For squashfs all needed functionality for idmapped mounts is already
implemented by the generic handlers in the VFS. Thus, it is sufficient
to just enable the corresponding FS_ALLOW_IDMAP flag to support
idmapped mounts.

We use this for unprivileged (user namespaced) containers based on
squashfs images as rootfs in GyroidOS.

A simple test using the mount-idmapped tool executed as user with
uid=1000 looks as follows:

$ mkdir test
$ echo "test" > test/test_file
$ mksquashfs test/ fs.img
$ sudo mkdir /mnt/test
$ sudo mkdir /mnt/mapped
$ sudo mount fs.img -o loop /mnt/test/
$ sudo ./mount-idmapped --map-mount b:1000:2000:1 /mnt/test/ /mnt/mapped/

$ mount | tail -n2
fs.img on /mnt/test type squashfs (ro,relatime,errors=continue)
fs.img on /mnt/mapped type squashfs (ro,relatime,idmapped,errors=continue)

$ ls -lan /mnt/test/
total 5
drwxr-xr-x 2 1000 1000   32 Okt 24 13:36 .
drwxr-xr-x 6    0    0 4096 Okt 24 13:38 ..
-rw-r--r-- 1 1000 1000    5 Okt 24 13:36 test_file

$ ls -lan /mnt/mapped/
total 5
drwxr-xr-x 2 2000 2000   32 Okt 24 13:36 .
drwxr-xr-x 6    0    0 4096 Okt 24 13:38 ..
-rw-r--r-- 1 2000 2000    5 Okt 24 13:36 test_file
Signed-off-by: Michael Weiß <michael.weiss@aisec.fraunhofer.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>