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

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Date:   Wed, 10 Apr 2019 15:21:15 -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 3:18 PM Sinan Kaya <Okaya@...nel.org> wrote:
>
> On 4/10/2019 6:04 PM, Kees Cook wrote:
>
> >> 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
> >
>
> It makes sense for a general purpose operating system. It doesn't apply
> to my limited case where I'm very concerned about image size.

Gotcha.

>
> >> 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
>
> If the idea is to just show, nothing should happen based on
> DEBUG_KERNEL, right?
>
> No default selection as in FTRACE, no c/S file changes in the code
> path as Mathieu identified.

Yeah, this is clearly a problem. I hadn't looked for code #ifdefs :(

> I can go after individual enables if you agree assuming Mathieu will
> go after the changes in the other email. Let me know otherwise.

How about you split it, but make DEBUG_KERNEL be "default EXPERT" that
way enabling EXPERT will enable DEBUG_KERNEL still in the default
case?

-- 
Kees Cook