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?
--
Josh
