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

[Eric Biggers]

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@xxxxxx>

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

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