Re: [PATCH v8 06/20] 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() never sets bits 31 and 63, causing kvm_random_u64_in_range() to have severe blind spots.
--

commit 5a3dcffb3cb35a622072ea5194b7875f774cb3a7
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) to return a random u64 
within an inclusive range.

> 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);

[Severity: Medium]
Will this result in severe blind spots within the generated range?

Looking at how the random value is constructed:

tools/testing/selftests/kvm/include/test_util.h:kvm_random_u64() {
    return ((u64)kvm_random_u32(state) << 32) | kvm_random_u32(state);
}

And considering kvm_random_u32() limits its return value with a modulo of 
((u32)(1 << 31) - 1), it appears bits 31 and 63 will always remain zero.

Does this mean the subsequent modulo operation in this function:

> +
> +	range = max - min;
> +	if (range == ULLONG_MAX)
> +		return value;
> +
> +	return min + (value % (range + 1));
> +}

will fail to cover large portions of requested ranges? For example, if a 
test requests a 4GB range, could this only ever return values in the lower 
2GB of that range?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260626213534.3866178-1-seanjc@google.com?part=6