linux-kernel - Re: [PATCH v5 00/21] kprobes: Unify kretprobe trampoline handlers and make kretprobe lockless

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Date:   Wed, 2 Sep 2020 15:42:52 +0200
From:   peterz@...radead.org
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Eddy_Wu@...ndmicro.com, x86@...nel.org, davem@...emloft.net,
        rostedt@...dmis.org, naveen.n.rao@...ux.ibm.com,
        anil.s.keshavamurthy@...el.com, linux-arch@...r.kernel.org,
        cameron@...dycamel.com, oleg@...hat.com, will@...nel.org,
        paulmck@...nel.org
Subject: Re: [PATCH v5 00/21] kprobes: Unify kretprobe trampoline handlers
 and make kretprobe lockless

On Wed, Sep 02, 2020 at 10:19:26PM +0900, Masami Hiramatsu wrote:
> On Wed, 2 Sep 2020 11:36:13 +0200
> peterz@...radead.org wrote:
> 
> > On Wed, Sep 02, 2020 at 05:17:55PM +0900, Masami Hiramatsu wrote:
> > 
> > > > Ok, but then lockdep will yell at you if you have that enabled and run
> > > > the unoptimized things.
> > > 
> > > Oh, does it warn for all spinlock things in kprobes if it is unoptimized?
> > > Hmm, it has to be noted in the documentation.
> > 
> > Lockdep will warn about spinlocks used in NMI context that are also used
> > outside NMI context.
> 
> OK, but raw_spin_lock_irqsave() will not involve lockdep, correct?

It will. The distinction between spin_lock and raw_spin_lock is only
that raw_spin_lock stays a spinlock on PREEMPT_RT, while spin_lock will
turn into a (PI) mutex in that case.

But both will call into lockdep. Unlike local_irq_disable() and
raw_local_irq_disable(), where the latter will not. Yes your prefixes
are a mess :/

> > Now, for the kretprobe that kprobe_busy flag prevents the actual
> > recursion self-deadlock, but lockdep isn't smart enough to see that.
> > 
> > One way around this might be to use SINGLE_DEPTH_NESTING for locks when
> > we use them from INT3 context. That way they'll have a different class
> > and lockdep will not see the recursion.
> 
> Hmm, so lockdep warns only when it detects the spinlock in NMI context,
> and int3 is now always NMI, thus all spinlock (except raw_spinlock?)
> in kprobe handlers should get warned, right?
> I have tested this series up to [16/21] with optprobe disabled, but
> I haven't see the lockdep warnings.

There's a bug, that might make it miss it. I have a patch. I'll send it
shortly.

> > pre_handler_kretprobe() is always called from INT3, right?
> 
> No, not always, it can be called from optprobe (same as original code
> context) or ftrace handler.
> But if you set 0 to /proc/sys/debug/kprobe_optimization, and compile
> the kernel without function tracer, it should always be called from
> INT3.

D'oh, ofcourse! Arguably I should make the optprobe context NMI like
too.. but that's for another day.