linux-kernel - Re: [RFC] [PATCH 1/2 v2] x86: introduce int3-based instruction patching

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Date:	Thu, 11 Jul 2013 10:35:44 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Jason Baron <jbaron@...hat.com>,
	Borislav Petkov <bpetkov@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH 1/2 v2] x86: introduce int3-based instruction
 patching

On Wed, 2013-07-10 at 14:36 -0700, H. Peter Anvin wrote:
> On 07/10/2013 02:31 PM, Jiri Kosina wrote:
> > 
> > If any CPU instruction execution would collide with the patching,
> > it'd be trapped by the int3 breakpoint and redirected to the provided
> > "handler" (which would typically mean just skipping over the patched
> > region, acting as "nop" has been there, in case we are doing nop -> jump
> > and jump -> nop transitions).
> > 
> 
> I'm wondering if it would be easier/more general to just return to the
> instruction.  The "more general" bit would allow this to be used for
> other things, like alternatives, and perhaps eventually dynamic call
> patching.
> 
> Returning to the instruction will, in effect, be a busy-wait for the
> faulted CPU until the patch is complete; more or less what stop_machine
> would do, but only for a CPU which actually strays into the affected region.
> 

Wont work for ftrace, as it patches all functions, it even patches
functions used to do the changes. Thus, it would cause a deadlock if a
breakpoint were to spin till the changes were finished.

-- Steve


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