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

From: Doug Anderson <dianders@chromium.org>
To: Will Deacon <will@kernel.org>
Cc: Jason Wessel <jason.wessel@windriver.com>,
	Daniel Thompson <daniel.thompson@linaro.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andy Gross <agross@kernel.org>,
	kgdb-bugreport@lists.sourceforge.net,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-serial@vger.kernel.org, Sumit Garg <sumit.garg@linaro.org>,
	Jonathan Corbet <corbet@lwn.net>, Ingo Molnar <mingo@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Frank Rowand <frowand.list@gmail.com>,
	bp@alien8.de, Bjorn Andersson <bjorn.andersson@linaro.org>,
	Jiri Slaby <jslaby@suse.com>,
	Alexios Zavras <alexios.zavras@intel.com>,
	Allison Randal <allison@lohutok.net>,
	Dave Martin <Dave.Martin@arm.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	James Morse <james.morse@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	jinho lim <jordan.lim@samsung.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 05/11] arm64: Add call_break_hook() to early_brk64() for early kgdb
Date: Mon, 11 May 2020 15:45:02 -0700	[thread overview]
Message-ID: <CAD=FV=W1F-B7SUwxebhhH2HS+fN4sYv4RHvvKud5a+00J0T=SA@mail.gmail.com> (raw)
In-Reply-To: <20200511145908.GA22040@willie-the-truck>

Hi,

On Mon, May 11, 2020 at 7:59 AM Will Deacon <will@kernel.org> wrote:
>
> Hi Doug,
>
> 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(). */

By moving debug_traps_init() early we're violating that comment.  Do I
just remove that comment, or was there a good reason for it?  ...or am
I reading it wrong and I should have read it as if it said:

/* NOTE: this registration happens early, before debug_traps_init(). */

...then removing it is fine.  Maybe that's right?

I coded this up and put it on the Chrome OS gerrit at
<https://crrev.com/c/2195061>.  I'm happy to post this on the list as
a loner patch to replace this one or spin the whole series depending
on what people want.

-Doug