Re: [XEN PATCH 9/9] xen/compat: address Rule 10.1 for macros CHECK_SIZE

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

On 10/10/2023 18:00, Andrew Cooper wrote:
> On 10/10/2023 9:02 am, Stefano Stabellini wrote:
> > On Fri, 6 Oct 2023, Nicola Vetrini wrote:
> > > The essential type of the result of an inequality operator is
> > > essentially boolean, therefore it shouldn't be used as an argument of
> > > the multiplication operator, which expects an integer.
> > >
> > > Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
> > > ---
> > >  xen/include/xen/compat.h | 10 ++++++----
> > >  1 file changed, 6 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/xen/include/xen/compat.h b/xen/include/xen/compat.h
> > > index f2ce5bb3580a..5ffee6a9fed1 100644
> > > --- a/xen/include/xen/compat.h
> > > +++ b/xen/include/xen/compat.h
> > > @@ -151,12 +151,14 @@ CHECK_NAME_(k, n, T)(k xen_ ## n *x, \
> > >      return x == c; \
> > >  }
> > >
> > > +#define SIZE_NEQUAL(a, b) \
> > > +    (sizeof(a) != sizeof(b) ? 1 : 0)
> > >  #define CHECK_SIZE(name) \
> > > -    typedef int CHECK_NAME(name, S)[1 - (sizeof(xen_ ## name ## _t) 
> > > != \
> > > -                                         sizeof(compat_ ## name ## 
> > > _t)) * 2]
> > > +    typedef int CHECK_NAME(name, S)[1 - (SIZE_NEQUAL(xen_ ## name ## 
> > > _t, \
> > > +                                                     compat_ ## name 
> > > ## _t)) * 2]
> > >  #define CHECK_SIZE_(k, n) \
> > > -    typedef int CHECK_NAME_(k, n, S)[1 - (sizeof(k xen_ ## n) != \
> > > -                                          sizeof(k compat_ ## n)) * 
> > > 2]
> > > +    typedef int CHECK_NAME_(k, n, S)[1 - (SIZE_NEQUAL(k xen_ ## n, \
> > > +                                                      k compat_ ## 
> > > n)) * 2]
> > I think this style is easier to read but I'll let the x86 maintainers
> > decide
> >
> >     typedef int CHECK_NAME(name, S)[(sizeof(xen_ ## name ## _t) == \
> >                                      sizeof(compat_ ## name ## _t)) ? 
> > 1 : -1]
> > Also am I reading this correctly that we are using -1 as array index? 
> > I
> > must have made a calculation mistake?
> This is a NIH BUILD_BUG_ON().  It should be rewritten as
>
> BUILD_BUG_ON(sizeof(xen ...) != sizeof(compat ...));
>
> which will use _Static_assert() in modern compilers.
>
> ~Andrew
Ok, thanks.

--
Nicola Vetrini, BSc
Software Engineer, BUGSENG srl (https://bugseng.com)