linux-kernel - Re: [PATCH v1] init: Do not select DEBUG

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Date:   Wed, 10 Apr 2019 15:04:24 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sinan Kaya <Okaya@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Adrian Reber <adrian@...as.de>,
        Richard Guy Briggs <rgb@...hat.com>
Subject: Re: [PATCH v1] init: Do not select DEBUG_KERNEL by default

On Wed, Apr 10, 2019 at 2:54 PM Sinan Kaya <Okaya@...nel.org> wrote:
>
> On 4/10/2019 5:45 PM, Kees Cook wrote:
> > On Wed, Apr 10, 2019 at 2:26 PM Sinan Kaya <okaya@...nel.org> wrote:
> >>
> >> We can't seem to have a kernel with CONFIG_EXPERT set but
> >> CONFIG_DEBUG_KERNEL unset these days.
> >>
> >> While some of the features under the CONFIG_EXPERT require
> >> CONFIG_DEBUG_KERNEL, it doesn't apply for all features.
> >>
> >> The meaning of CONFIG_EXPERT and CONFIG_DEBUG_KERNEL has been
> >> mixed here.
> >
> > I don't agree: the point of EXPERT is to show _everything_, which
> > means DEBUG_KERNEL should be selected to show those options as well. I
> > think this is fine as-is. What is the problem you want to solve?
> >
> > I think of it as low (nothing selected) medium (DEBUG_KERNEL) and high
> > (EXPERT and DEBUG_KERNEL). So EXPERT enables DEBUG_KERNEL too.
> >
>
> Sure, let's see if there is a better option.
>
> I don't want any of the debug features in my kernel but still
> need all the expert features. My kernel is considered a production
> kernel. I don't really want to ship all the good debug enables.

Production kernels enable it. e.g. Ubuntu:
$ grep '\bCONFIG_DEBUG_KERNEL\b' /boot/config-$(uname -r)
CONFIG_DEBUG_KERNEL=y

> On the other hand, I need the features under CONFIG_EXPERT to have
> a functional system.
>
> Let's take "multiple users" as an example.
>
> What's the point of having a kernel without multiple users? :)
>
> I don't see the relationship between CONFIG_DEBUG and CONFIG_EXPERT
> as none of the features except KALLSYMS depend on it. If there was
> a compile time dependency, I'd say move it to the things that need
> it as this patch suggests.

CONFIG_DEBUG_KERNEL mainly only enables the visibility of various
other options. I can only find two instances of it controlling a
"default", and one is overridden by CONFIG_SMP on alpha. :)

$ git grep -B2 'default.*DEBUG_KERNEL'
arch/alpha/Kconfig.debug-config MATHEMU
arch/alpha/Kconfig.debug-       tristate "Kernel FP software
completion" if DEBUG_KERNEL && !SMP
arch/alpha/Kconfig.debug:       default y if !DEBUG_KERNEL || SMP
--
kernel/trace/Kconfig-menuconfig FTRACE
kernel/trace/Kconfig-   bool "Tracers"
kernel/trace/Kconfig:   default y if DEBUG_KERNEL

What do you see enabled that CONFIG_DEBUG_KERNEL enables that you don't want?

-- 
Kees Cook