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

[Tamir Duberstein]

On Fri, Apr 11, 2025 at 9:29 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> On Fri, Apr 11, 2025 at 3:09 PM Tamir Duberstein <tamird@xxxxxxxxx> wrote:
> >
> > 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 _`?
>
> As far as I can tell, it should work with const pointers too. The rhs
> of the last assignment gets downgraded back to a const pointer in that
> case.
>
> > In any case, I hope we agree that this can be
> > done separately.
>
> Yes, since the feature does not exist today, it can happen separately.

Sent as another patch:
https://lore.kernel.org/all/20250411-b4-container-of-type-check-v1-1-08262ef67c95@xxxxxxxxx/.

Thanks for the suggestion!
Tamir