linux-kernel - i386: Why putting __USER_DS in kernel threads stack initialization?

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Message-ID: <20070318225831.GB3802@Ahmed>
Date:	Mon, 19 Mar 2007 00:58:31 +0200
From:	"Ahmed S. Darwish" <darwish.07@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: i386: Why putting __USER_DS in kernel threads stack initialization?

Hi list,

Reading the kernel threads initialization code I see:

int kernel_thread(...) {

	struct pt_regs regs;
	memset(&regs, 0, sizeof(regs));
	[...]
**	regs.xds = __USER_DS;
**	regs.xes = __USER_DS;
	[...]
	/* Ok, create the new process.. */
	return do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, \
	       	       0, NULL, NULL);

Continuing with the code, the threads stack (beginning from %esp) is
initialized with the passed *regs from do_fork:

int copy_thread(..., struct task_struct *p, struct pt_regs *regs) {

	struct pt_regs * childregs;
	struct task_struct *tsk;
 	childregs = task_pt_regs(p);
**	*childregs = *regs;
	[...]
** 	p->thread.esp = (unsigned long) childregs;

So the question is what will a _kernel_ thread do with the Usermode Segment
address ?

Thanks,

P.S. I've tried commenting out both lines which led to a non functional init,
Also setting them to __USER_DS made init start but stopped issuing the error:
`Panic: Segment violation at 0x8049798 - Sleeping for 30 seconds'

-- 
Ahmed S. Darwish
http://darwish.07.googlepages.com

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