Re: [PATCH] KVM: x86: fix shift out of bounds reported by UBSAN

[Paolo Bonzini]

On 22/12/20 19:31, David Laight wrote:
> > 	/*
> > 	 * Use 2ULL to incorporate the necessary +1 in the shift; adding +1 in
> > 	 * the shift count will overflow SHL's max shift of 63 if s=0 and e=63.
> > 	 */
> A comment of the desired output value would be more use.
> I think it is:
> 	return 'e-s' ones followed by 's' zeros without shifting by 64.

What about a mix of the two:

	/*
	 * Return 'e-s' ones followed by 's' zeros.  Note that the
	 * apparently obvious 1ULL << (e - s + 1) can shift by 64 if
	 * s=0 and e=63, which is undefined behavior.
	 */

Paolo