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

david laight
2025-11-27T10:11:45Z

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

> From: Ard Biesheuvel <ardb@kernel.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@kernel.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)); \