linux-kernel - Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

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Date:   Thu, 29 Oct 2020 10:37:44 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>, Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        live-patching@...r.kernel.org
Subject: Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the
 ftrace callback

On Thu, 29 Oct 2020 14:51:06 +0100 (CET)
Miroslav Benes <mbenes@...e.cz> wrote:

> > index b552cf2d85f8..6c0164d24bbd 100644
> > --- a/kernel/livepatch/patch.c
> > +++ b/kernel/livepatch/patch.c
> > @@ -45,9 +45,13 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> >  	struct klp_ops *ops;
> >  	struct klp_func *func;
> >  	int patch_state;
> > +	int bit;
> >  
> >  	ops = container_of(fops, struct klp_ops, fops);
> >  
> > +	bit = ftrace_test_recursion_trylock();
> > +	if (bit < 0)
> > +		return;  
> 
> This means that the original function will be called in case of recursion. 
> That's probably fair, but I'm wondering if we should at least WARN about 
> it.

It's probably what happens today. But if you add a WARN_ON_ONCE() it may
not hurt.

I also plan on adding code that reports when recursion has happened,
because even if it's not a problem, recursion adds extra overhead.

-- Steve