linux-kernel - Re: [PATCH 08/24] Allow a 32bit ABI to use the naming of the 64bit ABI syscalls to avoid confusion of not splitting the registers

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Date:	Wed, 01 Oct 2014 16:00:54 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Catalin Marinas <catalin.marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Pinski <apinski@...ium.com>,
	"pinskia@...il.com" <pinskia@...il.com>
Subject: Re: [PATCH 08/24] Allow a 32bit ABI to use the naming of the 64bit ABI syscalls to avoid confusion of not splitting the registers

On Wednesday 01 October 2014 13:42:27 Catalin Marinas wrote:
> On Thu, Sep 04, 2014 at 11:11:04AM +0100, Arnd Bergmann wrote:
> > On Wednesday 03 September 2014 14:19:02 Andrew Pinski wrote:
> > > + * For 32bit abis where 64bit can be passed via one
> > > + * register, use the same naming as the 64bit ones
> > > + * as they will only have a 64 bit off_t.
> > >   */
> > > -#if __BITS_PER_LONG == 64 && !defined(__SYSCALL_COMPAT)
> > > +#if (__BITS_PER_LONG == 64 && !defined(__SYSCALL_COMPAT)) || \
> > > +       defined(__ARCH_WANT_64BIT_SYSCALLS)
> > 
> > I'm not sure if __ARCH_WANT_64BIT_SYSCALLS is the best name for
> > this, since it's really only about off_t. It took me a while
> > to understand what you are doing here.
> 
> I'm not sure I fully get it yet. So with this change, we avoid using
> syscall numbers like __NR_ftruncate64 in favour of __NR_ftruncate. Why?
> (maybe there's a valid reason, just not getting it).

glibc depends on the name to decide which calling conventions it
uses. I assume this is the same on IPL32 ARM.

The general rule is that on a 32-bit architecture, __NR_ftruncate refers
to the system call that takes a 32-bit off_t argument, while __NR_ftruncate64
refers to the syscall that takes a 64-bit loff_t.

I would assume that the new ABI does not actually allow using 32-bit off_t
in applications (that would be silly) and defaults to using 64-bit offsets,
but it still needs to generate the right system calls.

> Either way, ILP32 would still end up calling sys_ftruncate64() (rather
> than the native sys_ftruncate()).

sys_ftruncate64 does not exist in 64-bit kernels, it can either call
compat_sys_ftruncate64_wrapper or sys_ftruncate. I'd assume it would
call the latter and pass a single 64-bit register, but that is another
matter.

	Arnd
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