linux-kernel - Re: [PATCH 00/17] Add memberof(), split some headers, and slightly simplify code

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Date:   Fri, 19 Nov 2021 17:25:11 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>, LKML <linux-kernel@...r.kernel.org>,
        Ajit Khaparde <ajit.khaparde@...adcom.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Bjorn Andersson <bjorn.andersson@...aro.org>,
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        David Sterba <dsterba@...e.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Jason Wang <jasowang@...hat.com>,
        Jitendra Bhivare <jitendra.bhivare@...adcom.com>,
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Subject: Re: [PATCH 00/17] Add memberof(), split some headers, and slightly
 simplify code

On Fri, Nov 19, 2021 at 5:12 PM Alejandro Colomar (man-pages)
<alx.manpages@...il.com> wrote:
>
> On 11/19/21 16:57, Arnd Bergmann wrote:
> >
> > From what I can tell, linux/stddef.h is tiny, I don't think it's really
> > worth optimizing this part. I have spent some time last year
> > trying to untangle some of the more interesting headers, but ended
> > up not completing this as there are some really hard problems
> > once you start getting to the interesting bits.
>
> In this case it was not about being worth it or not,
> but that the fact that adding memberof() would break,
> unless I use 0 instead of NULL for the implementation of memberof(),
> which I'm against, or I split stddef.
>
> If I don't do either of those,
> I'm creating a circular dependency,
> and it doesn't compile.

Sorry for missing the background here, but I don't see what that
dependency is. If memberof() is a macro, then including the definition
should not require having the NULL definition first, you just need to
have both at the time you use it.

> > The main issue here is that user space code should not
> > include anything outside of include/uapi/ and arch/*/include/uapi/
>
> Okay.  That's good to know.
>
> So everything can use uapi code,
> and uapi code can only use uapi code,
> right?

Correct.

> > offsetof() is defined in include/linux/stddef.h, so this is by
> > definition not accessible here. It appears that there is also
> > an include/uapi/linux/stddef.h that is really strange because
> > it includes linux/compiler_types.h, which in turn is outside
> > of uapi/. This should probably be fixed.
>
> I see.
> Then,
> perhaps it would be better to define offsetof() _only_ inside uapi/,
> and use that definition from everywhere else,
> and therefore remove the non-uapi version,
> right?

No, because the user-space <stddef.h> provided by the compiler
also includes an offsetof() definition. In the uapi/ namespace, the
kernel must only provide definitions that do not clash with anything
in user space.

        Arnd