Messages in this thread | | | Date | Tue, 27 Oct 2020 20:20:09 +0100 | From | Peter Zijlstra <> | Subject | Re: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no longer reported in dr6 |
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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.
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