linux-kernel - Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi

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Date:   Fri, 29 Jan 2021 14:01:03 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Nikolay Borisov <nborisov@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter()
 with nmi_enter()")

On Fri, 29 Jan 2021 18:59:43 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, Jan 29, 2021 at 09:45:48AM -0800, Alexei Starovoitov wrote:
> > Same things apply to bpf side. We can statically prove safety for
> > ftrace and kprobe attaching whereas to deal with NMI situation we
> > have to use run-time checks for recursion prevention, etc.  
> 
> I have no idea what you're saying. You can attach to functions that are
> called with random locks held, you can create kprobes in some very
> sensitive places.
> 
> What can you staticlly prove about that?

I think the main difference is, if you attach a kprobe or ftrace function,
you can theoretically analyze the location before you do the attachment.

Does, the NMI context mean "in_nmi()" returns true? Because there's cases
in ftrace callbacks where that is checked (like the stack tracer). And
having ftrace return true for "in_nmi()" will break a lot of existing
utilities.

-- Steve