linux-kernel - Re: [PATCH v4 15/16] ARM: add uprobes support

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Date:	Mon, 23 Dec 2013 16:32:09 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Jon Medhurst (Tixy)" <tixy@...aro.org>
Cc:	David Long <dave.long@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	Russell King <linux@....linux.org.uk>,
	Rabin Vincent <rabin@....in>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	davem@...emloft.net, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 15/16] ARM: add uprobes support

On 12/20, Jon Medhurst (Tixy) wrote:
>
> On Sun, 2013-12-15 at 23:08 -0500, David Long wrote:
> > From: "David A. Long" <dave.long@...aro.org>
> >
> > Caveats:
> >
> >  - Thumb is not supported
> >  - XOL abort/trap handling is not implemented
>
> I shall repeat my comment from version one of the patch...
>
> What are the consequences of this, e.g. is it possible for a probe to
> get stuck in an infinite loop of faulting? I hope there are no integrity
> issues for the kernel itself.

Unless there is something arm-specific, the kernel should be fine. The
task should notice a signal after handle_singlestep() before it returns
to the user-mode.

But of course the probed application can be confused. And I have no idea
what arm does if, say, a single-stepped xol insn triggers the page fault.
We need to change instruction_pointer() back to the faulted (probed) insn
if the page fault fails. But since because arch_uprobe_xol_was_trapped()
is not implemented this won't happen.

Oleg.

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