linux-kernel - Re: [tip: x86/entry] x86/entry: Treat BUG/WARN as NMI-like entries

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Date:   Mon, 15 Jun 2020 10:06:20 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-tip-commits@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86 <x86@...nel.org>
Subject: Re: [tip: x86/entry] x86/entry: Treat BUG/WARN as NMI-like entries

On Mon, Jun 15, 2020 at 7:50 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Jun 12, 2020 at 07:50:08PM -0000, tip-bot2 for Andy Lutomirski wrote:
> > +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)) {
>
> vmlinux.o: warning: objtool: exc_invalid_op()+0x47: call to probe_kernel_read() leaves .noinstr.text section
>
> > +             enum bug_trap_type type;
> > +
> > +             nmi_enter();
> > +             instrumentation_begin();
> > +             trace_hardirqs_off_finish();
> > +             type = report_bug(regs->ip, regs);
> > +             if (regs->flags & X86_EFLAGS_IF)
> > +                     trace_hardirqs_on_prepare();
> > +             instrumentation_end();
> > +             nmi_exit();
> > +
> > +             if (type == BUG_TRAP_TYPE_WARN) {
> > +                     /* Skip the ud2. */
> > +                     regs->ip += LEN_UD2;
> > +                     return;
> > +             }
> > +
> > +             /*
> > +              * Else, if this was a BUG and report_bug returns or if this
> > +              * was just a normal #UD, we want to continue onward and
> > +              * crash.
> > +              */
> > +     }
> > +
> > +     rcu_exit = idtentry_enter_cond_rcu(regs);
> > +     instrumentation_begin();
> >       handle_invalid_op(regs);
> > +     instrumentation_end();
> > +     idtentry_exit_cond_rcu(regs, rcu_exit);
> >  }
>
>
> For now something like so will do, but we need a DEFINE_IDTENTRY_foo()
> for the whole:
>
>         if (user_mode()) {
>                 rcu = idtentry_enter_cond_rcu()
>                 foo_user()
>                 idtentry_exit_cond_rcu(rcu);
>         } else {
>                 nmi_enter();
>                 foo_kernel()
>                 nmi_exit()
>         }
>
> thing, we're repeating that far too often.
>
>

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

> ---
>
> 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.

--Andy