linux-kernel - Re: + fully-honor-vdso_enabled.patch added to -mm tree

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Message-ID: <20070303173834.GB249@tv-sign.ru>
Date:	Sat, 3 Mar 2007 20:38:34 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	John Reiser <jreiser@...Wagon.com>
Cc:	Paul Mundt <lethal@...ux-sh.org>, Andi Kleen <ak@...e.de>,
	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: + fully-honor-vdso_enabled.patch added to -mm tree

On 03/02, John Reiser wrote:
>
> Oleg Nesterov wrote:
> 
> > Still, I don't understand why we don't pass NEW_AUX_ENT(AT_SYSINFO) when
> > vdso_enabled == 0. We don't need linux-gate.so to use __kernel_vsyscall,
> > we have FIX_VDSO. In that case we should s/PAGE_KERNEL_RO/PAGE_READONLY/
> > of course. I guess the reason is some magic in glibc.
> 
> In the past there were problems where user code could not single-step
> (or even read the code of) such system calls.  Can these two operations
> be performed today with what you propose, particularly if FIX_VDSO is
> near 0xfffff000 ?

No, we can't COW the gate page, it lives in swapper_pg_dir. I am not sure
why we could not read it, we have gate_vma for that.

So, the reason is debugger. Thanks.

Oleg.

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