linux-kernel - Re: [patch V4 part 4 07/24] x86/traps: Split int3 handler up

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Date:   Wed, 13 May 2020 22:03:13 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [patch V4 part 4 07/24] x86/traps: Split int3 handler up

On Tue, May 5, 2020 at 7:16 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> For code simplicity split up the int3 handler into a kernel and user part
> which makes the code flow simpler to understand.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  arch/x86/kernel/traps.c |   67 +++++++++++++++++++++++++++---------------------
>  1 file changed, 39 insertions(+), 28 deletions(-)
>
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -564,6 +564,35 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_pr
>         cond_local_irq_disable(regs);
>  }
>
> +static bool do_int3(struct pt_regs *regs)
> +{
> +       int res;
> +
> +#ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
> +       if (kgdb_ll_trap(DIE_INT3, "int3", regs, 0, X86_TRAP_BP,
> +                        SIGTRAP) == NOTIFY_STOP)
> +               return true;
> +#endif /* CONFIG_KGDB_LOW_LEVEL_TRAP */
> +
> +#ifdef CONFIG_KPROBES
> +       if (kprobe_int3_handler(regs))
> +               return true;
> +#endif
> +       res = notify_die(DIE_INT3, "int3", regs, 0, X86_TRAP_BP, SIGTRAP);
> +
> +       return res == NOTIFY_STOP;
> +}
> +
> +static void do_int3_user(struct pt_regs *regs)
> +{
> +       if (do_int3(regs))
> +               return;
> +
> +       cond_local_irq_enable(regs);
> +       do_trap(X86_TRAP_BP, SIGTRAP, "int3", regs, 0, 0, NULL);
> +       cond_local_irq_disable(regs);
> +}
> +
>  DEFINE_IDTENTRY_RAW(exc_int3)
>  {
>         /*
> @@ -581,37 +610,19 @@ DEFINE_IDTENTRY_RAW(exc_int3)
>          * because the INT3 could have been hit in any context including
>          * NMI.
>          */
> -       if (user_mode(regs))
> +       if (user_mode(regs)) {
>                 idtentry_enter(regs);
> -       else
> -               nmi_enter();
> -
> -       instr_begin();
> -#ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
> -       if (kgdb_ll_trap(DIE_INT3, "int3", regs, 0, X86_TRAP_BP,
> -                               SIGTRAP) == NOTIFY_STOP)
> -               goto exit;
> -#endif /* CONFIG_KGDB_LOW_LEVEL_TRAP */
> -
> -#ifdef CONFIG_KPROBES
> -       if (kprobe_int3_handler(regs))
> -               goto exit;
> -#endif
> -
> -       if (notify_die(DIE_INT3, "int3", regs, 0, X86_TRAP_BP,
> -                       SIGTRAP) == NOTIFY_STOP)
> -               goto exit;
> -
> -       cond_local_irq_enable(regs);
> -       do_trap(X86_TRAP_BP, SIGTRAP, "int3", regs, 0, 0, NULL);
> -       cond_local_irq_disable(regs);
> -
> -exit:
> -       instr_end();
> -       if (user_mode(regs))
> +               instr_begin();
> +               do_int3_user(regs);
> +               instr_end();
>                 idtentry_exit(regs);
> -       else
> +       } else {
> +               nmi_enter();
> +               instr_begin();
> +               do_int3(regs);

I think you should be checking the return value here.  Presumably this
should die() if it's not handled, since otherwise it will just
infinite loop.

> +               instr_end();
>                 nmi_exit();
> +       }
>  }
>
>  #ifdef CONFIG_X86_64
>