linux-kernel - Re: [RFC PATCH] UAPI: Document auxvec AT_* namespace policy and note reservations

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Date:   Thu, 17 May 2018 16:40:19 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Michal Simek <monstr@...str.eu>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Albert Ou <albert@...ive.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>
Subject: Re: [RFC PATCH] UAPI: Document auxvec AT_* namespace policy and note reservations

Dave Martin <Dave.Martin@....com> writes:

> There are constraints on defining AT_* auxvec tags that are not
> obvious to the casual maintainer of either the global
> <uapi/linux/auxvec.h> or the arch-specific headers.  This is likely
> to lead to mistakes.  (I certainly fell foul of it...)

Thanks for cleaning this up.

It looks like us (powerpc) / me is the main offender here.

My excuse is it was glibc folk who asked us to add all those new AT_
entries in the first place. </buckpassing>

> For the benefit of future maintainers, this patch collects the
> relevant information in one place, documenting how the namespace
> needs to be managed, and noting all the values currently in use.
>
> Maintaining a global list may result in some merge conflicts, but
> AT_* values are not added frequently.  I'm open to suggestions on
> the best approach.

Yeah I agree with Rich that having a global list would be best. That is
the most reliable to make people think twice about adding new entries.

> I also assume that values 38 and 39 may have been used for
> historical purposes, such as an architecture that is no longer
> supported.  If they have definitely never been used for anything,
> they could be removed from the "reserved" list.

I don't know why we added the new entries starting at 40, maybe Ben
remembers. Quite likely it was just an accident.

I don't see any sign of 38 or 39 in glibc history.

cheers