On Mon, Oct 26, 2020 at 10:18 AM Peter Zijlstra <pet...@infradead.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 <pet...@infradead.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
