mkfs.ubifs problem when output file really isn't in the UBIFS root directory

[Marcus Prebble]

Hi,

We are working on a project using UBI/UBIFS and I have run into a
problem with mkfs.ubifs which I am hoping someone can advise on.
Apologies if I have already missed the answer to this in the archives
somewhere.

A requirement of the project is to take an image of the root filesystem
(/) which is implemented as a static UBI read only volume. Obviously its
not possible to unmount /, but it seems to be possible to re-mount the
UBI volume, read only, onto another location e.g. /tmp/mnt/new_rootfs.

The problem arises when running mkfs.ubifs is that it fails saying that
the output file cannot be in the UBIFS root directory.
This sounds like a reasonable constraint if the output file really was
in the root, but this is not the case for us seeing root is mounted
somewhere belowtmp. To give a concrete example of the command we are
running:

mkfs.ubifs --output=/tmp/rootfs.img  --leb-size=129024
--root=/tmp/rootfs_mnt --min-io-size=2048 --max-leb-cnt=147 

Where /dev/ubi0_11 is mounted read-only on both / and /tmp/rootfs_mnt/.

Having a look at the source for mkfs.ubifs it seems that the function
in_path(..) is the problem - it works up the directory tree using ../
until it either reaches the top_dir (/) or the inode number/dev match in
which case it fails. It doesn't seem that this section of code has
changed in the latest version of mtd-utils (we are running 1.4.3-2) and
I wonder if this is a bug in in_path(..) or a genuine safeguard against
screwing something up? I suspect the former for when I disabled this
check in mkfs.ubifs.c the resulting image was completely OK.

Kind regards,

Marcus Prebble
Axis Communications AB