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

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Message-ID: <20201026155521.GQ2594@hirez.programming.kicks-ass.net>
Date:   Mon, 26 Oct 2020 16:55:21 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kyle Huey <me@...ehuey.com>
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 07:33:08AM -0700, Kyle Huey wrote:
> 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 :)

Urgh, now I have to try and remember how all that worked again ...

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")


Just to clarify, the sequence is something like:

 - tracer: ptrace(PTRACE_SINGLESTEP)
 - tracee: #DB, DR6 contains DR_STEP
 - tracer: ptrace_get_debugreg(6)

?

You're right that that would be broken, let me try and figure out what
the best way would be 'fix' that.

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