linux-kernel - Re: [PATCH 20/25] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

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Date:	Wed, 11 May 2016 12:55:01 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Yury Norov <ynorov@...iumnetworks.com>
Cc:	"Zhangjian (Bamvor)" <bamvor.zhangjian@...wei.com>,
	linux-doc@...r.kernel.org,
	Andrew Pinski <Andrew.Pinski@...iumnetworks.com>,
	catalin.marinas@....com, heiko.carstens@...ibm.com,
	Hanjun Guo <guohanjun@...wei.com>, joseph@...esourcery.com,
	linux-arch@...r.kernel.org, linux-s390@...r.kernel.org,
	"jijun (D)" <jijun2@...wei.com>, Prasun.Kapoor@...iumnetworks.com,
	schwab@...e.de, agraf@...e.de, pinskia@...il.com,
	klimov.linux@...il.com, broonie@...nel.org,
	linux-arm-kernel@...ts.infradead.org, Nathan_Lynch@...tor.com,
	linux-kernel@...r.kernel.org, Andrew Pinski <apinski@...ium.com>,
	schwidefsky@...ibm.com, christoph.muellner@...obroma-systems.com
Subject: Re: [PATCH 20/25] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

On Wednesday 11 May 2016 11:04:38 Yury Norov wrote:
> On Wed, May 11, 2016 at 10:04:16AM +0800, Zhangjian (Bamvor) wrote:
> [...]
> 
> > >>Ok, I will test the ltp syscall test.
> > >>With this changes, the issue I mentioned should be fixed. But we still
> > >>use mmap2 syscall for ILP32 application when we pass the offset instead
> > >>of page offset. Is it correct?
> > >
> > >I don't remember. It's probably not important whether we have the shift
> > >in there, as long as it's independent of the actual kernel page size and
> > >user space and kernel agree on the calling conventions.
> > Well. I am ok with where to shift the pages size because we get the same
> > result. I was just thinking if we should get rid of the name of mmap2 in our
> > ILP32 porting. Actually, it is mmap but we name it as mmap2. User may confused
> > if they do not know the implementations.
> > 
> 
> This is what generic unistd.h does. If you want to change it, you'd
> change each arch that uses generic unistd.h.
> 

Generic unistd.h has this:

#ifdef __SYSCALL_COMPAT
#define __SC_COMP_3264(_nr, _32, _64, _comp) __SYSCALL(_nr, _comp)
#else
#define __SC_COMP_3264(_nr, _32, _64, _comp) __SC_3264(_nr, _32, _64)
#endif

#define __NR3264_mmap 222
__SC_3264(__NR3264_mmap, sys_mmap2, sys_mmap)


#if __BITS_PER_LONG == 64 && !defined(__SYSCALL_COMPAT)
#define __NR_mmap __NR3264_mmap
#else
#define __NR_mmap2 __NR3264_mmap
#endif

So by default we get __NR_mmap2 and sys_mmap2 on 32-bit ABIs, but
__NR_mmap and sys_mmap on 64-bit ABIs, as it should be.

The problem is that arch/arm64/kernel/sys_ilp32.c now overrides
this to use __NR_mmap2 with sys_mmap, so we have a mismatch. I think
we should either override both the implementation and the number,
or neither of them.

I think what happened is that you chose to override sys_mmap because
sys_mmap2 sometimes takes an off_t argument, however the kernel
just treats it as 'unsigned long', with a range of 44 bits for the
offset. Using sys_mmap with a __kernel_loff_t argument gives
us 64-bit range, but if we do that, I think it should be __NR_mmap.

	Arnd