linux-kernel - Re: [RFC/RFT PATCH 3/6] random: Use u32 to keep track of batched entropy generation

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Message-ID: <20251127101129.204c6c5a@pumpkin>
Date: Thu, 27 Nov 2025 10:11:29 +0000
From: david laight <david.laight@...box.com>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-hardening@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>, Kees Cook
 <kees@...nel.org>, Ryan Roberts <ryan.roberts@....com>, Will Deacon
 <will@...nel.org>, Arnd Bergmann <arnd@...db.de>, Jeremy Linton
 <jeremy.linton@....com>, Catalin Marinas <Catalin.Marinas@....com>, Mark
 Rutland <mark.rutland@....com>, "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [RFC/RFT PATCH 3/6] random: Use u32 to keep track of batched
 entropy generation

On Thu, 27 Nov 2025 10:22:30 +0100
Ard Biesheuvel <ardb+git@...gle.com> wrote:

> From: Ard Biesheuvel <ardb@...nel.org>
> 
> The batched entropy containers each have a generation field, to keep
> track of the base_crng generation from which it was last reseeded.
> 
> This use case does not require all bits of the unsigned long to be
> stored: storing only 32 bits is sufficient to determine whether or not
> we're at most 4 billion generations behind, which seems ample.
> 
> So use an unsigned int instead: this will allow a future patch to treat
> the generation and position as a single 64-bit quantity, which can be
> used locklessly in a compare-and-exchange() operation.

Probably best to use a u32.
While it will always(?) be the same as 'unsigned int' it is more
descriptive.

> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  drivers/char/random.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index b8b24b6ed3fe..0e04bc60d034 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -507,7 +507,7 @@ struct batch_ ##type {								\
>  	 */									\
>  	type entropy[CHACHA_BLOCK_SIZE * 3 / (2 * sizeof(type))];		\
>  	local_lock_t lock;							\
> -	unsigned long generation;						\
> +	unsigned int generation;						\
>  	unsigned int position;							\
>  };										\
>  										\
> @@ -521,7 +521,7 @@ type get_random_ ##type(void)							\
>  	type ret;								\
>  	unsigned long flags;							\
>  	struct batch_ ##type *batch;						\
> -	unsigned long next_gen;							\
> +	unsigned int next_gen;							\
>  										\
>  	warn_unseeded_randomness();						\
>  										\
> @@ -533,7 +533,7 @@ type get_random_ ##type(void)							\
>  	local_lock_irqsave(&batched_entropy_ ##type.lock, flags);		\
>  	batch = raw_cpu_ptr(&batched_entropy_##type);				\
>  										\
> -	next_gen = READ_ONCE(base_crng.generation);				\
> +	next_gen = (unsigned int)READ_ONCE(base_crng.generation);		\

Isn't that cast pointless?

	David

>  	if (batch->position >= ARRAY_SIZE(batch->entropy) ||			\
>  	    next_gen != batch->generation) {					\
>  		_get_random_bytes(batch->entropy, sizeof(batch->entropy));	\