linux-kernel - Re: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR

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Date:   Mon, 26 Oct 2020 09:08:38 -0700
From:   Kyle Huey <me@...ehuey.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     open list <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Robert O'Callahan" <rocallahan@...il.com>,
        Alexandre Chartre <alexandre.chartre@...cle.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>,
        Joel Fernandes <joel@...lfernandes.org>,
        Steven Rostedt <rostedt@...dmis.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: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no
 longer reported in dr6

On Mon, Oct 26, 2020 at 8:55 AM Peter Zijlstra <peterz@...radead.org> wrote:
> Urgh, now I have to try and remember how all that worked again ...

Sorry.

> I suspect it's either one (or both) of the last two:
>
>   f4956cf83ed1 ("x86/debug: Support negative polarity DR6 bits")
>   d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to thread.virtual_dr6")

I think it's the latter, particularly the removal of this assignment[0]

> Just to clarify, the sequence is something like:
>
>  - tracer: ptrace(PTRACE_SINGLESTEP)
>  - tracee: #DB, DR6 contains DR_STEP
>  - tracer: ptrace_get_debugreg(6)

Right.

> Also, can you confirm that pthread_set_debugreg(6) should not do
> anything useful?

I don't believe it did anything useful.

- Kyle

[0] https://github.com/torvalds/linux/commit/d53d9bc0cf78#diff-51ce909c2f65ed9cc668bc36cc3c18528541d8a10e84287874cd37a5918abae5L790