Re: [PATCH v6 04/18] 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() silently fails to cover the requested range because it relies on kvm_random_u64(), which leaves bits 31 and 63 perpetually zero.
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commit 7a6b1751be67e53b72972ac1a299690dc79f537f
Author: Josh Hilke <jrhilke@google.com>

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

Introduce kvm_random_u64_in_range(state, min, max). This function returns a
random u64 in the inclusive range of [min, max] using a struct kvm_random_state.

> 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)
[ ... ]
> +/* 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 reliably cover the requested range?

Since kvm_random_u64() constructs its return value by concatenating two
outputs from kvm_random_u32(), and kvm_random_u32() only returns values up
to 0x7FFFFFFE, bits 31 and 63 of the 64-bit output are always 0.

When this skewed 64-bit value is used to bound the output range, significant
portions of the requested range become completely unreachable. For example, in
a 32-bit range, values >= 0x80000000 will never be generated.

> +}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260610005338.2967132-1-seanjc@google.com?part=4