[PATCH 26/27] random: factor out a __limit_random_u32_below helper

[David Laight]

On Wed, 11 Mar 2026 15:29:35 -0700
Eric Biggers <ebiggers at kernel.org> wrote:

> On Wed, Mar 11, 2026 at 08:03:58AM +0100, Christoph Hellwig wrote:
> > Factor out the guts of __get_random_u32_below into a new helper,
> > so that callers with their own prng state can reuse this code.
> > 
> > Signed-off-by: Christoph Hellwig <hch at lst.de>  
> 
> I think I'd prefer that the test just uses the mod operation instead,
> like many of the existing tests do:
> 
>     prandom_u32_state(&rng) % ceil

Or possibly what the old code used:
	(prandom_u32_state(&rnd) * (u64)ceil) >> 32

Which distributes the values evenly across the range although
some values happen 1 more time than others.
I suspect that is good enough for a lot of the users of the cryptographic
random number generator as well.

	David

> 
> Yes, when ceil isn't a power of 2 the result isn't uniformly
> distributed.  But that's perfectly fine for these tests, especially with
> the values of ceil being used being far smaller than U32_MAX.
> 
> There's been an effort to keep the cryptographic random number generator
> (drivers/char/random.c and include/linux/random.h) separate from the
> non-cryptographic random number generator (lib/random32.c and
> include/linux/prandom.h).  This patch feels like it's going in a
> slightly wrong direction, where random.c gains a function that's used
> with both cryptographic and non-cryptographic random numbers.
> 
> And if someone actually needs a fully unform distribution, then they'd
> probably want cryptographic random numbers as well.
> 
> So I'm not sure the proposed combination of "fully uniform
> non-cryptographic random numbers" makes much sense.
> 
> Plus the '% ceil' implementation is much easier to understand.
> 
> - Eric
>