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

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Message-ID: <alpine.LNX.2.00.1307112141151.29788@pobox.suse.cz>
Date:	Thu, 11 Jul 2013 21:43:42 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Borislav Petkov <bpetkov@...e.de>,
	linux-kernel@...r.kernel.org, Jason Baron <jbaron@...mai.com>
Subject: Re: [RFC] [PATCH 1/2 v2] x86: introduce int3-based instruction
 patching

On Thu, 11 Jul 2013, Steven Rostedt wrote:

[ .. snip .. ]
> > +	smp_wmb();
> > +
> > +	text_poke(addr, &int3, sizeof(int3));
> > +
> > +	if (len - sizeof(int3) > 0) {
> 
> I believe we need a sync here. Otherwise, if the instruction crosses
> cache lines, the original first byte could have been pulled in, and then
> after the text_poke() below, it gets the updated version, causing a
> crash on that CPU.
> 
> 		on_each_cpu(do_sync_core, NULL, 1);

Right you are.

OTOH we apparently don't need the one after the text_poke() below, as 
syncing the cores just after patching the first byte afterwards provides 
safe enough guard (at least according to hpa's words back in 2010 :) ).

Will change it for next respin of the patchset, thanks for review.

-- 
Jiri Kosina
SUSE Labs
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