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

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Message-ID: <CAJ-ks9mDUb2zS=9Ojx5GFnmsqKBdKerZoPL86LhLNnKndfpaag@mail.gmail.com>
Date: Tue, 11 Feb 2025 12:12:04 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Gary Guo <gary@...yguo.net>
Cc: Danilo Krummrich <dakr@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, DJ Delorie <dj@...hat.com>, 
	Eric Blake <eblake@...hat.com>, Paul Eggert <eggert@...ucla.edu>, 
	Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	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-man@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] rust: alloc: satisfy POSIX alignment requirement

On Tue, Feb 11, 2025 at 11:52 AM Gary Guo <gary@...yguo.net> wrote:
>
> On Mon, 10 Feb 2025 09:55:19 -0500
> Tamir Duberstein <tamird@...il.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@...nel.org>
> > Signed-off-by: Tamir Duberstein <tamird@...il.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();

Yep this works. Thanks Gary.