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Date:   Tue, 8 Jan 2019 11:56:48 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Andrea Righi <righi.andrea@gmail.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, peterz@infradead.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/2] kprobes: Fix kretprobe incorrect stacking order
 problem
Message-Id: <20190108115648.5ecf625bf5477ad3f3148b80@kernel.org>
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Hi Andrea and Steve,

On Mon, 7 Jan 2019 22:34:39 +0100
Andrea Righi <righi.andrea@gmail.com> wrote:

> On Mon, Jan 07, 2019 at 04:28:33PM -0500, Steven Rostedt wrote:
> > On Mon, 7 Jan 2019 22:19:04 +0100
> > Andrea Righi <righi.andrea@gmail.com> wrote:
> > 
> > > > > If we put a kretprobe to raw_spin_lock_irqsave() it looks like
> > > > > kretprobe is going to call kretprobe...  
> > > > 
> > > > Right, but we should be able to add some recursion protection to stop
> > > > that. I have similar protection in the ftrace code.  
> > > 
> > > If we assume that __raw_spin_lock/unlock*() are always inlined a
> > 
> > I wouldn't assume that.
> > 
> > > possible way to prevent this recursion could be to use directly those
> > > functions to do locking from the kretprobe trampoline.
> > > 
> > > But I'm not sure if that's a safe assumption... if not I'll see if I can
> > > find a better solution.
> > 
> > All you need to do is have a per_cpu variable, where you just do:
> > 
> > 	preempt_disable_notrace();
> > 	if (this_cpu_read(kprobe_recursion))
> > 		goto out;
> > 	this_cpu_inc(kprobe_recursion);
> > 	[...]
> > 	this_cpu_dec(kprobe_recursion);
> > out:
> > 	preempt_enable_notrace();
> > 
> > And then just ignore any kprobes that trigger while you are processing
> > the current kprobe.
> > 
> > Something like that. If you want (or if it already happens) replace
> > preempt_disable() with local_irq_save().
> 
> Oh.. definitely much better. I'll work on that and send a new patch.
> Thanks for the suggestion!

Thank you for pointing it out, 

Since we already have current_kprobe per_cpu, it can be done by setting up
a dummy kprobe on it. I'll add that in v2 series.

Actually, this bug has been introduced a long time ago by me... when I
introduced asm-coded kretprobe-trampoline. Before that, kretprobe trampoline
handler uses a kprobe to hook it, so the 2nd kretprobe must be skipped
automatically.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>