linux-kernel - Re: [PATCH v6 14/19] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

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Date:	Wed, 2 Dec 2015 00:29:04 +0300
From:	Yury Norov <ynorov@...iumnetworks.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	<catalin.marinas@....com>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <pinskia@...il.com>,
	<ddaney.cavm@...il.com>, <jan.dakinevich@...il.com>,
	<Prasun.Kapoor@...iumnetworks.com>,
	<christoph.muellner@...obroma-systems.com>,
	<philipp.tomsich@...obroma-systems.com>, <broonie@...nel.org>,
	<andrey.konovalov@...aro.org>, <Nathan_Lynch@...tor.com>,
	<agraf@...e.de>, <bamvor.zhangjian@...wei.com>,
	<klimov.linux@...il.com>, <joseph@...esourcery.com>,
	<schwab@...e.de>
Subject: Re: [PATCH v6 14/19] arm64:ilp32: add sys_ilp32.c and a separate
 table (in entry.S) to use it

> > +#define compat_sys_shmat               sys_shmat
> 
> What's special about compat_sys_shmat?
> 

It's about SHMLBA definition.
For aarch32 glibc defines it as (__getpagesize () << 2).
For ILP32 there's no definition, and so generic one is used: (__getpagesize ()).

In kernel, for ARM64, COMPAT_SHMLBA defined just as 0x4000. Both
compat and non-compat shmat syscalls pass identical arguments to
do_shmat, except shmlba. Effectively, library expects shmlba to
be 0x1000, as sys_shmat does. And compat_sys_shmat expects 0x4000.

I think, both kernel and library parts are to be fixed. In library
we'd use definition identical to ARM. For kernel we'd use compat
syscall.

My question. Why aarch64 defines COMPAT_SHMLBA as 0x4000? If there's
no specific reason for it, it looks like a bug, and we should
define it like in arch/arm:
        #define SHMLBA  (4 * PAGE_SIZE)          /* attach addr a multiple of this */

Maybe that's why AARCH32 is limited to 4K pages in config.

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