[Virtio-fs] [virtiofsd-rs] MR closed: Implement support for remote locks

[virtiofs-bot at sinrega.org]

Locks can either be managed locally by the guest kernel, or remotely
by the virtio-fs daemon. The latter option is more expensive, but
allows those locks to be visible outside the guest, a critical feature
if multiple actors (guests or other applications running on the host)
intend to access the same files using locking semantics.

This commit implements support for remote locks, both POSIX locks and
flock, adding the command line flag "remote-locks" to allow users to
configure the desired behavior from either "disabled" (the default
option), "enabled-safe" and "enabled-unsafe".

The reason for having two different "enabled" options, one "safe" and
another "unsafe", is because some locking operations are blocking,
meaning the thread servicing the request will be kept busy until the
lock is released. Thus, a malicious application running in the guest
may starve the virtio-fs daemon by issuing a number of blocking lock
operations.

The "safe" variant allows all locking operations, except the ones that
may be blocking, returning ENOTSUP for them, while the "unsafe" one
allows all operations with no exceptions.

Signed-off-by: Sergio Lopez <slp at redhat.com>
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/4