linux-kernel - PER_LINUX32, Was: [PATCH v2 21/31] arm64: 32-bit (compat) applications support

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Message-Id: <201208230646.31276.arnd@arndb.de>
Date:	Thu, 23 Aug 2012 06:46:30 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Will Deacon <will.deacon@....com>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jiri Kosina <jkosina@...e.cz>, x86@...nel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Andi Kleen <andi@...stfloor.org>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: PER_LINUX32, Was: [PATCH v2 21/31] arm64: 32-bit (compat) applications support

On Thursday 16 August 2012, Will Deacon wrote:
> On Wed, Aug 15, 2012 at 03:34:04PM +0100, Arnd Bergmann wrote:
> > On Tuesday 14 August 2012, Catalin Marinas wrote:
> > > +asmlinkage int compat_sys_personality(compat_ulong_t personality)
> > > +{
> > > +	int ret;
> > > +
> > > +	if (personality(current->personality) == PER_LINUX32 &&
> > > +		personality == PER_LINUX)
> > > +		personality = PER_LINUX32;
> > > +	ret = sys_personality(personality);
> > > +	if (ret == PER_LINUX32)
> > > +		ret = PER_LINUX;
> > > +	return ret;
> > > +}
> > 
> > Where did you get this from?
> > 
> > You should not need compat_sys_personality, just call the native function.
> 
> Hmm, but in that case an aarch32 application doing a personality(PER_LINUX)
> syscall will start seeing the wrong uname.

Coming back at this topic, I noticed another issue. Jiri Kosina
has recently posted patches to fix this function in the other architectures
in order to mask out the other personality bits, which is a correct fix,
but the above function is odd for other reasons.

* On MIPS, it is used only for compat tasks, like you have it above.
* On PA-RISC, it is used for native 32 bit tasks and for compat 32 bit tasks,
  but not for native 64 bit ones.
* On IA64, it was used for compat tasks (support for which has since
  been removed from the kernel), plus all 32 bit tasks would start with
  PER_LINUX32.
* On PowerPC, Sparc and s390, it is used for native 64 bit tasks and for
  compat 32 bit tasks, but not for native 32 bit ones.
* On Tile, it was never used.
* On x86_64, it used to be defined (copied from ia64) but not used
  throughout the git history.

The semantics of the function are also interesting: The intention seems
to be that to a compat task, PER_LINUX32 would appear as PER_LINUX.
The effect is that any process can set PER_LINUX32 but it can never
be unset except by a 64 bit MIPS or PA-RISC task.

Since x86_64 does not implement this behavior at all, I suspect that
there are now lots of things depending on not having it, while all
the other architectures might also have some (even predating the
x86_64 port) use cases that depend on depend on not being able to
observe PER_LINUX32 in 32 bit compat tasks.

I think we should try to agree on how this is all supposed to work
and use common code, either put the ppc/sparc/s390 version into
sys_personality, or remove all of them and just do what x86 and tile
do, using the regular sys_personality for all tasks.

	Arnd
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