Re: [PATCH 0/2] kprobes: Fix kretprobe incorrect stacking order problem
From: Masami Hiramatsu
Date: Mon Jan 07 2019 - 21:56:54 EST
Hi Andrea and Steve,
On Mon, 7 Jan 2019 22:34:39 +0100
Andrea Righi <righi.andrea@xxxxxxxxx> 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@xxxxxxxxx> 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
Masami Hiramatsu <mhiramat@xxxxxxxxxx>