Re: [PATCH v4] rust: alloc: satisfy POSIX alignment requirement

[Gary Guo <gary@garyguo.net>]

On Mon, 10 Feb 2025 09:55:19 -0500
Tamir Duberstein <tamird@gmail.com> wrote:

> ISO C's `aligned_alloc` is partially implementation-defined; on some
> systems it inherits stricter requirements from POSIX's `posix_memalign`.
> 
> This causes the call added in commit dd09538fb409 ("rust: alloc:
> implement `Cmalloc` in module allocator_test") to fail on macOS because
> it doesn't meet the requirements of `posix_memalign`.
> 
> Adjust the call to meet the POSIX requirement and add a comment. This fixes
> failures in `make rusttest` on macOS.
> 
> Fixes: dd09538fb409 ("rust: alloc: implement `Cmalloc` in module allocator_test")
> 
> Acked-by: Danilo Krummrich <dakr@kernel.org>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>
> ---
> Changes in v4:
> - Revert to `aligned_alloc` and correct rationale. (Miguel Ojeda)
> - Apply Danilo's Acked-by from v2.
> - Rebase on v6.14-rc2.
> - Link to v3: https://lore.kernel.org/r/20250206-aligned-alloc-v3-1-0cbc0ab0306d@gmail.com
> 
> Changes in v3:
> - Replace `aligned_alloc` with `posix_memalign` for portability.
> - Link to v2: https://lore.kernel.org/r/20250202-aligned-alloc-v2-1-5af0b5fdd46f@gmail.com
> 
> Changes in v2:
> - Shorten some variable names. (Danilo Krummrich)
> - Replace shadowing alignment variable with a second call to
>   Layout::align. (Danilo Krummrich)
> - Link to v1: https://lore.kernel.org/r/20250201-aligned-alloc-v1-1-c99a73f3cbd4@gmail.com
> ---
>  rust/kernel/alloc/allocator_test.rs | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
> index e3240d16040b..1c881ed73d79 100644
> --- a/rust/kernel/alloc/allocator_test.rs
> +++ b/rust/kernel/alloc/allocator_test.rs
> @@ -62,9 +62,30 @@ unsafe fn realloc(
>              ));
>          }
>  
> +        // ISO C (ISO/IEC 9899:2011) defines `aligned_alloc`:
> +        //
> +        // > The value of alignment shall be a valid alignment supported by the implementation
> +        // [...].
> +        //
> +        // As an example of the "supported by the implementation" requirement, POSIX.1-2001 (IEEE
> +        // 1003.1-2001) defines `posix_memalign`:
> +        //
> +        // > The value of alignment shall be a power of two multiple of sizeof (void *).
> +        //
> +        // and POSIX-based implementations of `aligned_alloc` inherit this requirement. At the time
> +        // of writing, this is known to be the case on macOS (but not in glibc).
> +        //
> +        // Satisfy the stricter requirement to avoid spurious test failures on some platforms.
> +        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())
> +        };

I think this can be more concisely expressed as

	let layout = layout.align_to(min_align)?.pad_to_align();

Best,
Gary

> +
>          // SAFETY: Returns either NULL or a pointer to a memory allocation that satisfies or
>          // exceeds the given size and alignment requirements.
> -        let dst = unsafe { libc_aligned_alloc(layout.align(), layout.size()) } as *mut u8;
> +        let dst = unsafe { libc_aligned_alloc(align, size) } as *mut u8;
>          let dst = NonNull::new(dst).ok_or(AllocError)?;
>  
>          if flags.contains(__GFP_ZERO) {
> 
> ---
> base-commit: a64dcfb451e254085a7daee5fe51bf22959d52d3
> change-id: 20250201-aligned-alloc-b52cb2353c82
> 
> Best regards,