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

[Andy Lutomirski <luto@kernel.org>]

On Mon, Aug 24, 2020 at 4:05 AM <peterz@infradead.org> wrote:
>
> On Sun, Aug 23, 2020 at 04:09:42PM -0700, Andy Lutomirski wrote:
> > On Fri, Aug 21, 2020 at 3:21 AM Peter Zijlstra <peterz@infradead.org> 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.
>
> Hurmph.. /me goes re-read the SDM.
>
> INT1 is a trap,
> instruction breakpoint is a fault
>
> So if you have:
>
>         INT1
> 1:      some-instr
>
> and set an X breakpoint on 1, we'll loose the INT1, right?
>
> And because INT1 is a trap, we can't easily decode the instruction
> stream either :-(
>
> Now, OTOH, I suppose you're right in that looking at DR6 early (before
> we let people muck about with it) is more reliable for detecting INT1.
> Esp since the hw_breakpoint crud can consume all bits.
>
> So I'll go fix that. What a giant pain in the ass all this is.
>
> > 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.
>
> I'm not convinced. Even with those patches applied we have to use
> debugreg6, and that code very much relies on clearing DR_TRAP_BITS
> early, and then having ptrace_triggered() re-set bits in it.
>
> This is so that hw_breakpoint handlers can use breakpoints in userspace
> without causing send_sigtrap() on them.

Ick.

Maybe we can rename this to thread->virtual_dr6.  So the traps.c
machinery would process dr6 (the actual value from hardware) and
generate virtual_dr6 to report to userspace.  And no one gets confused
about which is which, and no one ever consumes bits from the virtual
one.