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    SubjectRe: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no longer reported in dr6
    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).

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