Re: [PATCH RFC 01/18] rust: drm: ioctl: Add DRM ioctl abstraction

Karol Herbst
2023-03-09T20:40:48Z

On Thu, Mar 9, 2023 at 9:24 PM Faith Ekstrand
<faith.ekstrand@collabora.com> wrote:
>
> On Thu, 2023-03-09 at 15:04 +0900, Asahi Lina wrote:
> > On 08/03/2023 02.34, Björn Roy Baron wrote:
> > > > + // SAFETY: This is just the ioctl
> > > > argument, which hopefully has the right type
> > > > + // (we've done our best checking the
> > > > size).
> > >
> > > In the rust tree there is the ReadableFromBytes [1] trait which
> > > indicates that it is safe to read arbitrary bytes into the type.
> > > Maybe you could add it as bound on the argument type when it lands
> > > in rust-next? This way you can't end up with for example a struct
> > > containing a bool with the byte value 2, which is UB.
> >
> > There's actually a much bigger story here, because that trait isn't
> > really very useful without a way to auto-derive it. I need the same
> > kind
> > of guarantee for all the GPU firmware structs...
> >
> > There's one using only declarative macros [1] and one using proc
> > macros
> > [2]. And then, since ioctl arguments are declared in C UAPI header
> > files, we need a way to be able to derive those traits for them...
> > which
> > I guess means bindgen changes?
>
> It'd be cool to be able to auto-verify that uAPI structs are all
> tightly packed and use the right subset of types. Maybe not possible
> this iteration but it'd be cool to see in future. I'd like to see it
> for C as well, ideally.
>
> ~Faith
>

I'm sure that with a macro you could verify that a struct definition
doesn't contain any gaps, just not sure on how one would enforce that.
Could add a trait which can only be implemented through a proc_macro?
Maybe we can have a proc_macro ensuring no gaps? Would be cool tech to
have indeed.