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From:   Kyle Huey <me@kylehuey.com>
Date:   Mon, 26 Oct 2020 09:08:38 -0700
Message-ID: <CAP045Aoe2531x+dGcEHHiUyhzfz3GbKOeY0jmktbw6C1UK1Nvw@mail.gmail.com>
Subject: Re: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no
 longer reported in dr6
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     open list <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Robert O'Callahan" <rocallahan@gmail.com>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Brian Gerst <brgerst@gmail.com>,
        Andy Lutomirski <luto@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Daniel Thompson <daniel.thompson@linaro.org>
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On Mon, Oct 26, 2020 at 8:55 AM Peter Zijlstra <peterz@infradead.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