Re: [PATCH 2/2] src/vfs: skip whiteout-device fixtures on overlayfs

On Mon, Jun 15, 2026 at 5:33 PM Christian Brauner <[email protected]> wrote:
>
> The idmapped mount tests create a whiteout device,
> mknod(S_IFCHR, makedev(0, 0)), as a fixture: a whiteout device is the
> only character device an unprivileged caller may create, since it is
> exempt from the CAP_MKNOD check. overlayfs reserves a 0:0 character
> device as its on-disk whiteout marker, so ovl_mknod() returns -EPERM
> and the fixture cannot be created, failing generic/633, generic/696 and
> generic/697 on overlayfs.
>
> Add an is_overlayfs() helper and skip the whiteout-device fixtures and
> the checks that depend on them on overlayfs. The makedev(5, 1)
> "creation must fail" probes are unaffected and keep running. No-op on
> every other filesystem.

I have no strong objection to this patch but there was actually a request to
support makedev(0, 0) in overlayfs for creating overlayfs on an overlayfs root,
so perhaps it would be better to probe for makedev(0, 0) by fs owner instead
of the is_overlayfs() check.

Thanks,
Amir.