linux-kernel - Re: [PATCH] rust: retain pointer mut-ness in `container

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Message-ID: <CAJ-ks9kEMwpDoys=RJGfWvyLdRco_Lm1984KSmju7mio5_5_WA@mail.gmail.com>
Date: Fri, 11 Apr 2025 08:25:49 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: retain pointer mut-ness in `container_of!`

On Fri, Apr 11, 2025 at 5:02 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> On Wed, Apr 09, 2025 at 10:43:16AM -0400, Tamir Duberstein wrote:
> > Avoid casting the input pointer to `*const _`, allowing the output
> > pointer to be `*mut` if the input is `*mut`. This allows a number of
> > `*const` to `*mut` conversions to be removed at the cost of slightly
> > worse ergonomics when the macro is used with a reference rather than a
> > pointer; the only example of this was in the macro's own doctest.
> >
> > Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
> > Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> > Signed-off-by: Tamir Duberstein <tamird@...il.com>
> > ---
> > This patch is extracted from 3 other series to reduce duplication.
> > ---
> >  rust/kernel/lib.rs    |  5 ++---
> >  rust/kernel/rbtree.rs | 23 ++++++++++-------------
> >  2 files changed, 12 insertions(+), 16 deletions(-)
> >
> > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> > index de07aadd1ff5..1df11156302a 100644
> > --- a/rust/kernel/lib.rs
> > +++ b/rust/kernel/lib.rs
> > @@ -190,7 +190,7 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
> >  /// }
> >  ///
> >  /// let test = Test { a: 10, b: 20 };
> > -/// let b_ptr = &test.b;
> > +/// let b_ptr: *const _ = &test.b;
> >  /// // SAFETY: The pointer points at the `b` field of a `Test`, so the resulting pointer will be
> >  /// // in-bounds of the same allocation as `b_ptr`.
> >  /// let test_alias = unsafe { container_of!(b_ptr, Test, b) };
> > @@ -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)*`?

Perhaps you could send a separate patch implementing this.

Tamir