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From:   Nicolas Boichat <drinkcat@chromium.org>
Date:   Thu, 25 Jun 2020 09:32:19 +0800
Message-ID: <CANMq1KAVXvMJpNXQoBPMPmSF=vPphvCC_SCFsX03CkR6PY7AKA@mail.gmail.com>
Subject: Re: [PATCH] kernel/trace: Add TRACING_ALLOW_PRINTK config option
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     lkml <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Peter Zijlstra <peterz@infradead.org>,
        Vinod Koul <vkoul@kernel.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Tiezhu Yang <yangtiezhu@loongson.cn>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Guilherme G . Piccoli" <gpiccoli@canonical.com>,
        Will Deacon <will@kernel.org>,
        Douglas Anderson <dianders@chromium.org>,
        Guenter Roeck <groeck@chromium.org>
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On Thu, Jun 25, 2020 at 12:04 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Wed, 24 Jun 2020 16:45:24 +0800
> Nicolas Boichat <drinkcat@chromium.org> wrote:
>
> > trace_printk is only meant as a debugging tool, and should never be
> > compiled into production code without source code changes, as
> > indicated by the warning that shows up on boot if any trace_printk
> > is called:
> >  **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
> >  **                                                      **
> >  ** trace_printk() being used. Allocating extra memory.  **
> >  **                                                      **
> >  ** This means that this is a DEBUG kernel and it is     **
> >  ** unsafe for production use.                           **
> >
> > If this option is set to n, the kernel will generate a build-time
> > error if trace_printk is used.
> >
> > Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
>
> Interesting. Note, this will prevent modules with trace_printk from
> being loaded as well.

Err, all of these changes are in macros (nothing gets built in the
kernel), so this will prevent modules with trace_printk from being
_built_.

Now, if you set the option to =y when building the module (separately,
even though the rest of the kernel has =n) then I don't see why the
module could not be loaded.

>
> -- Steve