Re: Can overlayfs follow mounts in lowerdir?

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On Thu, Feb 22, 2024 at 07:19:00AM +0100, Rafał Miłecki wrote:
> Hi,
> I'm trying to use overlay to create temporary virtual root filesystem.
> I need a copy of / with custom files on top of it.
> To achieve that I used a simple mount like this:
> mount -t overlay overlay -o lowerdir=/,upperdir=/tmp/ov/upper,workdir=/tmp/ov/work /tmp/ov/virtual
> In /tmp/ov/virtual/ I can see my main filesystem and I can make temporary
> changes to it. Almost perfect!
> The problem are mounts. I have some standard ones:
> proc on /proc type proc (rw,nosuid,nodev,noexec,noatime)
> sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,noatime)
> tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
> They are not visible in my virtual root:
> # ls -l /tmp/ov/proc/
> # ls -l /tmp/ov/sys/
> # ls -l /tmp/ov/tmp/
> (all empty)
> Would that be possible to make overlayfs follow such mounts in lowerdir?

No, this doesn't work:

* overlayfs does clone mounts recursively
* procfs can't be used as a lower layer

So they would need to be bind-mounted on top of these locations.

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