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

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Date:   Thu, 17 May 2018 18:00:28 +0200
From:   Yann Droneaud <ydroneaud@...eya.com>
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>,
        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>,
        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>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH] UAPI: Document auxvec AT_* namespace policy and
 note reservations

Hi,

Le mercredi 16 mai 2018 à 15:20 +0100, Dave Martin a écrit :
> 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.
> 

Some of those AT_* values are described in getauxval(3) man-page:

http://man7.org/linux/man-pages/man3/getauxval.3.html

https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man3/g
etauxval.3?id=4eae8eb731386d81797d5c30365426722410874e

And glibc provides <elf.h> with definitions for almost all AT_*,
regardless of the current target architecture:

https://sourceware.org/git/?p=glibc.git;a=blob;f=elf/elf.h;h=954f3266f7
11ab83996670ea504a17dcf668e061;hb=23158b08a0908f381459f273a984c6fd32836
3cb#l1135

Also, despite not being listed as a reserved namespace by POSIX, one
should try to avoid name collision with other AT_ constants,
those used with *at() functions (openat(), etc.):

- AT_EACCESS 
- AT_EMPTY_PATH
- AT_FDCWD
- AT_NO_AUTOMOUNT
- AT_REMOVEDIR
- AT_STATX_DONT_SYNC
- AT_STATX_FORCE_SYNC
- AT_STATX_SYNC_AS_STAT
- AT_SYMLINK_FOLLOW
- AT_SYMLINK_NOFOLLOW

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/fcntl.h.html

http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.htm
l#tag_15_02_02

https://sourceware.org/git/?p=glibc.git;a=blob;f=io/fcntl.h;h=3d239e8f0
9f7ce0a3106621be327e1ea4cd1a3e7;hb=23158b08a0908f381459f273a984c6fd3283
63cb#l142

And there's also AT_ANYNET and AT_ANYNODE from ddp (aka. AppleTalk)

http://man7.org/linux/man-pages/man7/ddp.7.html

Regards.

-- 
Yann Droneaud
OPTEYA