Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()")

[Steven Rostedt]

On Fri, 29 Jan 2021 18:59:43 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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