Re: [PATCH v3 05/11] arm64: Add call_break_hook() to early_brk64() for early kgdb

[Will Deacon]

On Mon, May 11, 2020 at 03:45:02PM -0700, Doug Anderson wrote:
> On Mon, May 11, 2020 at 7:59 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > On Tue, Apr 28, 2020 at 02:13:45PM -0700, Douglas Anderson wrote:
> > > diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> > > index 48222a4760c2..59c353dfc8e9 100644
> > > --- a/arch/arm64/kernel/debug-monitors.c
> > > +++ b/arch/arm64/kernel/debug-monitors.c
> > > @@ -297,7 +297,7 @@ void unregister_kernel_break_hook(struct break_hook *hook)
> > >       unregister_debug_hook(&hook->node);
> > >  }
> > >
> > > -static int call_break_hook(struct pt_regs *regs, unsigned int esr)
> > > +int call_break_hook(struct pt_regs *regs, unsigned int esr)
> > >  {
> > >       struct break_hook *hook;
> > >       struct list_head *list;
> > > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> > > index cf402be5c573..a8173f0c1774 100644
> > > --- a/arch/arm64/kernel/traps.c
> > > +++ b/arch/arm64/kernel/traps.c
> > > @@ -1044,6 +1044,9 @@ int __init early_brk64(unsigned long addr, unsigned int esr,
> > >       if ((comment & ~KASAN_BRK_MASK) == KASAN_BRK_IMM)
> > >               return kasan_handler(regs, esr) != DBG_HOOK_HANDLED;
> > >  #endif
> > > +     if (call_break_hook(regs, esr) == DBG_HOOK_HANDLED)
> > > +             return 0;
> >
> > I think this just means we're not running debug_traps_init() early enough,
> > and actually the KASAN early handler is unnecessary too.
> >
> > If we call debug_traps_init() directly from setup_arch() and drop the
> > arch_initcall(), can we then drop early_brk64 entirely?
> 
> It seems to work in my testing.  ...but the worry I have is the
> comment right before trap_init().  It says:
> 
> /* This registration must happen early, before debug_traps_init(). */

I /think/ the reason for this is because debug_traps_init() replaces the
BRK vector, so if that runs before the break hooks have been registered
for e.g. BUG() then BUG() won't work during that window. Hmm, so dropping
early_brk64 is problematic after all. Damn.

Is trap_init() early enough for you? If so, we could call debug_traps_init()
from traps_init() after registering the break hooks.

Will