Re: [patch 4/8] x86/entry: Move irq tracing on syscall entry to C-code

[Andy Lutomirski]

> On Feb 29, 2020, at 6:44 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> ïThomas Gleixner <tglx@xxxxxxxxxxxxx> writes:
>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>>>>> On Feb 26, 2020, at 12:17 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>>>> ïOn Tue, Feb 25, 2020 at 09:43:46PM -0800, Andy Lutomirski wrote:
>>>>>> Your earlier patches suggest quite strongly that tracing isn't safe
>>>>>> until enter_from_user_mode().  But trace_hardirqs_off() calls
>>>>>> trace_irq_disable_rcuidle(), which looks [0] like a tracepoint.
>>>>>> 
>>>>>> Did you perhaps mean to do this *after* enter_from_user_mode()?
>>>>> 
>>>>> aside from the fact that enter_from_user_mode() itself also has a
>>>>> tracepoint, the crucial detail is that we must not trace/kprobe the
>>>>> function calling this.
>>>>> 
>>>>> Specifically for #PF, because we need read_cr2() before this. See later
>>>>> patches.
>>> 
>>> Indeed. Iâm fine with this patch, but I still donât understand what
>>> the changelog is about.
>> 
>> Yeah, the changelog is not really helpful. Let me fix that.
>> 
>>> And Iâm still rather baffled by most of the notrace annotations in the
>>> series.
>> 
>> As discussed on IRC, this might be too broad, but then I rather have the
>> actual C-entry points neither traceable nor probable in general and
>> relax this by calling functions which can be traced and probed.
>> 
>> My rationale for this decision was that enter_from_user_mode() is marked
>> notrace/noprobe as well, so I kept the protection scope the same as we
>> had in the ASM maze which is marked noprobe already.
> 
> I have second thoughts vs. tracing in this context.
> 
> While the tracer itself seems to handle this correctly, what about
> things like BPF programs which can be attached to tracepoints and
> function trace entries?

I think that everything using the tracing code, including BPF, should either do its own rcuidle stuff or explicitly not execute if weâre not in CONTEXT_KERNEL.  That is, we probably need to patch BPF.

> 
> Is that really safe _before_ context tracking has updated RCU state?
> 
> Thanks,
> 
>        tglx
> 
>