On Tue, Oct 27, 2020 at 11:00:52AM -0700, Andy Lutomirski wrote: > On Tue, Oct 27, 2020 at 1:19 AM Peter Zijlstra <[email protected]> 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.
