linux-kernel - Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

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Date:	Wed, 20 Jan 2010 20:58:26 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Jim Keniston <jkenisto@...ibm.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Avi Kivity <avi@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>, ananth@...ibm.com,
	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	utrace-devel <utrace-devel@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Maneesh Soni <maneesh@...ibm.com>,
	Mark Wielaard <mjw@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

> Re: rewriting instructions that use rip-relative addressing.  We do that
> now.  See handle_riprel_insn() in patch #2.  (As far as we can tell, it
> works, but we'd appreciate your review of it.)

Yes, but how do you get within 2GB of it? Add lots of holes
in the address space? 

> The instruction decoder is used only during instruction analysis, while
> registering the probe -- i.e., in kernel space.

Registering the user probe? That means if there's a buffer overflow
in there it would be exploitable.

> > 
> > In general the trend has been also to make traps faster in the CPU, make 
> > sure you're not optimizing for some old CPU here.
> 
> I won't argue with that.  What Avi seems to be proposing buys us a
> speedup, but at the cost of increased complexity -- among other things,
> splitting the instrumentation code between user space (in the "XOL" area
> -- which would then be used for much more than XOL instruction slots)

You can't have a single XOL area, at least not if you want to support
shared libraries on 64bit & rip relative.

> and kernel space.  The splitting would presumably be handled by
> higher-level code -- SystemTap, perf, or whatever.  It's a neat idea,
> but it seems like a v2 kind of feature.

I'm not sure it can even work, unless you severly limited the allowed
instructions.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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