Re: [PATCH v13 6/7] random: introduce generic vDSO getrandom() implementation

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Dec 21, 2022 at 03:23:26PM +0100, Jason A. Donenfeld wrote:
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 6425f5f838e0..660cd15b6228 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -60,6 +60,7 @@
>  #include <crypto/blake2s.h>
>  #ifdef CONFIG_VDSO_GETRANDOM
>  #include <vdso/getrandom.h>
> +#include <vdso/datapage.h>
>  #endif
>  #include <asm/archrandom.h>
>  #include <asm/processor.h>
> @@ -407,6 +408,9 @@ static void crng_reseed(struct work_struct *work)
>	/*
>	 * We copy the new key into the base_crng, overwriting the old one,
>	 * and update the generation counter. We avoid hitting ULONG_MAX,
>	 * because the per-cpu crngs are initialized to ULONG_MAX, so this
>	 * forces new CPUs that come online to always initialize.
>	 */
>	spin_lock_irqsave(&base_crng.lock, flags);
>	memcpy(base_crng.key, key, sizeof(base_crng.key));
>	next_gen = base_crng.generation + 1;
>  	if (next_gen == ULONG_MAX)
>  		++next_gen;
>  	WRITE_ONCE(base_crng.generation, next_gen);
> +#ifdef CONFIG_VDSO_GETRANDOM
> +	smp_store_release(&_vdso_rng_data.generation, next_gen + 1);
> +#endif

It's confusing that "uninitialized generation" is ULONG_MAX in the per-cpu
crngs, but 0 in the vdso_rng_data.  That results in a weird off-by one thing,
where the vdso_rng_data generation number has to be 1 higher.

Would it be possible to use 0 for both?

- Eric