linux-kernel - Re: [PATCH 2/2] [POWERPC] uprobes: powerpc port

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Date:	Fri, 8 Jun 2012 11:49:55 +0530
From:	Ananth N Mavinakayanahalli <ananth@...ibm.com>
To:	Michael Ellerman <michael@...erman.id.au>
Cc:	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>, oleg@...hat.com,
	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/2] [POWERPC] uprobes: powerpc port

On Fri, Jun 08, 2012 at 04:17:44PM +1000, Michael Ellerman wrote:
> On Fri, 2012-06-08 at 11:31 +0530, Ananth N Mavinakayanahalli wrote:
> > On Fri, Jun 08, 2012 at 03:51:54PM +1000, Michael Ellerman wrote:
> > > On Fri, 2012-06-08 at 10:06 +0530, Ananth N Mavinakayanahalli wrote:
> > > > On Wed, Jun 06, 2012 at 11:08:04AM -0700, Jim Keniston wrote:
> > > > > On Wed, 2012-06-06 at 15:05 +0530, Ananth N Mavinakayanahalli wrote:
> > > > > > On Wed, Jun 06, 2012 at 11:27:02AM +0200, Peter Zijlstra wrote:
> > > > > > > On Wed, 2012-06-06 at 14:51 +0530, Ananth N Mavinakayanahalli wrote:
> > > > 
> > > > ...
> > > > 
> > > > > > For the kernel, the only ones that are off limits are rfi (return from
> > > > > > interrupt), mtmsr (move to msr). All other instructions can be probed.
> > > > > > 
> > > > > > Both those instructions are supervisor level, so we won't see them in
> > > > > > userspace at all; so we should be able to probe all user level
> > > > > > instructions.
> > > > > 
> > > > > Presumably rfi or mtmsr could show up in the instruction stream via an
> > > > > erroneous or mischievous asm statement.  It'd be good to verify that you
> > > > > handle that gracefully.
> > > > 
> > > > That'd be flagged elsewhere, by the architecture itself -- you'd get a
> > > > privileged instruciton exception if you try execute any instruction not
> > > > part of the UISA. I therefore don't think its a necessary check in the
> > > > uprobes code.
> > > 
> > > But you're not executing the instruction, you're passing it to
> > > emulate_step(). Or am I missing something?
> > 
> > But MSR_PR=1 and hence emulate_step() will return -1 and hence we will
> > end up single-stepping using user_enable_single_step(). Same with rfid.
> 
> Right. But that was exactly Jim's point, you may be asked to emulate
> those instructions even though you wouldn't expect to see them in
> userspace code, so you need to handle it.
> 
> Luckily it looks like emulate_step() will do the right thing for you.
> It'd be good to test it to make 100% sure.

Sure. Will add that check and send v2.

Ananth

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