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From:   Nicolas Boichat <drinkcat@chromium.org>
Date:   Tue, 25 Aug 2020 08:38:27 +0800
Message-ID: <CANMq1KCQjX1632qyo5+8kv8TXNu_Onh4gz7oMx3rx79XQX2fqg@mail.gmail.com>
Subject: Re: [PATCH v5] kernel/trace: Add DISALLOW_TRACE_PRINTK make option
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Douglas Anderson <dianders@chromium.org>,
        Guenter Roeck <groeck@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        "Guilherme G. Piccoli" <gpiccoli@canonical.com>,
        Kars Mulder <kerneldev@karsmulder.nl>,
        Kees Cook <keescook@chromium.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tiezhu Yang <yangtiezhu@loongson.cn>,
        Will Deacon <will@kernel.org>, Yue Hu <huyue2@yulong.com>,
        linux-kbuild@vger.kernel.org, lkml <linux-kernel@vger.kernel.org>
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On Mon, Aug 24, 2020 at 9:30 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Mon, 24 Aug 2020 10:59:13 +0800
> Nicolas Boichat <drinkcat@chromium.org> wrote:
>
> > ---
> >

> > +ifeq ($(KBUILD_DISALLOW_TRACE_PRINTK),1)
> > +KBUILD_CFLAGS += -DDISALLOW_TRACE_PRINTK
> > +endif
> > +
> >  KBUILD_CFLAGS += $(DEBUG_CFLAGS)
> >  export DEBUG_CFLAGS
> >
>
>
> There's one more thing we need to do, is if you build without this option
> then build with it, you should trigger a full kernel rebuild.
> Otherwise, if you build without the option, then build with it, and it
> doesn't rebuild the tree, it wont catch anything.

This already works. I'll be honest, I'm not 100% sure why (and if
fully intentional)...

The CFLAGS end up in 3 generated assembly files:
# grep -R DISALLOW_TRACE_PRINTK * | grep -v ".cmd:"
arch/x86/kernel/asm-offsets.s:# -imultiarch x86_64-linux-gnu -D
__KERNEL__ -D DISALLOW_TRACE_PRINTK
kernel/bounds.s:# -imultiarch x86_64-linux-gnu -D __KERNEL__ -D
DISALLOW_TRACE_PRINTK
scripts/mod/devicetable-offsets.s:# -D DISALLOW_TRACE_PRINTK
(along with all *.cmd files)

and I suspect some/all of those force a complete kernel rebuild.

> -- Steve