Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3_stack member

[Andy Lutomirski]

> On Apr 28, 2019, at 12:43 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> On Sun, 28 Apr 2019 11:08:34 -0700
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> 
>>> 
>>> Perhaps adding another slot into pt_regs that gets used by int3 to
>>> store a slot to emulate a call on return?
>>> 
>>> 
>> 
>> Thatâs not totally nuts, although finding pt_regs isnât entirely trivial.
> 
> I meant on the int3 handler (which stores the pt_regs).

But thatâs below the stubâs RSP, so itâs toast if another interrupt happens. Or am I misunderstanding you?

> 
>> 
>> I still think I prefer an approach where we just emulate the call directly.
> 
> Then, on the return of int3, if there's anything in that slot, then we
> could possibly shift the exception handler frame (that was added by the
> hardware), insert the slot data into the top of the stack, and then
> call iret (which the int3 handler, would add the return ip to be the
> function being called), which would in essence emulate the call directly.

Oh, I get it.

I liked Joshâs old proposal of unconditionally shifting the #BP frame 8 bytes better. It will be interesting when kernel shadow stacks are thrown in the mix, but thatâs a problem for another day.