linux-kernel - Re: [PATCH] rust: alloc: satisfy `aligned

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Message-ID: <Z56Bq94kPaN9UX5H@cassiopeiae>
Date: Sat, 1 Feb 2025 21:18:51 +0100
From: Danilo Krummrich <dakr@...nel.org>
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>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: alloc: satisfy `aligned_alloc` requirements

Hi Tamir,

On Sat, Feb 01, 2025 at 01:58:10PM -0500, Tamir Duberstein wrote:
> The implementation added in commit dd09538fb409 ("rust: alloc: implement
> `Cmalloc` in module allocator_test") did not honor the documented
> requirements of `aligned_alloc`. These requirements may not be enforced
> on all system, but they are on macOS. Ensure that alignment is at least
> `sizeof(void *)` and round size up to the nearest multiple of that
> value.

Good catch!

> 
> Fixes: dd09538fb409 ("rust: alloc: implement `Cmalloc` in module allocator_test")
> 
> Signed-off-by: Tamir Duberstein <tamird@...il.com>
> ---
>  rust/kernel/alloc/allocator_test.rs | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
> index e3240d16040b..f360fc2e20f2 100644
> --- a/rust/kernel/alloc/allocator_test.rs
> +++ b/rust/kernel/alloc/allocator_test.rs
> @@ -62,9 +62,22 @@ unsafe fn realloc(
>              ));
>          }
>  
> +        // According to `man aligned_alloc`:
> +        //
> +        // aligned_alloc() returns a NULL pointer and sets errno to EINVAL if size is not an
> +        // integral multiple of alignment, or if alignment is not a power of 2 at least as large as
> +        // sizeof(void *).
> +        let alignment = layout.align();
> +        let minimum_alignment = core::mem::size_of::<*const crate::ffi::c_void>();
> +        let (alignment, size) = if alignment < minimum_alignment {
> +            (minimum_alignment, layout.size().div_ceil(minimum_alignment) * minimum_alignment)
> +        } else {
> +            (alignment, layout.size())
> +        };
> +

I think I prefer this to be slightly more compact:

   let min_align = core::mem::size_of::<*const crate::ffi::c_void>();
   let (align, size) = if layout.align() < min_align {
       (min_align, layout.size().div_ceil(min_align) * min_align)
   } else {
       (layout.align(), layout.size())
   };