linux-kernel - Re: [PATCH v6] rust: alloc: satisfy POSIX alignment requirement

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Message-ID: <CAJ-ks9mQG5pyEP+cnVxBkCDbfR_cecBZK0K456=Qw6Y03LuRvg@mail.gmail.com>
Date: Thu, 13 Feb 2025 06:33:58 -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 v6] rust: alloc: satisfy POSIX alignment requirement

On Thu, Feb 13, 2025 at 6:30 AM Gary Guo <gary@...yguo.net> wrote:
>
> On Thu, 13 Feb 2025 06:26:41 -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.
> >
> > Acked-by: Danilo Krummrich <dakr@...nel.org>
> > Fixes: dd09538fb409 ("rust: alloc: implement `Cmalloc` in module allocator_test")
> > Signed-off-by: Tamir Duberstein <tamird@...il.com>
> > ---
> > Changes in v6:
> > - Replace unsound use of build_error with map_err. (Danilo Krummrich)
>
> It's sound, just not correct.

Changed. I asked a tangential question in the last thread but: do you
think it's possible to make build_error work correctly on the host?

> > - Link to v5: https://lore.kernel.org/r/20250212-aligned-alloc-v5-1-c51e0b17dee9@gmail.com
> >
> > Changes in v5:
> > - Remove errant newline in commit message. (Miguel Ojeda)
> > - Use more succinct expression. (Gary Guo)
> > - Drop and then add Danilo's Acked-by again.
> > - Link to v4: https://lore.kernel.org/r/20250210-aligned-alloc-v4-1-609c3a6fe139@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 | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> >
> > diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
> > index e3240d16040b..e68775078e90 100644
> > --- a/rust/kernel/alloc/allocator_test.rs
> > +++ b/rust/kernel/alloc/allocator_test.rs
> > @@ -62,6 +62,24 @@ 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 layout = layout.align_to(min_align).map_err(|_| AllocError)?.pad_to_align();
> > +        let layout = layout.pad_to_align();
>
> You're doing two `pad_to_align`s.

What I get for going back and forth about 10 times. Thanks, Gary.

>
> 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;
> >
> > ---
> > base-commit: 8a5aae7dbbfb612509c8a2f112f7e0f79029ed45
> > change-id: 20250201-aligned-alloc-b52cb2353c82
> >
> > Best regards,
>