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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Thu, Jan 22, 2015 at 12:37 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Thu, 22 Jan 2015 12:24:47 -0800
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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?

>>
>> 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.

--Andy

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