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

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Message-ID: <20180516154957.GT7753@e103592.cambridge.arm.com>
Date:   Wed, 16 May 2018 16:49:59 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Rich Felker <dalias@...c.org>
Cc:     linux-kernel@...r.kernel.org, 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>,
        Michael Ellerman <mpe@...erman.id.au>,
        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>,
        "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

On Wed, May 16, 2018 at 11:29:13AM -0400, Rich Felker wrote:
> On Wed, May 16, 2018 at 03:20:47PM +0100, Dave Martin wrote:
> > 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...)
> > 
> > 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.
> > 
> > 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.
> 
> On the userspace side (elf.h) all the AT_* constants are in one file.
> Why don't we just do the same here and eliminate the
> arch/*/include/uapi/asm/auxvec.h files and likewise the need to
> manually maintain consistency of the comments about reservations?
> 
> If there are reasons not to do that, I'm not opposed to this patch
> as-is.

I agree, it would be better to merge them.

My concern was that the correct way to get these definitions from
userspace is very unclear, so there may be software out there
including <asm/auxvec.h> directly, which would now lack expected
definitions.

codesearch.debian.net shows no real hits for that, so maybe I'm too
paranoid.

Since <linux/auxvec.h> only contains #defines, it may be enough for arch
<asm/auxvec.h> headers to include <linux/auxvec.h>.

Thoughts?

Cheers
---Dave