Re: [syzbot] [mm?] KCSAN: data-race in mprotect_fixup / try_to_migrate_one

[Lorenzo Stoakes]

On Wed, Feb 05, 2025 at 05:25:16PM +0100, Marco Elver wrote:
> On Wed, 5 Feb 2025 at 16:51, Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> wrote:
> [...]
> > > [...]
> > > > I hate that we have these landmines waiting for us. Be good to find a way
> > > > to explicitly annotate this, or at least comment somehow.
> > > >
> > > > But agreed, probably adding a READ_ONCE()/WRITE_ONCE() is appropriate at
> > > > least for the proximate thing.
> > > >
> > > > It's a wonder these things don't trigger more, except you need probably
> > > > very precise timing to do it...
> > >
> > > They do trigger, but we don't send all of them to LKML.
> > > When we first introduced KCSAN, the notion of "data race" was still
> > > poorly understood. At the time we decided to pre-review a number of
> > > them (but our time to do so has been going down :-/), or let willing
> > > maintainers deal with them directly. A number of articles followed,
> >
> > We very much appreciate your efforts :)
> >
> > We are definitely willing to see these in mm, and as you can see from the
> > discussion here, the interaction between the rmap locks and other locks is
> > complicated (see also the docs I wrote on them at [0]).
>
> Tangentially, I've been trying to work out how to bring this [1] Clang
> feature to the kernel: it's more or less a simple "capability system"
> [2] to express "acquire this before doing that / don't hold this thing
> here / etc.". Locking rules are an obvious application. It's been on a
> number of people's radar over the years, but nothing materialized.
> Sparse's locking analysis is much weaker, nor easy (i.e. quick) to
> use.
>
> [1] https://clang.llvm.org/docs/ThreadSafetyAnalysis.html
> [2] https://www.cs.cornell.edu/talc/papers/capabilities.pdf
>
> The current work-in-progress is here:
> https://git.kernel.org/pub/scm/linux/kernel/git/melver/linux.git/log/?h=cap-analysis
> It lacks documentation, and proper commit messages, but is otherwise
> usable (see example enablements for kfence, kcov, and stackdepot and
> lib/test_capability-analysis.c).
> An official RFC will follow, but the hard part of writing
> documentation is in the works. ;-)
>
> There are also other questions, such as:  can a subset of the analysis
> be applied tree-wide (vs. current selective enablement), as it would
> help find more bugs faster.
> However, the reality of it is that using this system would be opting
> into a "dialect of C with capability analysis" with its own set of
> restrictions, and I don't know if everyone is willing to pay this
> cost.
>
> What I'd be curious about is, if some of the complex rules you mention
> above can be expressed so that Clang's "capability analysis" can point
> out some bugs. I suspect not everything can be expressed, but even if
> we get 50% there, we could catch a huge amount of bugs statically at
> compile-time.
>
> I let this cat out the bag, because this thread seems like a good way
> to get super-early high-level feedback. :-)
> It'll be a while before the first RFC.

By all means, let cats out of the bag, I am a cat enthusiast and prefer
them loose about the hoose :P

It's interesting, and I'm a big believer in declarative means of expressing
attributes like this such a preconditions. I mean we have some sparse stuff
for that scattered around, as well as lockdep obviously, so it's not
something that'd be crazily out of line.

It'd be nice to have something to say 'when we access the VMA this is the
VMA under an rmap lock', perhaps a means of casting a VMA to an equivalent
type or tagging that somehow.

I'm not sure if some semantic means of saying 'we must have the rmap lock
or a VMA lock here' is useful, as this is implicit with _any_ VMA access,
and perhaps anything that'd ultimately be all that useful would need more
complicated semantics.

It'd be good to explicitly pick up on a lack of READ_ONCE()/WRITE_ONCE() in
circumstances where an rmap lock could be involved, but again I'm not sure
if we could express it this way.

This is one I need to go off and think about, but I really appreciate the
suggestion and I for one at least am very open-minded to any tooling
-whatsoever- that can help us document, express, assert on and manage this
complicated locking logic.


>
> Thanks,
> -- Marco