linux-kernel - Re: [PATCH] rust: check type of `$ptr` in `container

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Message-ID: <D944ADIR8SAC.1CRNV5FJ99TFD@proton.me>
Date: Fri, 11 Apr 2025 21:11:59 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Tamir Duberstein <tamird@...il.com>, 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>, 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: check type of `$ptr` in `container_of!`

On Fri Apr 11, 2025 at 5:41 PM CEST, Tamir Duberstein wrote:
> On Fri, Apr 11, 2025 at 10:36 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>>
>> On Fri, Apr 11, 2025 at 4:31 PM Tamir Duberstein <tamird@...il.com> wrote:
>> >
>> > Add a compile-time check that `*$ptr` is of the type of `$type->$($f)*`.
>> >
>> > Suggested-by: Alice Ryhl <aliceryhl@...gle.com>
>> > Link: https://lore.kernel.org/all/CAH5fLgh6gmqGBhPMi2SKn7mCmMWfOSiS0WP5wBuGPYh9ZTAiww@mail.gmail.com/
>> > Signed-off-by: Tamir Duberstein <tamird@...il.com>
>> > ---
>> >  rust/kernel/lib.rs | 5 ++++-
>> >  1 file changed, 4 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
>> > index 1df11156302a..da9e36aa7967 100644
>> > --- a/rust/kernel/lib.rs
>> > +++ b/rust/kernel/lib.rs
>> > @@ -200,7 +200,10 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
>> >  macro_rules! container_of {
>> >      ($ptr:expr, $type:ty, $($f:tt)*) => {{
>> >          let offset: usize = ::core::mem::offset_of!($type, $($f)*);
>> > -        $ptr.byte_sub(offset).cast::<$type>()
>> > +        let container = $ptr.byte_sub(offset).cast::<$type>();
>> > +        fn assert_same_type<T>(_: T, _: T) {}
>> > +        assert_same_type($ptr, ::core::mem::addr_of!((*container).$($f)*).cast_mut());
>
> I noticed I accidentally sent `::core::mem::addr_of` instead of
> `::core::ptr::addr_of`; will fix once we agree below.
>
>> Perhaps it would be better to wrap the type check in an `if false` to
>> avoid evaluating the expressions at runtime?
>
> It's optimized out at O1: https://godbolt.org/z/44Go5xnWr. Is it worth it?

Wrapping in `if false` definitely doesn't hurt, since we get better
debug perf.

---
Cheers,
Benno