anatol/booster

Handle compressed root filesystem in ISO mode

Opened this issue · 9 comments

firasuke commented

Ok, consider the case the root filesystem was compressed using squashfs or erofs into a file called rootfs.erofs.

According to this, I think booster should check whether the root filesystem is compressed/squashed or not, if it is then an additional mount command should be run:

...
mount -t erofs -o ro /dev/some_loop_device /some_mount_directory
...

So in addition to checking for /dev/sr0 and mounting it, booster should also check whether another parameter is passed that specifies the location of the root filesystem on /dev/sr0 whether it was compressed or not (for example another root= as an argument for booster and not for cmdline)..

anatol commented

Booster needs to add erofs filesystem detection.

By the chance do you have exact instructions on how to create a bootable ISO with efofs & squashfs?

firasuke commented

First of all you need to have a root file system (you can grab one of alpine's mini root file system for convenience).

It comes in .tar.gz format, so you should extract that into a directory say rootfs, and then create an erofs out of it by running:

mkfs.erofs rootfs.erofs rootfs

Now going back to our ISO structure:

- boot/ (contains `vmlinuz`, `initramfs`, `grub/` or `isolinux/` files)
- fs/ (contains rootfs.erofs) (some distros call `fs/` as `live/`)

Now an initramfs should detect we are in ISO mode and attempt to mount /dev/sr0 as iso9660 which is what booster started doing as of #223, but that is only in the case the root file system was the root layout of /dev/sr0.

In our case here, the root file system is still in erofs format and simply mounting /dev/sr0 won't suffice, as booster now needs to do another mount for the erofs file system then switch_root into that or something similar:

losetup /dev/loop0 /run/iso/fs/rootfs (or location of rootfs.erofs)
mount -t erofs -o ro /dev/loop0 /mnt/fs

Loop device number (could be based on what loop device is available, and the location /mnt/fs could be in /run or whatever you see fit, as the above is just an example for the sake of explaining this).

Now having a read-only filesystem is not always a good idea, so we may need to have a tmpfs or overlayfs mounted to simulate read-write mode until system is shutdown.

Hope that clears things up a bit.

  • 👍1
firasuke commented

Any updates to this? as I am delaying the ISO release process until this feature is added :)

anatol commented

I just merged erofs fs detection code to master.

To get the schema you described working booster needs to mount a loop device on a file. So you can specify root=/rootfs.erofs and booster will handle mounting it.

Currently booster can mount a device file only, but it will be doable to add support for loopback devices in case if root is a regular file.

Are there any initramfs that already implement erofs root filesystems? I want to check their implementation as well.

firasuke commented

To get the schema you described working booster needs to mount a loop device on a file. So you can specify root=/rootfs.erofs and booster will handle mounting it.

Exactly!

Currently booster can mount a device file only, but it will be doable to add support for loopback devices in case if root is a regular file.

Ok, cool.

Are there any initramfs that already implement erofs root filesystems? I want to check their implementation as well.

I believe only Arch's ISO scripts support EROFS. Alpine's initramfs generation utility (mkinitfs) supports SquashFS (which providing support for is almost identical to EROFS).

firasuke commented

We should also handle the case of providing read-write support when in ISO mode (via overlayfs), which is also mentioned in #88.

firasuke commented

A proposal for the whole ISO / OverlayFS / SquashFS / EROFS situation (Getting booster to support handling ISOs with read-write support):

  • The following directories need to be created by booster beforehand:
/media/cdrom # Where /dev/sr0 will be mounted
/media/rootfs/lower # Where /dev/loop0 will be mounted (Overlayfs step 1)
/media/rootfs/upper # (Overlayfs step 2)
/media/rootfs/work # (Overlayfs step 3)
/new_root # Where chroot / switch_root will take place
  • The following modules might be needed beforehand:
erofs,iso9660,loop,overlay
  • /dev/sr0 should be mounted ro as type iso9660 on /media/cdrom (according to FHS 3.0):
mount -t iso9660 -o ro /dev/sr0 /media/cdrom
  • losetup should be run (the location of the compressed rootfs should be controlled by the variable root= which has recently added to booster):
losetup /dev/loop0 /media/cdrom/boot/rootfs.erofs
  • /dev/loop0 should be mounted ro as type erofs on /media/rootfs/lower:
mount -t erofs -o ro /dev/loop0 /media/rootfs/lower
  • overlay should be mounted rw as type overlay on /new_root with the following parameters (lowerdir=/media/rootfs/lower,upperdir=/media/rootfs/upper,workdir=/media/rootfs/work):
mount -t overlay -o lowerdir=/media/rootfs/lower,upperdir=/media/rootfs/upper,workdir=/media/rootfs/work overlay /new_root
firasuke commented

Any updates to this?

firasuke commented

What's the current situation on this?