Source path of loop-device

Use losetup's --list option:

$ losetup --list /dev/loop0
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE
/dev/loop0         0      0         0  0 /tmp/backing-file

If you only want the file, use the -O option to pick the column:

$ losetup --list --noheadings -O BACK-FILE /dev/loop0
/tmp/backing-file

This option is part of recent versions of util-linux. Earlier versions support only the -a option, which lists all active devices in a harder-to-process format:

$ losetup -a
/dev/loop0: []: (/tmp/backing-file)

Either way, it's not overly onerous to process however you want.