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

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Message-Id: <4EFF3B04-2C8A-4D63-BB63-B5804EBFFE2F@amacapital.net>
Date:   Sat, 29 Feb 2020 11:25:24 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <JGross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [patch 4/8] x86/entry: Move irq tracing on syscall entry to C-code



> On Feb 29, 2020, at 6:44 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> Thomas Gleixner <tglx@...utronix.de> writes:
>> Andy Lutomirski <luto@...capital.net> writes:
>>>>> On Feb 26, 2020, at 12:17 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>>>> On 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. I’m fine with this patch, but I still don’t understand what
>>> the changelog is about.
>> 
>> Yeah, the changelog is not really helpful. Let me fix that.
>> 
>>> And I’m 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 we’re 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
> 
>