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

[Qu Wenruo <wqu@suse.com>]

在 2025/10/14 12:27, André Almeida 写道:
> 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.

Yep, that's the btrfs' hack to allowing mounting cloned devices (as long 
as they are all single-device only btrfs)

Although I'm not a huge fan for that, without that you can not even 
mount any cloned btrfs in the first place.

> 
> 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.

Or, let btrfs to reject the cloned device in the first place.

> 
> I'm not sure which one would be better here, so I would like to hear some ideas
> on this.
> 
> Thanks!
> 	André
> 
> ---
> 
> To reproduce:
> 
> mkdir -p dir1 dir2
> 
> fallocate -l 300m ./disk1.img
> mkfs.btrfs -q -f ./disk1.img
> 
> # cloning the disks
> cp disk1.img disk2.img

If you really want to use the same copied fs, at least you can use
`btrfstune -m disk2.img` to change it to a new metadata uuid (without 
re-writing all metadata).

Then everything should work.

Thanks,
Qu
> sudo mount -o loop ./disk1.img dir1
> sudo mount -o loop ./disk2.img dir2
> 
> mkdir -p dir2/lower aux/upper aux/work
> 
> # this works
> sudo mount -t overlay -o lowerdir=dir2/lower,upperdir=aux/upper,workdir=aux/work,userxattr none dir2/lower
> 
> sudo umount dir2/lower
> sudo umount dir2
> 
> sudo mount -o loop ./disk2.img dir2
> 
> # this doesn't works
> sudo mount -t overlay -o lowerdir=dir2/lower,upperdir=aux/upper,workdir=aux/work,userxattr none dir2/lower
> 
> André Almeida (1):
>    ovl: Use fsid as unique identifier for trusted origin
> 
>   fs/overlayfs/copy_up.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
>