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From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	linux-tip-commits@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>, x86 <x86@kernel.org>
Subject: Re: [tip: x86/entry] x86/entry: Treat BUG/WARN as NMI-like entries
Date: Mon, 15 Jun 2020 21:44:58 +0200	[thread overview]
Message-ID: <20200615194458.GL2531@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CALCETrWhbg_61CTo9_T6s1NDFvOgUx7ebSzhXj7O_m8htePwKA@mail.gmail.com>

On Mon, Jun 15, 2020 at 10:06:20AM -0700, Andy Lutomirski wrote:
> On Mon, Jun 15, 2020 at 7:50 AM Peter Zijlstra <peterz@infradead.org> wrote:

> Hmm.  IMO you're making two changes here, and this is fiddly enough
> that it might be worth separating them for bisection purposes.

Sure, can do.

> > ---
> >
> > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> > index af75109485c26..a47e74923c4c8 100644
> > --- a/arch/x86/kernel/traps.c
> > +++ b/arch/x86/kernel/traps.c
> > @@ -218,21 +218,22 @@ static inline void handle_invalid_op(struct pt_regs *regs)
> >
> >  DEFINE_IDTENTRY_RAW(exc_invalid_op)
> >  {
> > -       bool rcu_exit;
> > -
> >         /*
> >          * Handle BUG/WARN like NMIs instead of like normal idtentries:
> >          * if we bugged/warned in a bad RCU context, for example, the last
> >          * thing we want is to BUG/WARN again in the idtentry code, ad
> >          * infinitum.
> >          */
> > -       if (!user_mode(regs) && is_valid_bugaddr(regs->ip)) {
> > -               enum bug_trap_type type;
> > +       if (!user_mode(regs)) {
> > +               enum bug_trap_type type = BUG_TRAP_TYPE_NONE;
> >
> >                 nmi_enter();
> >                 instrumentation_begin();
> >                 trace_hardirqs_off_finish();
> > -               type = report_bug(regs->ip, regs);
> > +
> > +               if (is_valid_bugaddr(regs->ip))
> > +                       type = report_bug(regs->ip, regs);
> > +
> 
> Sigh, this is indeed necessary.

:-)

> >                 if (regs->flags & X86_EFLAGS_IF)
> >                         trace_hardirqs_on_prepare();
> >                 instrumentation_end();
> > @@ -249,13 +250,16 @@ DEFINE_IDTENTRY_RAW(exc_invalid_op)
> >                  * was just a normal #UD, we want to continue onward and
> >                  * crash.
> >                  */
> > -       }
> > +               handle_invalid_op(regs);
> 
> But this is really a separate change.  This makes handle_invalid_op()
> be NMI-like even for non-BUG/WARN kernel #UD entries.  One might argue
> that this doesn't matter, and that's probably right, but I think it
> should be its own change with its own justification.  With just my
> patch, I intentionally call handle_invalid_op() via the normal
> idtentry_enter_cond_rcu() path.

All !user exceptions really should be NMI-like. If you want to go
overboard, I suppose you can look at IF and have them behave interrupt
like when set, but why make things complicated.

Anyway, let me to smaller and proper patches for this.

  reply	other threads:[~2020-06-15 19:45 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-12  3:26 [PATCH] " Andy Lutomirski
2020-06-12  4:13 ` Andy Lutomirski
2020-06-12 19:50 ` [tip: x86/entry] " tip-bot2 for Andy Lutomirski
2020-06-15 14:50   ` Peter Zijlstra
2020-06-15 17:06     ` Andy Lutomirski
2020-06-15 19:44       ` Peter Zijlstra [this message]
2020-06-15 21:08         ` Andy Lutomirski
2020-06-15 22:23           ` Peter Zijlstra
2020-06-15 22:46             ` Andy Lutomirski
2020-06-16 11:14               ` Peter Zijlstra

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