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

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Date:   Wed, 26 Feb 2020 09:17:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>
Subject: Re: [patch 4/8] x86/entry: Move irq tracing on syscall entry to
 C-code

On Tue, Feb 25, 2020 at 09:43:46PM -0800, Andy Lutomirski wrote:
> On 2/25/20 2:08 PM, Thomas Gleixner wrote:
> > Now that the C entry points are safe, move the irq flags tracing code into
> > the entry helper.
> 
> I'm so confused.
> 
> > 
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > ---
> >  arch/x86/entry/common.c          |    5 +++++
> >  arch/x86/entry/entry_32.S        |   12 ------------
> >  arch/x86/entry/entry_64.S        |    2 --
> >  arch/x86/entry/entry_64_compat.S |   18 ------------------
> >  4 files changed, 5 insertions(+), 32 deletions(-)
> > 
> > --- a/arch/x86/entry/common.c
> > +++ b/arch/x86/entry/common.c
> > @@ -57,6 +57,11 @@ static inline void enter_from_user_mode(
> >   */
> >  static __always_inline void syscall_entry_fixups(void)
> >  {
> > +	/*
> > +	 * Usermode is traced as interrupts enabled, but the syscall entry
> > +	 * mechanisms disable interrupts. Tell the tracer.
> > +	 */
> > +	trace_hardirqs_off();
> 
> 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.