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Date: Tue, 14 Jun 2016 11:59:40 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H . Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@kernel.org>,
        systemtap@sourceware.org,
        Linus Torvalds <torvalds@linux-foundation.org>, fenghua.yu@intel.com
Subject: Re: [PATCH tip/master] [BUGFIX] kprobes/x86: Fix to clear TF bit in
 fault-on-single-stepping
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* Steven Rostedt <rostedt@goodmis.org> wrote:

> On Mon, 13 Jun 2016 19:13:45 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> 
> > >   # cd /sys/kernel/debug/tracing
> > >   # echo p copy_user_enhanced_fast_string+5 > kprobe_events
> > >   # echo 1 > events/kprobes/enable
> > > 
> > > And you'll see a kernel panic on do_debug(), since the debug
> > > trap is not handled by kprobes.
> > > 
> > > To fix this problem, we just need to clear the TF bit when
> > > resetting running kprobe.
> > >   
> > 
> > This should definitely be marked for stable, and I bisected it all the
> > way down to this commit: f4cb1cc18f364d "x86-64, copy_user: Remove zero
> > byte check before copy user buffer."
> > 
> > I reverted that commit and sure enough, this bug goes away. I'm not
> > saying the revert should be done. I'm just doing an FYI, and showing how
> > changes that appear to be a nice clean up can have subtle effects. I'm
> > not even sure how that change caused this to be a problem with kprobes.
> > 
> 
> Nevermind, reverting that commit only moved the location of the
> "rep movsb" that you were placing the kprobe on. When I do:
> 
>   echo "p copy_user_enhanced_fast_string+9" > kprobe_events
> 
> I get the same result.
> 
> That means we need to make that stable tag even earlier.

I added:

  Cc: stable@vger.kernel.org # All the way back to ancient kernels

Thanks,

	Ingo