linux-kernel - Re: [PATCH v3] rust: check type of `$ptr` in `container

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Message-ID: <34457c78-fdcd-4f1b-a349-4ca9bcc2febc@nvidia.com>
Date: Sun, 27 Apr 2025 15:59:48 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Tamir Duberstein <tamird@...il.com>, 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>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] rust: check type of `$ptr` in `container_of!`

On 4/23/25 10:40 AM, Tamir Duberstein wrote:
...
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 1df11156302a..d14ed86efb68 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -198,9 +198,15 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
>   /// ```
>   #[macro_export]
>   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>()
> +    ($field_ptr:expr, $Container:ty, $($fields:tt)*) => {{
> +        let offset: usize = ::core::mem::offset_of!($Container, $($fields)*);
> +        let field_ptr = $field_ptr;
> +        let container_ptr = field_ptr.byte_sub(offset).cast::<$Container>();
> +        if false {

This jumped out at me. It's something that I'd like to recommend NOT
doing, here or anywhere else, because:

     a) Anything of the form "if false" will get removed by any compiler
        worthy of the name, especially in kernel builds.

     b) It is a "magic trick", in that the code is on the face of it,
     unnecessary. So that's not something that you would pick as your
     first choice anyway. But as I see now that Miguel has also pointed
     out, the -O2 optimization level that we build at makes it either
     unreliable or broken, so it's Bad Magic. :)

Anyway, I don't know where this pattern came from, but it's not a good
one for kernel builds.


thanks,
-- 
John Hubbard