Re: [PATCH] KVM: x86: fix undefined behavior in bit shift for __feature_bit

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 31, 2022, Gaosheng Cui wrote:
> Shifting signed 32-bit value by 31 bits is undefined, so changing
> significant bit to unsigned. The UBSAN warning calltrace like below:
> 
> UBSAN: shift-out-of-bounds in arch/x86/kvm/reverse_cpuid.h:101:11
> left shift of 1 by 31 places cannot be represented in type 'int'

PeterZ is contending that this isn't actually undefined behavior given how the
kernel is compiled[*].  That said, I would be in favor of replacing the open-coded
shift with BIT() to make the code a bit more self-documenting, and that would
naturally fix this maybe-undefined-behavior issue. 

[*] https://lore.kernel.org/all/Y1%2FAaJOcgIc%2FINtv@hirez.programming.kicks-ass.net

> ---
>  arch/x86/kvm/reverse_cpuid.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
> index a19d473d0184..ebd6b621d3b8 100644
> --- a/arch/x86/kvm/reverse_cpuid.h
> +++ b/arch/x86/kvm/reverse_cpuid.h
> @@ -98,7 +98,7 @@ static __always_inline u32 __feature_bit(int x86_feature)
>  	x86_feature = __feature_translate(x86_feature);
>  
>  	reverse_cpuid_check(x86_feature / 32);
> -	return 1 << (x86_feature & 31);
> +	return 1U << (x86_feature & 31);
>  }
>  
>  #define feature_bit(name)  __feature_bit(X86_FEATURE_##name)
> -- 
> 2.25.1
>