Re: [PATCH v3 1/3] rust: Introduce irq module

[Lyude Paul <lyude@redhat.com>]

On Thu, 2024-08-15 at 17:05 -0400, Lyude Paul wrote:
> The type system approach is slightly more complicated, but I'm now realizing
> it is probably the correct solution actually. Thanks for pointing that out!
> 
> So: Functions like wait_event_lock_interruptible_irq() work because they drop
> the spinlock in question before re-enabling interrupts, then re-disable
> interrupts and re-acquire the lock before checking the condition. This is
> where a soundness issue with my current series lies.
> 
> For the sake of explanation, let's pretend we have an imaginary rust function
> "irqs_on_and_sleep(irq: IrqDisabled<'_>)" that re-enables IRQs explicitly,
> sleeps, then turns them back on. This leads to a soundness issue if we have
> IrqDisabled be `Copy`:
> 
> with_irqs_disabled(|irq| {
>   let some_guard = some_spinlockirq.lock_with(irq);
>   // ^ Let's call this type Guard<'1, …>
> 
>   irqs_on_and_sleep(irq);
>   // ^ because `irq` is just copied here, the lifetime '1 doesn't end here.
>   // Since we re-enabled interrupts while holding a SpinLockIrq, we would
>   // potentially deadlock here.
> 
>   some_function(some_guard.some_data);
> });
> 
> So - I'm thinking we might want to make it so that IrqDisabled does not have
> `Copy` - and that resources acquired with it should share the lifetime of an
> immutable reference to it. Let's now pretend `.lock_with()` takes an &'1
> IrqDisabled, and the irqs_on_and_sleep() function from before returns an
> IrqDisabled.
> 
> with_irqs_disabled(|irq| { // <- still passed by value here
>   let some_guard = some_spinlockirq.lock_with(&irq); // <- Guard<'1, …>
> 
>   let irq = irqs_on_and_sleep(irq); // The lifetime of '1 ends here
> 
>   some_function(some_guard.some_data);
>   // Success! ^ this fails to compile, as '1 no longer lives long enough
>   // for the guard to still be usable.
>   // Deadlock averted :)
> )}
> 
> Then if we were to add bindings for things like
> wait_event_lock_interruptible_irq() - we could have those take both the
> IrqDisabled token and the Guard<'1, …> by value - and then return them
> afterwards. Which I believe would fix the soundness issue :)
> 
> How does that sound to everyone?

I should note though - after thinking about this for a moment, I realized that
there are still some issues with this. For instance: Since
with_irqs_disabled() can still be nested, a nested with_irqs_disabled() call
could create another IrqDisabled with its own lifetime - and thus we wouldn't
be able to do this same lifetime trick with any resources acquired outside the
nested call.

Granted - we -do- still have lockdep for this, so in such a situation with a
lockdep-enabled kernel we would certainly get a warning when this happens. I
think one option we might have if we wanted to go a bit further with safety
here: maybe we could do something like this:


pub fn with_irqs_disabled<T>(cb: impl for<'a> FnOnce(IrqDisabled<'a>) -> T) -> T {
  // With this function, we would assert that IRQs are not enabled at the start
  …
}

(I am a bit new to HRTBs, so the syntax here might not be right - but
hopefully you can still follow what I mean)

pub fn with_nested_irqs_disabled<T>(
  irq: impl for<'a> Option<&'a mut IrqDisabled<'a>>,
  cb: impl for<'a> FnOnce(IrqDisabled<'a>) -> T,
) -> T {
  // With this function, we would assert that IRQs are disabled 
  // if irq.is_some(), otherwise we would assert they're disabled
  // Since we require a mutable reference, this would still invalidate any 
  // borrows which rely on the previous IrqDisabled token
  …
}

Granted - I have no idea how ergonomic something like this would be since on
the C side of things: we don't really require that the user know the prior IRQ
state for things like irqsave/irqrestore functions.

> 
> > 
> > Regards,
> > Boqun
> > 
> > > > Or you're saying there could exist an `IrqDisabled<'a>` but the
> > > > interrupts are enabled?
> > > 
> > > No.
> > > 
> > > ---
> > > Cheers,
> > > Benno
> > > 
> > 
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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