On Thu, Aug 20, 2020 at 10:16:59AM -0500, Josh Poimboeuf wrote: > On Thu, Aug 20, 2020 at 05:08:41PM +0200, pet...@infradead.org wrote: > > On Thu, Aug 20, 2020 at 10:45:12AM -0400, Brian Gerst wrote: > > > On Thu, Aug 20, 2020 at 6:53 AM Peter Zijlstra <pet...@infradead.org> > > > wrote: > > > > > > > > > > > > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> > > > > --- > > > > arch/x86/kernel/traps.c | 24 ++++++++++++------------ > > > > 1 file changed, 12 insertions(+), 12 deletions(-) > > > > > > > > --- a/arch/x86/kernel/traps.c > > > > +++ b/arch/x86/kernel/traps.c > > > > @@ -820,18 +820,6 @@ static void handle_debug(struct pt_regs > > > > goto out; > > > > } > > > > > > > > - if (WARN_ON_ONCE((dr6 & DR_STEP) && !user_mode(regs))) { > > > > - /* > > > > - * Historical junk that used to handle SYSENTER > > > > single-stepping. > > > > - * This should be unreachable now. If we survive for a > > > > while > > > > - * without anyone hitting this warning, we'll turn this > > > > into > > > > - * an oops. > > > > - */ > > > > - tsk->thread.debugreg6 &= ~DR_STEP; > > > > - set_tsk_thread_flag(tsk, TIF_SINGLESTEP); > > > > - regs->flags &= ~X86_EFLAGS_TF; > > > > - } > > > > - > > > > si_code = get_si_code(tsk->thread.debugreg6); > > > > if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS) || > > > > user_icebp) > > > > send_sigtrap(regs, 0, si_code); > > > > @@ -874,6 +862,18 @@ static __always_inline void exc_debug_ke > > > > if (kprobe_debug_handler(regs)) > > > > goto out; > > > > > > > > + if (WARN_ON_ONCE(dr6 & DR_STEP)) { > > > > + /* > > > > + * Historical junk that used to handle SYSENTER > > > > single-stepping. > > > > + * This should be unreachable now. If we survive for a > > > > while > > > > + * without anyone hitting this warning, we'll turn this > > > > into > > > > + * an oops. > > > > + */ > > > > + dr6 &= ~DR_STEP; > > > > + set_thread_flag(TIF_SINGLESTEP); > > > > + regs->flags &= ~X86_EFLAGS_TF; > > > > + } > > > > + > > > > handle_debug(regs, dr6, false); > > > > > > > > out: > > > > > > Can this be removed or changed to a BUG()? The warning has been there > > > since 2016 and nobody has apparently complained about it. > > > > Something like: > > > > /* > > * The kernel doesn't do TF outside of Kprobes, so if we get > > * here, something's fishy. > > */ > > BUG_ON(dr6 & DR_STEP); > > > > ? > > What about kernel debuggers like kgdb and qemu gdb?
qemu-gdb stub should eat the event before it lands in the guest, kgdb should eat it through the notifier. Thank you! Because that means I got the order wrong :/