linux-kernel - Re: [PATCH v2 5/8] x86/debug: Simplify #DB signal code

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Message-ID: <CALCETrVBB3YNCqCQ6bFBrSM10OTe354zyh=WbPFaEBxK1_Sa7g@mail.gmail.com>
Date:   Sun, 23 Aug 2020 16:09:42 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Kyle Huey <me@...ehuey.com>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        "Robert O'Callahan" <rocallahan@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        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>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Daniel Thompson <daniel.thompson@...aro.org>
Subject: Re: [PATCH v2 5/8] x86/debug: Simplify #DB signal code

On Fri, Aug 21, 2020 at 3:21 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> Get rid of the two variables, avoid computing si_code when not needed
> and be consistent about which dr6 value is used.
>

> -       if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS) || user_icebp)
> -               send_sigtrap(regs, 0, si_code);
> +       /*
> +        * If dr6 has no reason to give us about the origin of this trap,
> +        * then it's very likely the result of an icebp/int01 trap.
> +        * User wants a sigtrap for that.
> +        */
> +       if (dr6 & (DR_STEP | DR_TRAP_BITS) || !dr6)
> +               send_sigtrap(regs, 0, get_si_code(dr6));

The old condition was ... || (actual DR6 value) and the new condition
was ... || (stupid notifier-modified DR6 value).  I think the old code
was more correct.

The right fix is to get rid of the notifier garbage.  Want to pick up
these two changes into your series:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry&id=b695a5adfdd661a10eead1eebd4002d08400e7ef
https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry&id=40ca016606bd39a465feaf5802a8dc3e937aaa06

And then there is no code left that modifies ->debugreg6 out from under you.

--Andy