linux-kernel - Re: [PATCH v2] rust: alloc: satisfy `aligned

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Message-ID: <Z6T5sOWXxmUsflW7@pollux>
Date: Thu, 6 Feb 2025 19:04:32 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: Tamir Duberstein <tamird@...il.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 <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 v2] rust: alloc: satisfy `aligned_alloc` requirements

On Thu, Feb 06, 2025 at 06:56:38PM +0100, Miguel Ojeda wrote:
> On Sun, Feb 2, 2025 at 12:27 PM Tamir Duberstein <tamird@...il.com> wrote:
> >
> > requirements of `aligned_alloc`. These requirements may not be enforced
> > on all systems, but they are on macOS. Ensure that alignment is at least
> 
> Which requirements? `aligned_alloc` comes from ISO C, and POSIX says
> it is aligned with it; i.e. the change to make it work in macOS seems
> fine, but please see below.
> 
> > +        // 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 *).
> 
> These requirements seem to come from the macOS man pages, not the
> actual specification. The C one seems required to fail on invalid
> alignments, but is the set of those the ones that macOS mentions? (It
> seems the history of the requirements of that function is convoluted
> and involves at least a DR, and glibc is very lax, more than
> apparently its docs say)

I previously checked man posix_memalign(3) and it says:

ERRORS
	EINVAL The alignment argument was not a power of two, or was not a
	       multiple of sizeof(void *).