Re: [PATCH v2 1/2] arch/Kconfig: Make CONFIG_CPU_SPECTRE available for all architectures

[Russell King (Oracle)]

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.

-- 
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