mkdir sysroot
dd if=/dev/urandom of=sysroot/myfile bs=1024 count=1024
virt-make-fs --format=raw --type=ext2 sysroot sysroot.ext2

$ mkdir mnt
$ # mount can deduce "-o loop -t ext2" nowadays.
$ sudo mount sysroot.ext2 mnt
$ # Mount worked fine.
$ cmp mnt/myfile sysroot/myfile
$ losetup
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE               DIO LOG-SEC
/dev/loop0         0      0         1  0 /home/ciro/sysroot.ext2   0     512
$ # I don't trust userland.
$ cat /sys/devices/virtual/block/loop0/loop/autoclear
1
$ sudo umount
$ losetp
$

$ sudo losetup /dev/loop0 sysroot.ext2
losetup: sysroot.ext2: Warning: file does not fit into a 512-byte sector; the end of the file will be ignored.
$ losetup
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE               DIO LOG-SEC
/dev/loop0         0      0         0  0 /home/ciro/sysroot.ext2   0     512
$ cat /sys/devices/virtual/block/loop0/loop/autoclear
0
$ sudo mount /dev/loop0 mnt
$ cmp mnt/myfile sysroot/myfile
$ sudo umount mnt
$ losetup
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE               DIO LOG-SEC
/dev/loop0         0      0         0  0 /home/ciro/sysroot.ext2   0     512
$ sudo mount /dev/loop0 mnt
$ sudo umount -d mnt
$ losetup

sudo mount /dev/loop0 mnt
sudo umount mnt
sudo losetup -d /dev/loop0

$ sudo losetup /dev/loop0 sysroot.ext2
$ sudo mount /dev/loop0 mnt
$ cat /sys/devices/virtual/block/loop0/loop/autoclear
0
$ sudo losetup -d /dev/loop0
$ cat /sys/devices/virtual/block/loop0/loop/autoclear
1
$ # Still mounted.
$ cmp mnt/myfile sysroot/myfile
$ sudo umount mnt
$ losetup
$

sudo strace mount sysroot.ext2 mnt

openat(AT_FDCWD, "/dev/loop0", O_RDWR|O_CLOEXEC) = 4
ioctl(4, LOOP_SET_STATUS64, {lo_offset=0, lo_number=0, lo_flags=LO_FLAGS_AUTOCLEAR, lo_file_name="/home/ciro/sysroot.ext2", ...}) = 0
mount("/dev/loop0", "/home/ciro/test/libguestfs/mnt", "ext2", MS_MGC_VAL, NULL) = 0