Re: [RFC][PATCH 4/7] x86/debug: Move historical SYSENTER junk into exc_debug_kernel()

[Brian Gerst]

On Thu, Aug 20, 2020 at 6:53 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  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.

--
Brian Gerst