Re: [RFC PATCH 0/2] libxl: support common cases without block script

[Jason Andryuk <jandryuk@xxxxxxxxx>]

On Wed, Apr 28, 2021 at 3:00 AM Demi Marie Obenour
<demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> When it comes to file-based block devices, the major difficulty is
> the extremely bad kernel API.  The only fully safe way to use loop
> devices is to use LOOP_CONFIGURE with LO_FLAGS_AUTOCLEAR and hold a
> file descriptor open to the device until another piece of code (either
> another userspace program or the kernel) has grabbed a reference to it.
> Everything else risks either using a freed loop device (that might now
> be attached to a different file) or risks leaking them on unclean exit.
> The only exception is if one can make certain assumptions, such as no
> other program freeing loop devices for the file in question.  This is
> a reasonable assumption for Qubes dom0, but neither for Qubes domU nor
> for Xen dom0 in general.  Nevertheless, this is effectively what the
> current block script does: if I understand the code correctly, there
> is a race where badly timed calls to losetup by another process could
> result in the block script freeing the wrong loop device.

I posted this a while ago, but didn't get any response:

https://lore.kernel.org/xen-devel/CAKf6xpv-U91nF2Fik7GRN3SFeOWWcdR5R+ZcK5fgojE+-D43sg@xxxxxxxxxxxxxx/

tl;dr: AFAICT, the block script check_sharing function doesn't work
for loop devices

Regards,
Jason