linux-kernel - Re: [PATCHv3 00/24] ILP32 support in ARM64

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Message-ID: <20150212161354.GT23507@brightrain.aerifal.cx>
Date:	Thu, 12 Feb 2015 11:13:54 -0500
From:	Rich Felker <dalias@...c.org>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"H.J. Lu" <hjl.tools@...il.com>,
	GNU C Library <libc-alpha@...rceware.org>,
	Andrew Pinski <pinskia@...il.com>, musl@...ts.openwall.com,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Pinski <apinski@...ium.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCHv3 00/24] ILP32 support in ARM64

On Thu, Feb 12, 2015 at 03:50:24PM +0000, Catalin Marinas wrote:
> On Wed, Feb 11, 2015 at 12:15:56PM -0800, Andy Lutomirski wrote:
> > On 02/11/2015 11:57 AM, H.J. Lu wrote:
> > >>>>trivially satisfied if you consider x32 and x86_64 separate
> > >>>>compilation environments, but it's not related to the core issue: that
> > >>>>the definition of timespec violates core (not obscure) requirements of
> > >>>>both POSIX and C11. At the time you were probably unaware of the C11
> > >>>>requirement. Note that it's a LOT harder to effect change in the C
> > >>>>standard, so even if the Austin Group would be amenable to changing
> > >>>>the requirement for timespec to allow something like nseconds_t,
> > >>>>getting WG14 to make this change to work around a Linux/glibc mistake
> > >>>>does not sound practical.
> > >>>
> > >>>That is very unfortunate.  I consider it is too late for x32 to change.
> > >>
> > >>Why? It's hardly an incompatible ABI change, as long as the
> > >>kernel/libc fills the upper bits (for old programs that read them
> > >>based on the old headers) when structs are read from the kernel to the
> > >>application, and ignores the upper bits (potentially set or left
> > >>uninitialized by the application) when strings are passed from
> > >>userspace to the kernel. Newly built apps using the struct definition
> > >>with 32-bit tv_nsec would need new libc to ensure that the high bits
> > >>aren't interpreted, but this could be handled by symbol versioning.
> > >>
> > >
> > >We have considered this option.  But since kernel wouldn't change
> > >tv_nsec/tv_usec handling just for x32, it wasn't selected.
> > 
> > Did anyone *ask* the kernel people (e.g. hpa)?
> 
> It seems so:
> 
> https://lkml.org/lkml/2011/8/31/244
> 
> Couple of more replies from hpa:
> 
> https://lkml.org/lkml/2011/8/31/261
> https://lkml.org/lkml/2012/2/8/408
> 
> It looks like hpa was going to talk the POSIX committee but I don't know
> what the conclusion was and didn't follow the thread (at the time I
> wasn't interested in ARM ILP32).

At this point POSIX committee is not sufficient. ISO C specifies
timespec now, and as Jens Gustedt mentioned (I don't think his reply
made it to the whole CC list; see the musl list archive here:
http://www.openwall.com/lists/musl/2015/02/11/21 ), it seems unlikely
that one could pose a convincing argument for this requirement to be
changed in the C language.

Rich
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