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

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Aug 23, 2012 at 07:46:30AM +0100, Arnd Bergmann wrote:
> 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.

IMHO, it makes sense to keep the compat_sys_personality() as implemented
above. You may want to start a chroot ARMv7 environment using "linux32"
but don't want some 32-bit app calling personality(PER_LINUX) (as that's
the default personality on an ARMv7 system) and unknowingly changing the
personality that you wanted to enforce via "linux32".

I agree with not setting the personality based on the ELF type, but
that's different from the compat_sys_personality().

> 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.

Late topic for the KS :).

I don't think we can move this behaviour to sys_personality. We may want
to add a generic compat_sys_personality() if we agree on the above
use-case.

-- 
Catalin