Re: [PATCH v2 03/23] kcsan: Avoid checking scoped accesses from nested contexts

Boqun Feng
2021-11-29T16:43:22Z

On Mon, Nov 29, 2021 at 11:57:30AM +0100, Marco Elver wrote:
> On Mon, Nov 29, 2021 at 04:47PM +0800, Boqun Feng wrote:
> > Hi Marco,
> >
> > On Thu, Nov 18, 2021 at 09:10:07AM +0100, Marco Elver wrote:
> > > Avoid checking scoped accesses from nested contexts (such as nested
> > > interrupts or in scheduler code) which share the same kcsan_ctx.
> > >
> > > This is to avoid detecting false positive races of accesses in the same
> >
> > Could you provide an example for a false positive?
> >
> > I think we do want to detect the following race:
> >
> > static int v = SOME_VALUE; // a percpu variable.
> > static int other_v = ... ;
> >
> > void foo(..)
> > {
> > int tmp;
> > int other_tmp;
> >
> > preempt_disable();
> > {
> > ASSERT_EXCLUSIVE_ACCESSS_SCOPED(v);
> > tmp = v;
> >
> > other_tmp = other_v; // int_handler() may run here
> >
> > v = tmp + 2;
> > }
> > preempt_enabled();
> > }
> >
> > void int_handler() // an interrupt handler
> > {
> > v++;
> > }
> >
> > , if I understand correctly, we can detect this currently, but with this
> > patch, we cannot detect this if the interrupt happens while we're doing
> > the check for "other_tmp = other_v;", right? Of course, running tests
> > multiple times may eventually catch this, but I just want to understand
> > what's this patch for, thanks!
>
> The above will still be detected. Task and interrupt contexts in this
> case are distinct, i.e. kcsan_ctx differ (see get_ctx()).
>

Ok, I was missing that.

> But there are rare cases where kcsan_ctx is shared, such as nested
> interrupts (NMI?), or when entering scheduler code -- which currently
> has a KCSAN_SANITIZE := n, but I occasionally test it, which is how I
> found this problem. The problem occurs frequently when enabling KCSAN in
> kernel/sched and placing a random ASSERT_EXCLUSIVE_ACCESS_SCOPED() in
> task context, or just enable "weak memory modeling" without this fix.
> You also need CONFIG_PREEMPT=y + CONFIG_KCSAN_INTERRUPT_WATCHER=y.
>

Thanks for the background, it's now more clear that the problem is
triggered ;-)

> The emphasis here really is on _shared kcsan_ctx_, which is not too
> common. As noted in the commit description, we need to "[...] setting up
> a watchpoint for a non-scoped (normal) access that also "conflicts" with
> a current scoped access."
>
> Consider this:
>
> static int v;
> int foo(..)
> {
> ASSERT_EXCLUSIVE_ACCESS_SCOPED(v);
> v++; // preempted during watchpoint for 'v++'
> }
>
> Here we set up a scoped_access to be checked for v. Then on v++, a
> watchpoint is set up for the normal access. While the watchpoint is set
> up, the task is preempted and upon entering scheduler code, we're still
> in_task() and 'current' is still the same, thus get_ctx() returns a
> kcsan_ctx where the scoped_accesses list is non-empty containing the
> scoped access for foo()'s ASSERT_EXCLUSIVE.
>
> That means, when instrumenting scheduler code or any other code called
> by scheduler code or nested interrupts (anything where get_ctx() still
> returns the same as parent context), it'd now perform checks based on
> the parent context's scoped access, and because the parent context also
> has a watchpoint set up on the variable that conflicts with the scoped
> access we'd report a nonsensical race.
>

Agreed.

> This case is also possible:
>
> static int v;
> static int x;
> int foo(..)
> {
> ASSERT_EXCLUSIVE_ACCESS_SCOPED(v);
> x++; // preempted during watchpoint for 'v' after checking x++
> }
>
> Here, all we need is for the scoped access to be checked after x++, end
> up with a watchpoint for it, then enter scheduler code, which then
> checked 'v', sees the conflicting watchpoint, and reports a nonsensical
> race again.
>

Just to be clear, in both examples, the assumption is that 'v' is a
variable that scheduler code doesn't access, right? Because if scheduler
code does access 'v', then it's a problem that KCSAN should report. Yes,
I don't know any variable that scheduler exports, just to make sure
here.

> By disallowing scoped access checking for a kcsan_ctx, we simply make
> sure that in such nested contexts where kcsan_ctx is shared, none of
> these nonsensical races would be detected nor reported.
>
> Hopefully that clarifies what this is about.
>

Make sense to me, thanks.

Regards,
Boqun

> Thanks,
> -- Marco