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 1:16 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Thu, 22 Jan 2015 12:58:00 -0800
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
>> 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:
>>
>> >>
>> >> 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.

Nope :(

In 3.20, there's likely to be logic that can handle a single level of
nesting as long as the outer one came from user space.

--Andy

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