linux-kernel - Re: PTRACE_SYSCALL && vsyscall (Was: arch_check_bp_in

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Message-ID: <20121204175933.GA11537@redhat.com>
Date:	Tue, 4 Dec 2012 18:59:33 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	u3557@...o.sublimeip.com
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Pedro Alves <palves@...hat.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: PTRACE_SYSCALL && vsyscall (Was: arch_check_bp_in_kernelspace:
	fix the range check)

On 12/03, u3557@...o.sublimeip.com wrote:
>
> > However. Of course it would be nice to avoid the new option. IMO it
> > would be better to do nothing ;) vsyscall is deprecated, and EMULATE
> > is x86-specific.
>
> The problem is that the current static glibc invokes the vsyscall page,

Yes I know.

Still I'd like to avoid to change the ptrace API, even if the change is
simple. This emulate_vsyscall() is too "exotic" imho.

> > You forgot again that EMULATE does not execute the code in the
> > vsyscall page.
>
> The beauty of using the x86 debug-registers, is that they do not
> trap the instruction, but rather the fact that the program-counter
> has a given value.

Yes, I understand, so DR_RW_EXECUTE should probably work. And I even
sent the patch (untested/uncompiled). But given that even the simple
bugfix which started this thread was ignored by maintainers, I am
not sure how we can convince them this change makes sense ;)

However. This looks like a hack to me, because this code is never
executed. But this is sudjective and I am not saying this can't work.
And yes, this doesn't add new ptrace hacks.

But If we want to allow to trace vsyscall's, hw bp doesn't look very
nice imo. HBP_NUM = 4 and you need to setup 3 bp's to trace them all.

And what about strace? It won't be easy to change it to use hwbp.

That is why I think PTRACE_SYSCALL should "simply work" somehow. And
so far I think that "just report syscall_exit with orig_ax = -1" is
the best (and simple) solution.

OK. We can do more. We can report both syscall_enter/exit and we can
change orig_ax/ax temporary to "fool" the tracer, so that everything
will look as a "normal" syscall. Like vsyscall_seccomp() does.

But this needs much more changes.

Oleg.

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