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

[Pawan Gupta]

On 29.10.2021 11:08, Russell King (Oracle) wrote:
> 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".

Looks like we are settling on unconditional 'default y' for now [1].
I have sent a v3 with 'default y' [2].

> 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.

Yes, existing configurations will have to toggle this manually. However,
many distros already have BPF_UNPRIV_DEFAULT_OFF=y in their
configuration.

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

Agree.

[1] https://lore.kernel.org/lkml/6130e55f-4d84-5ada-4e86-5b678e3eaf5e@xxxxxxxxxxxxx/
[2] https://lore.kernel.org/lkml/0ace9ce3f97656d5f62d11093ad7ee81190c3c25.1635535215.git.pawan.kumar.gupta@xxxxxxxxxxxxxxx/