Re: [RFC PATCH 1/9] ntsync: Introduce the ntsync driver and character device.

Elizabeth Figura
2024-01-24T22:56:27Z

On Wednesday, 24 January 2024 15:26:15 CST Andy Lutomirski wrote:
> On Tue, Jan 23, 2024 at 4:59 PM Elizabeth Figura
> <zfigura@codeweavers.com> wrote:
> >
> > ntsync uses a misc device as the simplest and least intrusive uAPI interface.
> >
> > Each file description on the device represents an isolated NT instance, intended
> > to correspond to a single NT virtual machine.
>
> If I understand this text right, and if I understood the code right,
> you're saying that each open instance of the device represents an
> entire universe of NT synchronization objects, and no security or
> isolation is possible between those objects. For single-process use,
> this seems fine. But fork() will be a bit odd (although NT doesn't
> really believe in fork, so maybe this is fine).
>
> Except that NT has *named* semaphores and such. And I'm pretty sure
> I've written GUI programs that use named synchronization objects (IIRC
> they were events, and this was a *very* common pattern, regularly
> discussed in MSDN, usenet, etc) to detect whether another instance of
> the program is running. And this all works on real Windows because
> sessions have sufficiently separated namespaces, and the security all
> works out about as any other security on Windows, etc. But
> implementing *that* on top of this
> file-description-plus-integer-equals-object will be fundamentally
> quite subject to one buggy program completely clobbering someone
> else's state.
>
> Would it make sense and scale appropriately for an NT synchronization
> *object* to be a Linux open file description? Then SCM_RIGHTS could
> pass them around, an RPC server could manage *named* objects, and
> they'd generally work just like other "Object Manager" objects like,
> say, files.

It's a sensible concern. I think when I discussed this with Alexandre
Julliard (the Wine maintainer, CC'd) the conclusion was this wasn't
something we were concerned about.

While the current model *does* allow for processes to arbitrarily mess
with each other, accidentally or not, I think we're not concerned with
the scope of that than we are about implementing a whole scheduler in
user space.

For one, you can't corrupt the wineserver state this way—wineserver
being sort of like a dedicated process that handles many of the things
that a kernel would, and so sometimes needs to set or reset events, or
perform NTSYNC_IOC_KILL_MUTEX, but never relies on ntsync object state.
Whereas trying to implement a scheduler in user space would involve the
wineserver taking locks, and hence other processes could deadlock.

For two, it's probably a lot harder to mess with that internal state
accidentally.

[There is also a potential problem where some broken applications
create a million (literally) sync objects. Making these into files runs
into NOFILE. We did specifically push distributions and systemd to
increase those limits because an older solution *did* use eventfds and
*did* run into those limits. Since that push was successful I don't
know if this is *actually* a concern anymore, but avoiding files is
probably not a bad thing either.]

--Zeb