linux-kernel - Re: [PATCH v2] rust: check type of `$ptr` in `container

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Message-ID: <Z_0V85LF2d4EquZQ@google.com>
Date: Mon, 14 Apr 2025 14:04:35 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Tamir Duberstein <tamird@...il.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 v2] rust: check type of `$ptr` in `container_of!`

On Sat, Apr 12, 2025 at 02:16:08PM -0400, Tamir Duberstein wrote:
> +    ($field_ptr:expr, $Container:ty, $($fields:tt)*) => {{
> +        let offset: usize = ::core::mem::offset_of!($Container, $($fields)*);
> +        let container_ptr = $field_ptr.byte_sub(offset).cast::<$Container>();
> +        if false {
> +            let field_ptr = ::core::ptr::addr_of!((*container_ptr).$($fields)*).cast_mut();
> +            [$field_ptr, field_ptr]; // typeof(`$ptr_to_field`) == typeof(`$Container.$($fields)*`)

This evaluates $field_ptr twice. The `if false` avoids bad stuff if the
expression has side-effects, but still seems sub-optimal.

Alice