linux-kernel - Re: [PATCH] rust: mm: Mark VmaNew as transparent

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Message-Id: <20250820162927.34201cfb395ec7319b15920a@linux-foundation.org>
Date: Wed, 20 Aug 2025 16:29:27 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Baptiste Lepers <baptiste.lepers@...il.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>, Lorenzo Stoakes
 <lorenzo.stoakes@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.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
 <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross
 <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, Jann Horn
 <jannh@...gle.com>, linux-mm@...ck.org, rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: mm: Mark VmaNew as transparent

On Tue, 12 Aug 2025 15:26:56 +0200 Baptiste Lepers <baptiste.lepers@...il.com> wrote:

> Unsafe code in VmaNew's methods assumes that the type has the same
> layout as the inner `bindings::vm_area_struct`. This is not guaranteed by
> the default struct representation in Rust, but requires specifying the
> `transparent` representation.
> 
> ...
>
> +++ b/rust/kernel/mm/virt.rs
> @@ -209,6 +209,7 @@ pub fn vm_insert_page(&self, address: usize, page: &Page) -> Result {
>  ///
>  /// For the duration of 'a, the referenced vma must be undergoing initialization in an
>  /// `f_ops->mmap()` hook.
> +#[repr(transparent)]
>  pub struct VmaNew {
>      vma: VmaRef,
>  }

Alice suggests that I add a cc:stable to this.  But I see nothing in
the changelog which explains why we're proposing a backport.

So please send us a description of the userspace-visible runtime
impact of this flaw and I'll paste it into the changelog, thanks.