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

[Andy Lutomirski]

> On Apr 28, 2019, at 10:51 AM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> On Sun, 28 Apr 2019 10:41:10 -0700
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> 
> 
>>> Note that at any given point
>>> in time, there can be at most four such call insn emulations pending:
>>> namely at most one per "process", "irq", "softirq" and "nmi" context.
>>> 
>> 
>> Thatâs quite an assumption. I think your list should also contain
>> exception, exceptions nested inside that exception, and machine
>> check, at the very least.  Iâm also wondering why irq and softirq are
>> treated separately.
> 
> 4 has usually been the context count we choose. But I guess in theory,
> if we get exceptions then I could potentially be more.
> 
> As for irq vs softirq, an interrupt can preempt a softirq. Interrupts
> are enabled while softirqs are running. When sofirqs run, softirqs are
> disabled to prevent nested softirqs. But interrupts are enabled again,
> and another interrupt may come in while a softirq is executing.
> 
>> 
>> All this makes me think that one of the other solutions we came up
>> with last time we discussed this might be better.
> 
> +100
> 
> 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 still think I prefer an approach where we just emulate the call directly.