linux-kernel - Re: [patch 13/16] x86/entry: Move irqflags and context tracking to C for simple idtentries

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Message-ID: <20200226092018.GR18400@hirez.programming.kicks-ass.net>
Date:   Wed, 26 Feb 2020 10:20:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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 13/16] x86/entry: Move irqflags and context tracking to C
 for simple idtentries

On Wed, Feb 26, 2020 at 09:05:38AM +0100, Peter Zijlstra wrote:
> On Tue, Feb 25, 2020 at 11:33:34PM +0100, Thomas Gleixner wrote:
> 
> > --- a/arch/x86/include/asm/idtentry.h
> > +++ b/arch/x86/include/asm/idtentry.h
> > @@ -7,14 +7,31 @@
> >  
> >  #ifndef __ASSEMBLY__
> >  
> > +#ifdef CONFIG_CONTEXT_TRACKING
> > +static __always_inline void enter_from_user_context(void)
> > +{
> > +	CT_WARN_ON(ct_state() != CONTEXT_USER);
> > +	user_exit_irqoff();
> > +}
> > +#else
> > +static __always_inline void enter_from_user_context(void) { }
> > +#endif
> > +
> >  /**
> >   * idtentry_enter - Handle state tracking on idtentry
> >   * @regs:	Pointer to pt_regs of interrupted context
> >   *
> > - * Place holder for now.
> > + * Invokes:
> > + *  - The hardirq tracer to keep the state consistent as low level ASM
> > + *    entry disabled interrupts.
> > + *
> > + *  - Context tracking if the exception hit user mode
> >   */
> >  static __always_inline void idtentry_enter(struct pt_regs *regs)
> >  {
> > +	trace_hardirqs_off();
> > +	if (user_mode(regs))
> > +		enter_from_user_context();
> >  }
> 
> So:
> 
> 	asm_exc_int3
> 	  exc_int3
> 	    idtentry_enter()
> 	      enter_from_user_context
> 	        if (context_tracking_enabled())
> 
> 	    poke_int3_handler();
> 
> Is, AFAICT, completely buggered.
> 
> You can't have a static_branch before the poke_int3_handler that deals
> with text_poke.

Forgot to say that this isn't new with this patch, just noticed it when
chasing after tracepoints.

After my patch series we can actually fix this by moving the
poke_int3_handler() before idtentry_enter().