Re: [PATCH v2 5/8] x86/debug: Simplify #DB signal code

[Andy Lutomirski]

On Fri, Aug 21, 2020 at 3:21 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Get rid of the two variables, avoid computing si_code when not needed
> and be consistent about which dr6 value is used.
>

> -       if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS) || user_icebp)
> -               send_sigtrap(regs, 0, si_code);
> +       /*
> +        * 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.
> +        * User wants a sigtrap for that.
> +        */
> +       if (dr6 & (DR_STEP | DR_TRAP_BITS) || !dr6)
> +               send_sigtrap(regs, 0, get_si_code(dr6));

The old condition was ... || (actual DR6 value) and the new condition
was ... || (stupid notifier-modified DR6 value).  I think the old code
was more correct.

The right fix is to get rid of the notifier garbage.  Want to pick up
these two changes into your series:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry&id=b695a5adfdd661a10eead1eebd4002d08400e7ef
https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/entry&id=40ca016606bd39a465feaf5802a8dc3e937aaa06

And then there is no code left that modifies ->debugreg6 out from under you.

--Andy