On Fri, Jun 14, 2024 at 03:09:22PM GMT, Hongbo Li wrote: > Hi everyone ! > > How can I create new file in idmapped mountpoint in ext4? > > I try to do the following test: > ``` > losetup /dev/loop1 ext4.img > mkfs.ext4 /dev/loop1 > mount /dev/loop1 /mnt/ext4 > ./mount-idmapped --map-mount b:0:1001:1 /mnt/ext4 /mnt/idmapped1 > cp testfile /mnt/idmapped1 > ``` > then it rebacks me: > ``` > cp: cannot create regular file '/mnt/idmapped1/testfile': Value too large > for defined data type > ``` > Did I use it incorrectly? You're setting up a mount in which uid 1001 maps to uid 0. So if you create files as uid 1001 they will map to uid 0 on-disk. But you haven't mapped uid 0 to anything so the mount doesn't allow the root user to create files. In order to that you could e.g., do: sudo mount --bind -o X-mount.idmap='0:1001:1 1001:0:1' /mnt/ex4/ /mnt/idmapped1/ If you now (as root) do cp testfile /mnt/idmapped1/ it will work and files created by root will show up as being owned by uid 1001 on disk.
