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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx > --- > 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 >
