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

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Message-ID: <Pine.LNX.4.61.0703190717240.19254@chaos.analogic.com>
Date:	Mon, 19 Mar 2007 07:23:25 -0400
From:	"linux-os \(Dick Johnson\)" <linux-os@...logic.com>
To:	"Ahmed S. Darwish" <darwish.07@...il.com>
Cc:	"Linux kernel" <linux-kernel@...r.kernel.org>
Subject: Re: i386: Why putting __USER_DS in kernel threads stack initialization?

On Sun, 18 Mar 2007, Ahmed S. Darwish wrote:

> 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

You might be confusing two routines. The kernel thread routine sets
DS and ES to the kernel data segment, __KERNEL_DS, not the user data
segment. This is so the kernel thread can access the kernel data. Note
that this is done by putting the values in the pt_regs structure so
it doesn't happen 'now', but after the fork.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.68 BogoMips).
New book: http://www.AbominableFirebug.com/
_
..

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