Re: [PATCH] rust: retain pointer mut-ness in `container_of!`

[Tamir Duberstein]

On Fri, Apr 11, 2025 at 8:38 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> On Fri, Apr 11, 2025 at 2:35 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
> >
> > On Fri Apr 11, 2025 at 2:25 PM CEST, Tamir Duberstein wrote:
> > > On Fri, Apr 11, 2025 at 5:02 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
> > >> On Wed, Apr 09, 2025 at 10:43:16AM -0400, Tamir Duberstein wrote:
> > >> > @@ -199,9 +199,8 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
> > >> >  #[macro_export]
> > >> >  macro_rules! container_of {
> > >> >      ($ptr:expr, $type:ty, $($f:tt)*) => {{
> > >> > -        let ptr = $ptr as *const _ as *const u8;
> > >> >          let offset: usize = ::core::mem::offset_of!($type, $($f)*);
> > >> > -        ptr.sub(offset) as *const $type
> > >> > +        $ptr.byte_sub(offset).cast::<$type>()
> > >> >      }}
> > >> >  }
> > >>
> > >> This implementation does not check the type of `ptr`. Would we not want
> > >> it to have the type of the field?
> > >
> > > I might be missing it but ISTM that the current implementation doesn't
> > > check that either.
> > >
> > > It's not obvious to me how you'd implement such a check; given `$ptr`
> > > and `$f`, how do you get your hands on the type of `$ptr->$($f)*`?
> >
> > I don't think it's possible with current rust, but maybe with field
> > projection (:
> >
> >     ($ptr:expr, $type:ty, $($f:tt)*) => {{
> >         // do not run this code, only use it for type-checking:
> >         let _ = || {
> >             let mut ptr = $ptr;
> >             ptr = $ptr.cast::<<field_of!($t, $($f)*) as Field>::Type>();
> >         };
> >         let offset: usize = ::core::mem::offset_of!($type, $($f)*);
> >         $ptr.byte_sub(offset).cast::<$type>()
> >     }}
>
> You can definitely implement the check with current Rust. You use
> addr_of! to create a raw pointer with the type of the field, and
> trigger a type error if `ptr` doesn't have the same type as that other
> pointer. Something along these lines would do it:
>
> let mut ptr = $ptr;
> let offset: usize = ::core::mem::offset_of!($type, $($f)*);
> let container = ptr.byte_sub(offset).cast::<$type>();
> if false {
>     ptr = ::core::ptr::addr_of!((*container).$($f)*).cast_mut();
> }
> container

It's a nice idea. Wouldn't it require `$ptr` to be `*mut _` and not
work with `*const _`? In any case, I hope we agree that this can be
done separately.