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

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Date:   Mon, 26 Oct 2020 11:36:41 -0700
From:   Kyle Huey <me@...ehuey.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 10:18 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Oct 26, 2020 at 10:12:30AM -0700, Kyle Huey wrote:
> > On Mon, Oct 26, 2020 at 9:55 AM Peter Zijlstra <peterz@...radead.org> 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);
> > > +
> > > +       /*
> > > +        * The SDM says "The processor clears the BTF flag when it
> > > +        * generates a debug exception."  Clear TIF_BLOCKSTEP to keep
> > > +        * TIF_BLOCKSTEP in sync with the hardware BTF flag.
> > > +        */
> > > +       clear_thread_flag(TIF_BLOCKSTEP);
> > > +
> > >         /*
> > >          * If dr6 has no reason to give us about the origin of this trap,
> > >          * then it's very likely the result of an icebp/int01 trap.
> >
> > This looks good to me (at least the non BTF parts), and I'll test it
> > shortly, but especially now that clearing virtual_dr6 is moved to
> > exc_debug_user I still don't see why it's not ok to copy the entire
> > dr6 value into virtual_dr6 unconditionally.  Any extraneous dr6 state
> > from an in-kernel #DB would have been picked up and cleared already
> > when we entered exc_debug_kernel.
>
> There is !ptrace user breakpoints as well. Why should we want potential
> random bits in dr6 ?
>
> Suppose perf and ptrace set a user breakpoint on the exact same
> instruction. The #DB fires and has two DR_TRAP# bits set. perf consumes
> one and ptrace consumes one.
>
> Only the ptrace one should be visible to ptrace, the perf one doesn't
> affect the userspace execution at all and shouldn't be visible.

Ok. Makes sense.

I can confirm that your second patch does fix the behavior I was
seeing and rr works again.

- Kyle