linux-kernel - Re: [PATCH v2 1/3] vdso: Switch get/put unaligned from packed struct to memcpy

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Message-ID: <20250620143425.6d654bf6@pumpkin>
Date: Fri, 20 Jun 2025 14:34:25 +0100
From: David Laight <david.laight.linux@...il.com>
To: Ian Rogers <irogers@...gle.com>
Cc: Eric Biggers <ebiggers@...gle.com>, Yuzhuo Jing <yuzhuo@...gle.com>,
 Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Vincenzo Frascino <vincenzo.frascino@....com>, Arnaldo Carvalho de Melo
 <acme@...hat.com>, Al Viro <viro@...iv.linux.org.uk>, Christophe Leroy
 <christophe.leroy@...roup.eu>, "Jason A. Donenfeld" <Jason@...c4.com>,
 linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2 1/3] vdso: Switch get/put unaligned from packed
 struct to memcpy

On Tue, 17 Jun 2025 13:53:18 -0700
Ian Rogers <irogers@...gle.com> wrote:

> Type punning is necessary for get/put unaligned but the use of a
> packed struct violates strict aliasing rules, requiring
> -fno-strict-aliasing to be passed to the C compiler. Switch to using
> memcpy so that -fno-strict-aliasing isn't necessary.
>....							\
> +/**
> + * __put_unaligned_t - write an unaligned value to memory.
> + * @type:	the type of the value to store.
> + * @val:	the value to store.
> + * @ptr:	the pointer to store to.
> + *
> + * Use memcpy to affect an unaligned type sized store avoiding undefined
> + * behavior from approaches like type punning that require -fno-strict-aliasing
> + * in order to be correct. The void* cast silences ubsan warnings.
> + */
> +#define __put_unaligned_t(type, val, ptr) do {				\
> +	type __put_unaligned_val = (val);				\
> +	__builtin_memcpy((void *)(ptr), &__put_unaligned_val,		\
> +			 sizeof(__put_unaligned_val));			\
>  } while (0)

Does that actually work?
If 'ptr' has type 'long *' gcc will (validly according to C) assume
it is aligned and will generate a misaligned memory write.
The (void *) cast make no difference.

Using (void *)(long)(ptr) might lose the alignment.
Otherwise you may need to use OPTIMISER_HIDE_VAR() and live
with the extra register-register move it tends to generate.

	David