Re: [PATCH 7/9] include: Upgrade <linux/typecheck.h>

[Simon Glass <sjg@chromium.org>]

Hi Pierre-Clément,

On Thu, 17 Mar 2022 at 05:46, Pierre-Clément Tosi <ptosi@google.com> wrote:
>
> On Wed, Mar 16, 2022 at 01:23:38PM -0600, Simon Glass wrote:
> > Hi,
> >
> > On Wed, 16 Mar 2022 at 09:41, Pierre-Clément Tosi <ptosi@google.com> wrote:
> > >
> > > Upgrade the header to version 5.16 of the kernel:
> > >
> > >     commit df0cc57e057f18e44dac8e6c18aba47ab53202f9
> > >
> > > Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>
> > > Cc: Simon Glass <sjg@chromium.org>
> > > Cc: Tom Rini <trini@konsulko.com>
> > > ---
> > >  include/linux/typecheck.h | 10 ++++++++++
> > >  1 file changed, 10 insertions(+)
> >
> > Reviewed-by: Simon Glass <sjg@chromium.org>
> >
> > But I don't understand how this works at all. Could you add a comment?
>
> This patch is a simple upgrade of the header. I needed it for another patch
> stack and, given how trivial and self-contained it is, I thought I would slip it
> into this one.
>
> If your "this" refers to the macro itself, an error should be generated by the
> compiler when dereferencing a non-pointer and it avoids introducing side effects
> through the use of __dummy but I'm not sure what the sizeof is there for; is it
> perhaps compliant for an implementation to keep generating code when
> encountering a dereferenced non-pointer (leading to a bogus result, from UB) so
> that sizeof forces it to depend on that expression being of a valid type? Or is
> it to do with catching variables of incomplete type e.g. arrays of unknown size?
> It was introduced by commit cb0f80039fb7 ("bitops: Add non-atomic bitops for
> pointers"), which doesn't provide much information either.

Hmm OK. The Linux people are quite averse to comments. If you have
time you could send a patch to comment the macro and see if someone
responds and dispels the uncertainty.

>
> >
> > >
> > > diff --git a/include/linux/typecheck.h b/include/linux/typecheck.h
> > > index eb5b74a575..46b15e2aae 100644
> > > --- a/include/linux/typecheck.h
> > > +++ b/include/linux/typecheck.h
> > > @@ -1,3 +1,4 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > >  #ifndef TYPECHECK_H_INCLUDED
> > >  #define TYPECHECK_H_INCLUDED
> > >
> > > @@ -21,4 +22,13 @@
> > >         (void)__tmp; \
> > >  })
> > >
> > > +/*
> > > + * Check at compile time that something is a pointer type.
> > > + */
> > > +#define typecheck_pointer(x) \
> > > +({     typeof(x) __dummy; \
> > > +       (void)sizeof(*__dummy); \
> > > +       1; \
> > > +})
> > > +
> > >  #endif         /* TYPECHECK_H_INCLUDED */
> > > --
> > > 2.35.1.723.g4982287a31-goog
Regards,
Simon