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

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Date:   Mon, 26 Oct 2020 07:33:08 -0700
From:   Kyle Huey <me@...ehuey.com>
To:     open list <linux-kernel@...r.kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Cc:     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: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no longer
 reported in dr6

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