Re: Instrumentation and RCU

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Mar 10, 2020, at 8:18 PM, Masami Hiramatsu mhiramat@kernel.org wrote:
[...]
 
>> An approach where the "in_tracer" flag is tested and set by the instrumentation
>> (function tracer, kprobes, tracepoints) would work here. Let's say the beginning
>> of the int3 ISR is part of the code which is invisible to instrumentation, and
>> before we issue rcu_nmi_enter(), we handle the in_tracer flag:
>> 
>> rcu_nmi_enter();
>>  <int3>
>>     (recursion_ctx->in_tracer == false)
>>     set recursion_ctx->in_tracer = true
>>     do_int3() {
>>        rcu_nmi_enter();
>>          <int3>
>>             if (recursion_ctx->in_tracer == true)
>>                 iret
>> 
>> We can change "in_tracer" for "in_breakpoint", "in_tracepoint" and
>> "in_function_trace" if we ever want to allow different types of instrumentation
>> to nest. I'm not sure whether this is useful or not through.
> 
> Kprobes already has its own "in_kprobe" flag, and the recursion path is
> not so simple. Since the int3 replaces the original instruction, we have to
> execute the original instruction with single-step and fixup.
> 
> This means it involves do_debug() too. Thus, we can not do iret directly
> from do_int3 like above, but if recursion happens, we have no way to
> recover to origonal execution path (and call BUG()).

I think that all the code involved when hitting a breakpoint which would
be the minimal subset required to act as if the kprobe was not there in the
first place (single-step, fixup) should be hidden from kprobes
instrumentation. I suspect this is the current intent today with noprobe
annotations, but Thomas' proposal brings this a step further.

However, any other kprobe code (and tracer callbacks) beyond that
minimalistic "effect-less" kprobe could be protected by a
per-recursion-context in_kprobe flag.

> As my previous email, I showed a patch which is something like
> "bust_kprobes()" for oops path. That is not safe but no other way to escape
> from this recursion hell. (Maybe we can try to call it instead of calling
> BUG() so that the kernel can continue to run, but I'm not sure we can
> safely make the pagetable to readonly again.)

As long as we provide a minimalistic "effect-less" kprobe implementation
in a non-instrumentable section which can be used whenever we are in a
recursion scenario, I think we could achieve something recursion-free without
requiring a bust_kprobes() work-around.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com