Mounting as <user>, a loop still assigns root ownership

Question

I am trying to set up a virtual drive from a file. This file will then be written to a flash device (not relevant). Because creating and manipulating the virtual drive will be in a script, I need to do it in user space, i.e., not as root. The script is for building and creating an image for a flash device; so, running as root will be problematic.

In order to mount the file as a virtual drive, I added the following line to /etc/fstab:

/home/user/drive.img /home/user/mnt ext4 loop,rw,user,noauto,noexec 0 0

The problem is that when I mount the virtual drive, root takes ownership of ~/mnt, defeating the purpose of mounting it as a regular user.

I know that other file systems allow you to mount while specifying the uid/gid, but the virtual drive must be ext4 to be compatible with an existing process. I tried udisksctl, but it requires root authentication for loopback.

I am going to try mounting then changing ownership (as root) but never unmount it. I will do a 'sync' then take a snapshot of the virtual drive. I do not like it because it is not clean, but it may work for now.

Answer 1

The step you haven't mentioned is how you created the ext4 filesystem, which is the source of the problem. Using mkfs.ext4 /home/user/drive.img will create a root inode owned by root, so when you mount it, it will still belong to root.

The solution is to add option -E root_owner to make it belong to the user running mkfs.ext4, or even -E root_owner=$uid:$gid for some explicit numeric user and group id. (Another solution is to use debugfs (package e2fsprogs for Fedora) to edit the inode.) This example worked for me:

uid=$(id -u)
gid=$(id -g)
rm -f /tmp/ext4fs
truncate -s 50M /tmp/ext4fs
if true
then mkfs.ext4 -E root_owner=$uid:$gid /tmp/ext4fs
else mkfs.ext4 /tmp/ext4fs
     debugfs -w -R "set_inode_field . uid $uid" /tmp/ext4fs
     debugfs -w -R "set_inode_field . gid $gid" /tmp/ext4fs
fi
# echo '/tmp/ext4fs /tmp/mymnt ext4 loop,rw,user,noauto,noexec' >>/etc/fstab
mkdir -p /tmp/mymnt
mount /tmp/ext4fs
ls -lRa /tmp/mymnt
touch /tmp/mymnt/afile
ls -l /tmp/mymnt
umount /tmp/ext4fs

On mount the ls shows the mount point as

drwxr-xr-x  3 meuh  users  1024 May 15 21:04 .

and allows me to create a file there.