linux-kernel - Re: [PATCH v2 1/2] arch/Kconfig: Make CONFIG_CPU

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Date:   Fri, 29 Oct 2021 11:08:35 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Colin Ian King <colin.king@...onical.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        YiFei Zhu <yifeifz2@...inois.edu>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Wang Kefeng <wangkefeng.wang@...wei.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, bpf@...r.kernel.org
Subject: Re: [PATCH v2 1/2] arch/Kconfig: Make CONFIG_CPU_SPECTRE available
 for all architectures

On Thu, Oct 28, 2021 at 12:36:58PM -0700, Pawan Gupta wrote:
> Isn't ARM already using CPU_SPECTRE for selecting things:
> 
> 	config HARDEN_BRANCH_PREDICTOR
> 	     bool "Harden the branch predictor against aliasing attacks" if EXPERT
> 	     depends on CPU_SPECTRE
> 
> This was the whole motivation for doing the same for x86.
> 
> Adding a condition for all architectures is also okay, but its going to
> a little messier:
> 
> 	 config BPF_UNPRIV_DEFAULT_OFF
> 	        default y if X86 || ARM || ...

It doesn't have to be (but sadly we end up repeating "DEFAULT"):

config BPF_UNPRIV_DEFAULT_OFF_DEFAULT
	bool

config BPF_UNPRIV_DEFAULT_OFF
	bool "Disable unprivileged BPF by default"
	default BPF_UNPRIV_DEFAULT_OFF_DEFAULT

Then architectures can select BPF_UNPRIV_DEFAULT_OFF_DEFAULT if they
wish this to be defaulted to "yes".

However, please note that this has limited use given that the
BPF_UNPRIV_DEFAULT_OFF option has been around for a while now. Any
existing configuration that mentions this symbol will override any
default specified in the Kconfig files if the option is user-visible.

So, IMHO, defaults need to be set correctly from the point in time
that the option is introduced.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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