Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 27 Oct 2020 20:20:09 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Kyle Huey <me@kylehuey.com>,
        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>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Daniel Thompson <daniel.thompson@linaro.org>
Subject: Re: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no
 longer reported in dr6
Message-ID: <20201027192009.GQ2628@hirez.programming.kicks-ass.net>
References: <CAP045Ar5CtqknH66i5ti6xOvo9cC9ib5v-5+3fFKcp_DW91hYw@mail.gmail.com>
 <20201026155521.GQ2594@hirez.programming.kicks-ass.net>
 <20201026160513.GC2651@hirez.programming.kicks-ass.net>
 <CAP045ApB_9h5Pp=a0L+taA6qFURrR6Se+W77Vb7A_VOWJNKfng@mail.gmail.com>
 <20201026163100.GR2594@hirez.programming.kicks-ass.net>
 <20201026165519.GD2651@hirez.programming.kicks-ass.net>
 <CALCETrVwzcpk88jWeNb+iCGBFsyzgbZ0E9_x330A2P-CMzSr4g@mail.gmail.com>
 <20201027081932.GY2611@hirez.programming.kicks-ass.net>
 <CALCETrUcx_GaYhUbdc82faeftn+PssdWbWSPWELZ7Npcrbd92w@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CALCETrUcx_GaYhUbdc82faeftn+PssdWbWSPWELZ7Npcrbd92w@mail.gmail.com>
Precedence: bulk

On Tue, Oct 27, 2020 at 11:00:52AM -0700, Andy Lutomirski wrote:
> On Tue, Oct 27, 2020 at 1:19 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Mon, Oct 26, 2020 at 04:30:32PM -0700, Andy Lutomirski wrote:
> >
> > > > @@ -935,6 +936,26 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
> > > >         irqentry_enter_from_user_mode(regs);
> > > >         instrumentation_begin();
> > > >
> > > > +       /*
> > > > +        * Clear the virtual DR6 value, ptrace routines will set bits here for
> > > > +        * things we want signals for.
> > > > +        */
> > > > +       current->thread.virtual_dr6 = 0;
> > > > +
> > > > +       /*
> > > > +        * If PTRACE requested SINGLE(BLOCK)STEP, make sure to reflect that in
> > > > +        * the ptrace visible DR6 copy.
> > > > +        */
> > > > +       if (test_thread_flag(TIF_BLOCKSTEP) || test_thread_flag(TIF_SINGLESTEP))
> > > > +               current->thread.virtual_dr6 |= (dr6 & DR_STEP);
> > >
> > > I'm guessing that this would fail a much simpler test, though: have a
> > > program use PUSHF to set TF and then read out DR6 from the SIGTRAP.  I
> > > can whip up such a test if you like.
> >
> > Kyle also mentioned it. The reason I didn't do that is because ptrace()
> > didn't set the TF, so why should it see it in ptrace_get_debugreg(6) ?
> 
> I assume you already figured this out, but my specific concern is with
> the get_si_code(dr6) part -- that's sent directly to the task being
> debugged or debugging itself (and, sadly, to ptrace, and who knows
> what debuggers do).

Right, so for a task doing TF on its own, DR_STEP should remain set in
our on-stack dr6 variable, nothing should consume it.

So the get_si_code(dr6) should be identical. So the only difference is
if DR_STEP is visible in ptrace or not.