Amir Goldstein <amir73il@...il.com>
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>, 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 Tue, Oct 14, 2025 at 3:57 AM André Almeida <andrealmeid@...lia.com> 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.

FWIW this operation already exists in export_operations.
It is currently only used by pnfs and only implemented by xfs.
I would nor object for overlayfs to use this method if implemented
and fall back to copying uuid directly from s_uuid
(better yet make it a vfs helper)
Note that commit
8f720d9f892e0 xfs: publish UUID in struct super_block
was done for a similar reason.
The xfs mount option nouuid is the poor man's solution for
mounting cloned disk images.

Thanks,
Amir.