linux-kernel - Re: Oops caused by race between livepatch and ftrace

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Date:   Tue, 21 May 2019 09:16:29 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Johannes Erdfelt <johannes@...felt.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jessica Yu <jeyu@...nel.org>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Ingo Molnar <mingo@...hat.com>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Oops caused by race between livepatch and ftrace

On Mon, May 20, 2019 at 05:39:10PM -0400, Steven Rostedt wrote:
> On Mon, 20 May 2019 16:19:31 -0500
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index a12aff849c04..8259d4ba8b00 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -34,6 +34,7 @@
> >  #include <linux/hash.h>
> >  #include <linux/rcupdate.h>
> >  #include <linux/kprobes.h>
> > +#include <linux/memory.h>
> >  
> >  #include <trace/events/sched.h>
> >  
> > @@ -2610,10 +2611,12 @@ static void ftrace_run_update_code(int command)
> >  {
> >  	int ret;
> >  
> > +	mutex_lock(&text_mutex);
> > +
> 
> Hmm, this may blow up with lockdep, as I believe we already have a
> locking dependency of:
> 
>  text_mutex -> ftrace_lock
> 
> And this will reverses it. (kprobes appears to take the locks in this
> order).
> 
> Perhaps have live kernel patching grab ftrace_lock?

Where does kprobes call into ftrace with the text_mutex?  I couldn't
find it.

-- 
Josh