Re: [PATCH v5 04/21] KVM: selftests: Add helper to generate random u64 in range [min,max]

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] kvm_random_u64_in_range() cannot generate values in the upper half of large ranges and introduces severe modulo bias due to underlying PRNG limitations.
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commit f54822168923546305cbe06e95c821b08928d279
Author: Josh Hilke <jrhilke@google.com>

KVM: selftests: Add helper to generate random u64 in range [min,max]

This commit introduces kvm_random_u64_in_range() to generate a random 64-bit
integer within an inclusive range using a kvm_random_state structure.

> diff --git a/tools/testing/selftests/kvm/lib/test_util.c b/tools/testing/selftests/kvm/lib/test_util.c
> index e98ca7ef439c4..e208a57f190cf 100644
> --- a/tools/testing/selftests/kvm/lib/test_util.c
> +++ b/tools/testing/selftests/kvm/lib/test_util.c

[ ... ]

> @@ -42,6 +42,24 @@ u32 kvm_random_u32(struct kvm_random_state *state)
>  	return state->seed;
>  }
>  
> +/* Returns a random u64 in the inclusive range [min, max] */
> +u64 kvm_random_u64_in_range(struct kvm_random_state *state, u64 min,
> +			    u64 max)
> +{
> +	u64 value;
> +	u64 range;
> +
> +	TEST_ASSERT(min <= max, "PEBKAC, min = 0x%lx, max = 0x%lx", min, max);
> +
> +	value = kvm_random_u64(state);
> +
> +	range = max - min;
> +	if (range == ULLONG_MAX)
> +		return value;
> +
> +	return min + (value % (range + 1));

[Severity: Medium]
Will this return values uniformly across the entire 64-bit range?

Because kvm_random_u32() uses a Lehmer PRNG with a modulus of 2^31 - 1, its
maximum return value is 0x7FFFFFFE. When kvm_random_u64() concatenates two
of these values, the maximum possible output is 0x7FFFFFFE7FFFFFFE, meaning
bits 31 and 63 are never set.

Due to this restricted PRNG space, using the modulo operator here means the
function cannot generate values in the upper half of large ranges (above
0x7FFFFFFE7FFFFFFE). It also introduces significant modulo bias for ranges
that don't evenly divide this artificial maximum, preventing uniform coverage
of the requested [min, max] range.

> +}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260604020143.748245-1-jrhilke@google.com?part=4