linux-kernel - Re: [PATCH v3 05/11] arm64: Add call_break_hook() to early

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Date:   Tue, 12 May 2020 08:35:53 +0100
From:   Will Deacon <will@...nel.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Gross <agross@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Catalin Marinas <catalin.marinas@....com>,
        linux-serial@...r.kernel.org, Sumit Garg <sumit.garg@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Frank Rowand <frowand.list@...il.com>, bp@...en8.de,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Jiri Slaby <jslaby@...e.com>,
        Alexios Zavras <alexios.zavras@...el.com>,
        Allison Randal <allison@...utok.net>,
        Dave Martin <Dave.Martin@....com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        James Morse <james.morse@....com>,
        Mark Rutland <mark.rutland@....com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        jinho lim <jordan.lim@...sung.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 05/11] arm64: Add call_break_hook() to early_brk64()
 for early kgdb

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@...nel.org> 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