Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups

[Andy Lutomirski]

On Sun, Feb 28, 2016 at 9:28 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> While I was doing all this, I also noticed that DR6 and BTF handling
> in do_debug was a bit off.  Two of the patches in here try to fix it
> up.

It's worth noting that do_debug is really quite screwed up with or
without this patchset applied.  For example:

    /*
     * Let others (NMI) know that the debug stack is in use
     * as we may switch to the interrupt stack.
     */
    debug_stack_usage_inc();

    /* It's safe to allow irq's after DR6 has been saved */
    preempt_disable();
    cond_local_irq_enable(regs);

This has never really been valid.  It should be guarded by an
if(user_mode(regs)).  And we need to kill the die_notifier garbage in
here -- it makes it basically impossible to understand what's going
on.

--Andy