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

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Date:   Tue, 21 May 2019 10:42:04 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 Tue, 21 May 2019 09:16:29 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> > 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.

Hmm, maybe it doesn't. I was looking at the arm_kprobe_ftrace() but
it doesn't call it with text_mutex().

Maybe it is fine, but we had better perform a lot of testing with
lockdep on to make sure.

-- Steve