linux-kernel - Re: Q: a bogus set_fs(USER_DS) in setup_frame/setup_rt

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Date:	Tue, 17 Jul 2007 09:05:57 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...sign.ru>
cc:	Chuck Ebbert <76306.1226@...puserve.com>,
	Ingo Molnar <mingo@...e.hu>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: a bogus set_fs(USER_DS) in setup_frame/setup_rt_frame ?



On Tue, 17 Jul 2007, Oleg Nesterov wrote:
>
> I am really puzzled by set_fs(USER_DS) in setup_frame/setup_rt_frame.
> 
> How is it possible that current->addr_limit != USER_DS ? If this _is_
> possible, how can can we trust the result of access_ok() above?

Heh. I think it's entirely historical.

Please realize that the whole reason that function is called "set_fs()" is 
that it literally used to set the %fs segment register, not 
"->addr_limit".

So I think the "set_fs(USER_DS)" is there _only_ to match the other

        regs->xds = __USER_DS;
        regs->xes = __USER_DS;
        regs->xss = __USER_DS;
        regs->xcs = __USER_CS;

things, and never mattered. And now it matters even less, and has been 
copied to all other architectures where it is just totally insane.

		Linus
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