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

[Sean Christopherson <seanjc@google.com>]

On Thu, Jun 04, 2026, sashiko-bot@kernel.org wrote:
> > @@ -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.

Not sure how much I trust AI's ability to reason about math, given past "bug"
reports.  Regardless, this is a problem to solve in the future.