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

[Alice Ryhl]

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