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

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Date:	Sat, 3 Mar 2007 01:18:54 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	John Reiser <jreiser@...Wagon.com>
Cc:	Andi Kleen <ak@...e.de>, Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>,
	Paul Mundt <lethal@...ux-sh.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:
> > John Reiser wrote:
> 
> >>+	switch (vdso_enabled) {
> >>+	case 0:  /* none */
> >>+		return 0;
> > 
> > 
> > This means we don't initialize mm->context.vdso and ->sysenter_return.
> > 
> > Is it ok? For example, setup_rt_frame() uses VDSO_SYM(&__kernel_rt_sigreturn),
> > sysenter_past_esp pushes ->sysenter_return on stack.
> 
> Paul Mundt has commented on setup_rt_frame() and provided a patch which
> bullet-proofs that area.  I will include that patch into the next revision.

Confused. I still think his patch incomplete. Don't we need the same check
in setup_rt_frame() ?

> The value of ->sysenter_return is interpreted in user space by the
> sysexit instruction; nobody else cares what the value is.  The kernel
> is not required to provide a good value when vdso_enabled is zero,

Yes sure.

> Both context.vdso and sysenter_return could be set to zero whenever
> vdso_enabled is zero; those two values might even be defaulted.
> I'll add such a change to the next revision of the patch, if you'll
> defend it against claims of "unnecessary code."

context.vdso == NULL after mm_alloc(). I don't see a "good" arch dependent
function to clear ->sysenter_return (if we really need this). May be
flush_thread().

> > Note also that load_elf_binary does
> > 
> > 	arch_setup_additional_pages()
> > 	create_elf_tables()
> > 
> > , looks like application can crash after exec if vdso_enabled changes from 0
> > to 1 in between.
> 
> Correct.  Changing vdso_enabled from 0 to non-zero must be prepared
> to lose this race if it is not prevented.  Ordinarily it won't matter
> because the administrator will perform such changes at a "quiet" time.

I agree, this problem is mostly theoretical, but I believe it is hardly
possible to document what the "quiet" time is ;)

How about

	 #define ARCH_DLINFO
	-do if (vdso_enabled) {
	+do if (VDSO_BASE) {
?

Oleg.

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