Loopback with offset

[tsemczyszyn]

Is it possible to mount a loop device with a given offset? If so how might I do that?

[mmstick]

https://docs.rs/sys-mount/latest/sys_mount/struct.MountBuilder.html

Try passing the options to the data parameter.

[tsemczyszyn]

I tried that but no luck. It looks like the offset actually has to be specified when the loop device is being set up. From an strace on my program I see this before the mount syscall:

ioctl(8, LOOP_SET_STATUS64, {lo_device=makedev(0, 0), lo_inode=0, lo_rdevice=makedev(0, 0), lo_offset=0, lo_sizelimit=0, lo_number=0, lo_encrypt_type=LO_CRYPT_NONE, lo_encrypt_key_size=0, lo_flags=0, lo_file_name="", lo_crypt_name="", lo_encrypt_key="", lo_init=[0, 0]}) = 0

If I look at the same strace from the mount command instead of my program I see this:

ioctl(4, LOOP_SET_STATUS64, {lo_offset=0x20100000, lo_number=0, lo_flags=LO_FLAGS_AUTOCLEAR, lo_file_name="/my/disk/image.img", ...}) = 0

So my guess is that the mount command detects that offset parameter in the options and feeds it into the loop device creation.