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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 02, 2022 at 06:54:22PM +0100, Paolo Bonzini wrote:
> On 10/31/22 21:54, Peter Zijlstra wrote:
> > > 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
> > I'm definitely in favour of updating this code; both your suggestion and
> > hpa's suggestion look like sane changes. But I do feel that whatever
> > UBSAN thing generated this warning needs to be fixed too.
> > 
> > I'm fine with the compiler warning about this code -- but it must not
> > claim undefined behaviour given the compiler flags we use.
> 
> Yes, the compiler is buggy here (see old bug report for GCC, now fixed, at
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68418).
> 
> I cannot even reproduce the problem with the simple userspace testcase
> 
> #include <stdlib.h>
> int main(int argc) {
> 	int i = argc << 31;
> 	exit(i < 0);
> }
> 
> on either GCC 12 or clang 15.

Perhaps we should have the UBSAN splat include the compiler-version
used... because clearly someone is using ancient crap here.