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From:   Kyle Huey <me@kylehuey.com>
Date:   Mon, 26 Oct 2020 07:33:08 -0700
Message-ID: <CAP045Ar5CtqknH66i5ti6xOvo9cC9ib5v-5+3fFKcp_DW91hYw@mail.gmail.com>
Subject: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no longer
 reported in dr6
To:     open list <linux-kernel@vger.kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc:     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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After resuming a ptracee with PTRACE_SINGLESTEP, in the following
ptrace stop retrieving the dr6 value for the tracee gets a value that
does not include DR_STEP (it is in fact always DR6_RESERVED). I
bisected this to the 13cb73490f475f8e7669f9288be0bcfa85399b1f merge. I
did not bisect further.

I don't see any handling to ever set DR_STEP in virtual_dr6, so I
think this code is just broken.

Sorry for not testing this when I was CCd on the original patch series :)

- Kyle