linux-kernel - Re: new execve/kernel

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Date:	Wed, 17 Oct 2012 16:27:06 +0200
From:	Michal Simek <monstr@...str.eu>
To:	Jonas Bonn <jonas@...thpole.se>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Haavard Skinnemoen <hskinnemoen@...il.com>,
	Mike Frysinger <vapier@...too.org>,
	Jesper Nilsson <jesper.nilsson@...s.com>,
	David Howells <dhowells@...hat.com>,
	Tony Luck <tony.luck@...el.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Hirokazu Takata <takata@...ux-m32r.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Richard Kuo <rkuo@...eaurora.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Lennox Wu <lennox.wu@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Paul Mundt <lethal@...ux-sh.org>,
	Chris Zankel <chris@...kel.net>,
	Chris Metcalf <cmetcalf@...era.com>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	Guan Xuetao <gxt@...c.pku.edu.cn>
Subject: Re: new execve/kernel_thread design

2012/10/17 Jonas Bonn <jonas@...thpole.se>:
> On 17 October 2012 00:35, Al Viro <viro@...iv.linux.org.uk> wrote:
>>
>> Not even a tentative patchset: hexagon, openrisc, tile, xtensa.
>>
>
> I did most of the OpenRISC conversion last weekend... the
> kernel_thread bits work fine but I end up with the init thread dying
> with what I've got now for kernel_execve.  Once I've got that sorted
> out, I'll pass this along to you.

I am testing the Microblaze conversion and I see the similar problem
with GENERIC_KERNEL_EXECVE
(commit: http://git.kernel.org/?p=linux/kernel/git/viro/signal.git;a=commit;h=6aa044199aed5b541eba7fe7f25efdfb3a655a58)

I have look at the patch and I have found this.
(From description above: a kernel thread can become a userland
process.  The primitive is kernel_execve())

In  init/main.c:795/run_init_process() kernel_execve is called.
In old style, kernel_execve is called which runs microblaze
kernel_execve which calls __NR_execve as syscall.
In entry.S user exception detects that jump comes from kernel space
and save pt_regs on the current stack
and calls sys_execve and then microblaze_execve with 4th argument
which is pointer to pt_regs, etc.

In the patch above there is directly used current_pt_regs() function
which works good for newly created threads
when pt_regs are exactly in current_pt_regs() position but not for
pt_regs which are saved on the stack
which is the init task case.

Also this is the reason why microblaze has implementation for calling
_user_exception from the kernel space.
I believe that it is called just once for /init.

My question is how should /init be called? Because I need to save
pt_regs to current_pt_regs() position where
generic kernel_execve expects it.

Thanks,
Michal

-- 
Michal Simek, Ing. (M.Eng)
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel 2.6 Microblaze Linux - http://www.monstr.eu/fdt/
Microblaze U-BOOT custodian
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