linux-kernel - Re: new execve/kernel

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Message-ID: <20121017160702.GY2616@ZenIV.linux.org.uk>
Date:	Wed, 17 Oct 2012 17:07:03 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Michal Simek <monstr@...str.eu>
Cc:	Jonas Bonn <jonas@...thpole.se>, 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

On Wed, Oct 17, 2012 at 04:27:06PM +0200, Michal Simek wrote:

> 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.

init_task does *not* do kernel_execve().  It's PID 0, not PID 1.
init is spawned by it.

> 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.

What happens during boot is this:
	* init_task (not to be confused with init) is used as current during
infrastructure initializations.  Once everything needed for scheduler and
for working fork is set, we spawn two threads - future init and future
kthreadd.  The last thing we do with init_task is telling init that kthreadd
has been spawned.  After that init_task turns itself into an idle thread.
	* future init waits for kthreadd to be spawned (it would be more
natural to fork them in opposite order, but we want init to have PID 1 -
too much stuff in userland depends on that).  Then it does the rest of
initialization, including setting up initramfs contents.  And does
kernel_execve() on /init.  Note that this is a task that had been created
by kernel_thread() and is currently in function called from
ret_from_kernel_thread().  Its kernel stack has been set up by copy_thread().
That's where pt_regs need to be set up; note that they'll be passed to
start_thread() before you return to userland.  If there are any magic bits
in pt_regs needed by return-from-syscall code, set them in kthread case of
copy_thread().
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