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Re: [PATCH tip/master] [BUGFIX] kprobes/x86: Fix to clear TF bit in fault-on-single-stepping
- From: Steven Rostedt <rostedt at goodmis dot org>
- To: Masami Hiramatsu <mhiramat at kernel dot org>
- Cc: Ingo Molnar <mingo at redhat dot com>, linux-kernel at vger dot kernel dot org, Peter Zijlstra <peterz at infradead dot org>, Ananth N Mavinakayanahalli <ananth at linux dot vnet dot ibm dot com>, Thomas Gleixner <tglx at linutronix dot de>, "H . Peter Anvin" <hpa at zytor dot com>, Andy Lutomirski <luto at kernel dot org>, systemtap at sourceware dot org, Linus Torvalds <torvalds at linux-foundation dot org>, fenghua dot yu at intel dot com
- Date: Mon, 13 Jun 2016 19:20:19 -0400
- Subject: Re: [PATCH tip/master] [BUGFIX] kprobes/x86: Fix to clear TF bit in fault-on-single-stepping
- Authentication-results: sourceware.org; auth=none
- References: <20160611140648 dot 25885 dot 37482 dot stgit at devbox> <20160613191345 dot 6bdeb535 at gandalf dot local dot home>
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.
-- Steve