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

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Message-ID: <xn4j14nq63.fsf@greed.delorie.com>
Date: Sat, 08 Feb 2025 15:21:24 -0500
From: DJ Delorie <dj@...hat.com>
To: Alejandro Colomar <alx@...nel.org>
Cc: miguel.ojeda.sandonis@...il.com, eblake@...hat.com, will.newton@...aro.org,
        eggert@...ucla.edu, linux-man@...r.kernel.org, dakr@...nel.org,
        tamird@...il.com, ojeda@...nel.org, alex.gaynor@...il.com,
        boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
        benno.lossin@...ton.me, a.hindborg@...nel.org, aliceryhl@...gle.com,
        tmgross@...ch.edu, rust-for-linux@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rust: alloc: satisfy `aligned_alloc` requirements

Alejandro Colomar <alx@...nel.org> writes:
> -  aligned_alloc()
>	It makes sure that the input is a power of two, or it fails.
>
> -  posix_memalign()
>
> 	. . . It requires that the input is power of
> 	two,
>
> I wonder why glibc silently overaligns aligned_alloc() without reporting
> an error for an alignment of 2, while it reports an error for an
> alignment of 3.  It doesn't make much sense at first glance.  No
> standard seems to require that, so it looks like an arbitrary choice.

Because 2 is a power of two, but 3 isn't.  No power of two is a multiple
of 3.

GNU malloc only supports alignments that are powers of two.  The
resulting addresses might *happen* to be multiples of other numbers, but
you cannot request that.

As for why, ask Posix:

"If the value of alignment is not a valid alignment supported by the
 implementation, a null pointer shall be returned."
 [EINVAL]
    The value of alignment is not a valid alignment supported by the
    implementation."