Racy loop device reuse logic

[Jan Kara <jack@suse.cz>]

Hello,

Tetsuo has been doing some changes to the loop device shutdown in the
kernel and that broke LTP that is doing essentially the following loop:

while :; do mount -o loop,ro isofs.iso isofs/; umount isofs/; done

And this loop is broken because of a subtle interaction with systemd-udev
that also opens the loop device. The race seems to be in mount(8) handling
itself and the altered kernel timing makes it happen. It look like:

bash					systemd-udev
  mount -o loop,ro isofs.iso isofs/
    /dev/loop0 is created and bound to isofs.iso, autoclear is set for
    loop0
  					opens /dev/loop0
  umount isofs/
  loop0 still lives because systemd-udev still has device open
  mount -o loop,ro isofs.iso isofs/
    gets to mnt_context_setup_loopdev()
      loopcxt_find_overlap()
      sees loop0 is still valid and with proper parameters
      reuse = true;
					close /dev/loop0
					  last fd closed => loop0 is
					    cleaned up
      loopcxt_get_fd()
        opens loop0 but it is no longer the device we wanted!
    calls mount(2) which fails because we cannot read from the loop device

It seems to me that mnt_context_setup_loopdev() should actually recheck
that loop device parameters still match what we need after opening
/dev/loop0 (if LOOP_GET_STATUS ioctl succeeds on the fd, you are guaranteed
the loop device is in that state and will not be torn down under your
hands). What do you think?

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR