linux-kernel - Re: [PATCH v3 2/3] prandom: Convert prandom_u32_state() to __always

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Message-ID: <aW4G0PBEwx1RjbOl@J2N7QTR9R3>
Date: Mon, 19 Jan 2026 10:26:24 +0000
From: Mark Rutland <mark.rutland@....com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Huacai Chen <chenhuacai@...nel.org>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Paul Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Kees Cook <kees@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	"Jason A. Donenfeld" <Jason@...c4.com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Jeremy Linton <jeremy.linton@....com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
	linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
	linux-s390@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3 2/3] prandom: Convert prandom_u32_state() to
 __always_inline

On Fri, Jan 02, 2026 at 01:11:53PM +0000, Ryan Roberts wrote:
> We will shortly use prandom_u32_state() to implement kstack offset
> randomization and some arches need to call it from non-instrumentable
> context. Given the function is just a handful of operations and doesn't
> call out to any other functions, let's take the easy path and make it
> __always_inline.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>

I see there were some comments about keeping an out-of-line wrapper.
With or without that, this looks good to me, and either way:

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.

> ---
>  include/linux/prandom.h | 19 ++++++++++++++++++-
>  lib/random32.c          | 19 -------------------
>  2 files changed, 18 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/prandom.h b/include/linux/prandom.h
> index ff7dcc3fa105..e797b3709f5c 100644
> --- a/include/linux/prandom.h
> +++ b/include/linux/prandom.h
> @@ -17,7 +17,24 @@ struct rnd_state {
>  	__u32 s1, s2, s3, s4;
>  };
>  
> -u32 prandom_u32_state(struct rnd_state *state);
> +/**
> + * prandom_u32_state - seeded pseudo-random number generator.
> + * @state: pointer to state structure holding seeded state.
> + *
> + * This is used for pseudo-randomness with no outside seeding.
> + * For more random results, use get_random_u32().
> + */
> +static __always_inline u32 prandom_u32_state(struct rnd_state *state)
> +{
> +#define TAUSWORTHE(s, a, b, c, d) ((s & c) << d) ^ (((s << a) ^ s) >> b)
> +	state->s1 = TAUSWORTHE(state->s1,  6U, 13U, 4294967294U, 18U);
> +	state->s2 = TAUSWORTHE(state->s2,  2U, 27U, 4294967288U,  2U);
> +	state->s3 = TAUSWORTHE(state->s3, 13U, 21U, 4294967280U,  7U);
> +	state->s4 = TAUSWORTHE(state->s4,  3U, 12U, 4294967168U, 13U);
> +
> +	return (state->s1 ^ state->s2 ^ state->s3 ^ state->s4);
> +}
> +
>  void prandom_bytes_state(struct rnd_state *state, void *buf, size_t nbytes);
>  void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state);
>  
> diff --git a/lib/random32.c b/lib/random32.c
> index 24e7acd9343f..d57baf489d4a 100644
> --- a/lib/random32.c
> +++ b/lib/random32.c
> @@ -42,25 +42,6 @@
>  #include <linux/slab.h>
>  #include <linux/unaligned.h>
>  
> -/**
> - *	prandom_u32_state - seeded pseudo-random number generator.
> - *	@state: pointer to state structure holding seeded state.
> - *
> - *	This is used for pseudo-randomness with no outside seeding.
> - *	For more random results, use get_random_u32().
> - */
> -u32 prandom_u32_state(struct rnd_state *state)
> -{
> -#define TAUSWORTHE(s, a, b, c, d) ((s & c) << d) ^ (((s << a) ^ s) >> b)
> -	state->s1 = TAUSWORTHE(state->s1,  6U, 13U, 4294967294U, 18U);
> -	state->s2 = TAUSWORTHE(state->s2,  2U, 27U, 4294967288U,  2U);
> -	state->s3 = TAUSWORTHE(state->s3, 13U, 21U, 4294967280U,  7U);
> -	state->s4 = TAUSWORTHE(state->s4,  3U, 12U, 4294967168U, 13U);
> -
> -	return (state->s1 ^ state->s2 ^ state->s3 ^ state->s4);
> -}
> -EXPORT_SYMBOL(prandom_u32_state);
> -
>  /**
>   *	prandom_bytes_state - get the requested number of pseudo-random bytes
>   *
> -- 
> 2.43.0
>