Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to be preempted

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 22 Jan 2015 12:58:00 -0800
Andy Lutomirski <luto@amacapital.net> wrote:

> On Thu, Jan 22, 2015 at 12:37 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> > On Thu, 22 Jan 2015 12:24:47 -0800
> > Andy Lutomirski <luto@amacapital.net> wrote:
> >
> >> > Also, please remove the "notrace", because function tracing goes an
> >> > extra step to not require RCU being visible. The only thing you get
> >> > with notrace is not being able to trace an otherwise traceable function.
> >> >
> >>
> >> Is this also true for kprobes?  And can kprobes nest inside function
> >> tracing hooks?
> >
> > No, kprobes are a bit more fragile than function tracing or tracepoints.
> >
> > And nothing should nest inside a function hook (except for interrupts,
> > they are fine).
> >
> 
> But kprobes do nest inside interrupts, right?

A kprobe being called while a function trace is happening is fine, but
you should not have the kprobe set directly inside the function trace
callback code. Because that means a kprobe could happen anywhere
function tracing is happening (for instance, in NMI context).

> 
> >>
> >> The other issue, above and beyond RCU, is that we can't let kprobes
> >> run on the int3 stack.  If Xen upcalls can happen when interrupts are
> >> off, then we may need this protection to prevent that type of
> >> recursion.  (This will be much less scary in 3.20, because userspace
> >> int3 instructions will no longer execute on the int3 stack.)
> >
> > Does this execute between the start of the int3 interrupt handler and
> > the call of do_int3()?
> 
> I doubt it.
> 
> The thing I worry about is that, if do_int3 nests inside itself by any
> means (e.g. int3 sends a signal, scheduling for whatever reason
> (really shouldn't happen, but I haven't looked that hard)), then we're
> completely hosed -- the inner int3 will overwrite the outer int3's
> stack frame.  Since I have no idea what Xen upcalls do, I don't know
> whether they can fire inside do_int3.

I thought there's logic in the do_int3 handler (in the assembly code)
that can handle nested int3s.

I'm not sure what xen does though.

-- Steve