[REGRESSION] erofs: new file-backed stacking limit breaks container overlay use case in 6.12.63

[Alekséi Naidénov]

Hello,

I am reporting a regression in the 6.12 stable series related to EROFS
file-backed mounts.

After updating from Linux 6.12.62 to 6.12.63, a previously working setup
using OSTree-backed composefs mounts as Podman rootfs no longer works.

The regression appears to be caused by the following commit:

  34447aeedbaea8f9aad3da5b07030a1c0e124639 ("erofs: limit the level of fs
stacking for file-backed mounts")
  (backport of upstream commit d53cd891f0e4311889349fff3a784dc552f814b9)

## Setup description

We use OSTree to materialize filesystem trees, which are mounted via
composefs (EROFS + overlayfs) as a read-only filesystem. This mounted
composefs tree is then used as a Podman rootfs, with Podman mounting a
writable overlayfs on top for each container.

This setup worked correctly on Linux 6.12.62 and earlier.

In short, the stacking looks like:

  EROFS (file-backed)
    -> composefs (EROFS + overlayfs with ostree repo as datadir, read-only)
        -> Podman rootfs overlays (RW upperdir)

There is no recursive or self-stacking of EROFS.

## Working case (6.12.62)

The composefs mount exists and Podman can successfully start a container
using it as rootfs.

Example composefs mount:

    ❯ mount | grep
a31550cc69eef0e3227fa700623250592711fdfd51b5403a74288b55e89e7e8c
    a31550cc69eef0e3227fa700623250592711fdfd51b5403a74288b55e89e7e8c on
/home/growler/.local/share/containers/ostree/a31550cc69eef0e3227fa700623250592711fdfd51b5403a74288b55e89e7e8c
type overlay
(ro,noatime,lowerdir+=/proc/self/fd/10,datadir+=/proc/self/fd/7,redirect_dir=on,metacopy=on)

(lowedir is a handle for the erofs file-backed mount, datadir is a handle
for the ostree repository objects directory)

Running Podman:

  ❯ podman run --rm -it --rootfs
$HOME/.local/share/containers/ostree/a31550cc69eef0e3227fa700623250592711fdfd51b5403a74288b55e89e7e8c:O
bash -l
  root at d691e785bba3:/# uname -a
  Linux d691e785bba3 6.12.62 #1-NixOS SMP PREEMPT_DYNAMIC Fri Dec 12
17:37:22 UTC 2025 x86_64 GNU/Linux
  root at d691e785bba3:/#

(succeed)

## Failing case (6.12.63)

After upgrading to 6.12.63, the same command fails when Podman tries to
create the writable overlay on top of the composefs mount.

Error:

    ❯ podman run --rm -it --rootfs
$HOME/.local/share/containers/ostree/a31550cc69eef0e3227fa700623250592711fdfd51b5403a74288b55e89e7e8c:O
bash -l
    Error: rootfs-overlay: creating overlay failed
"/home/growler/.local/share/containers/ostree/a31550cc69eef0e3227fa700623250592711fdfd51b5403a74288b55e89e7e8c"
from native overlay: mount
overlay:/home/growler/.local/share/containers/storage/overlay-containers/a0851294d6b5b18062d4f5316032ee84d7bae700ea7d12c5be949d9e1999b0a1/rootfs/merge,
flags: 0x4, data:
lowerdir=/home/growler/.local/share/containers/ostree/a31550cc69eef0e3227fa700623250592711fdfd51b5403a74288b55e89e7e8c,upperdir=/home/growler/.local/share/containers/storage/overlay-containers/a0851294d6b5b18062d4f5316032ee84d7bae700ea7d12c5be949d9e1999b0a1/rootfs/upper,workdir=/home/growler/.local/share/containers/storage/overlay-containers/a0851294d6b5b18062d4f5316032ee84d7bae700ea7d12c5be949d9e1999b0a1/rootfs/work,userxattr:
invalid argument
    ❯ uname -a
    Linux ci-node-09 6.12.63 #1-NixOS SMP PREEMPT_DYNAMIC Thu Dec 18
12:55:23 UTC 2025 x86_64 GNU/Linux

## Expected behavior

Using a composefs (EROFS + overlayfs) read-only mount as the lowerdir for a
container rootfs overlay should continue to work as it did in 6.12.62.

## Actual behavior

Overlayfs mounting fails with EINVAL when stacking on top of the composefs
mount backed by EROFS.

## Notes

The setup does not involve recursive EROFS mounting or unbounded stacking
depth. It appears the new stacking limit rejects this valid and previously
supported container use case.

Please let me know if further details or testing would be helpful.

Thank you,
-- 
 Alekséi Nadénov
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