Help with EROFS for specific test scenario

[Gao Xiang]

Hi Fred,

On 2024/9/21 00:11, Fred Lotter wrote:
> Hi,
> 
> In the context of this page:
> https://erofs.docs.kernel.org/en/latest/merging.html <https://erofs.docs.kernel.org/en/latest/merging.html>
> 
> I am trying to experiment with EROFS where I want to try something crazy like the following setup:
> 
> /dev/mmcblk0p3:
> |
> EROFS root image
> |
> --------
> |
> EROFS second image
> |
> --------
> 
> I wanted to have a primate root EROFS filesystem written at the start of a partition. Then I would like to "append" files to the immutable root EROFS filesystem, by adding a concatenated EROFS filesystem after the root filesystem, with an external device reference pointing to the root EROFS filesystem.

Thanks for your question.

> 
> My idea in my head was then to boot the Linux kernel with something like:
> 
> root=/dev/mmcblk0p3 rootfstype=erofs rootoptions=device=/dev/mmcblk0p3,offset=<root size>
> 
> 1. Is it possible to have the "blobdevice" point to a complete EROFS filesystem?

If there are two partitions, currently EROFS kernel runtime supports
your requirement in practice, for example:

  /dev/mmcblk0p3:
  |
  EROFS root image
  |
  -------- /dev/mmcblk0p4:
  |
  EROFS second image
  |
  --------

You could boot with:
root=/dev/mmcblk0p4 rootfstype=erofs rootoptions=device=/dev/mmcblk0p3

to get the incremental filesystem and use:

root=/dev/mmcblk0p3 rootfstype=erofs

to boot the original filesystem.

The two images are both usable for booting.

The `offset` option is only supported by the loop device, and parsed by
util-linux `mount` command, as in:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/block/loop.c?h=v6.11#n52

So if you'd like to use the same partition, I guess it's still possible
as long as you have a minimal initramfs to generate a special loop
device with a specific offset, like:

losetup -ooffset=<root size> /dev/loop0 /dev/mmcblk0p3
mount -odevice=/dev/mmcblk0p3 /dev/loop0 /

for example.  I'm not sure if it meets your requirement.

In short, the on-disk format supports your requirement, and
kernel runtime supports your requirement too, but just with
two independent devices (or loop devices with `offset`).

I'm not sure EROFS modules needs to support `offset`
semantics since other filesystems don't support this too.

> 
> 2. If yes, is there userspace support for creating this setup?

Unfortunately, currently mkfs.erofs incremental build doesn't
support this mode, but it can be implemented in the next
erofs-utils 1.9 version.

Thanks,
Gao Xiang

> 
> Kind Regards,
> Fred
> 
> 
> 
>