Re: [PATCH v13 0/2] arm64: Enable BTI for the executable as well as the interpreter

Jeremy Linton
2022-04-20T13:39:34Z

Hi,

On 4/20/22 06:57, Mark Brown wrote:
> On Wed, Apr 20, 2022 at 10:57:30AM +0100, Catalin Marinas wrote:
>> On Wed, Apr 20, 2022 at 10:36:13AM +0100, Will Deacon wrote:
>
>>> Kees, please can you drop this series while Catalin's alternative solution
>>> is under discussion (his Reviewed-by preceded the other patches)?
>
>>> https://lore.kernel.org/r/20220413134946.2732468-1-catalin.marinas@arm.com
>
>>> Both series expose new behaviours to userspace and we don't need both.
>
>> I agree. Even though the patches have my reviewed-by, I think we should
>> postpone them until we figure out a better W^X solution that does not
>> affect BTI (and if we can't, we revisit these patches).
>
> Indeed. I had been expecting this to follow the pattern of the previous
> nine months or so and be mostly ignored for the time being while
> Catalin's new series goes forward. Now that it's applied it might be
> worth keeping the first patch still in case someone else needs it but
> the second patch can probably wait.
>
>> Arguably, the two approaches are complementary but the way this series
>> turned out is for the BTI on main executable to be default off. I have a
>> worry that the feature won't get used, so we just carry unnecessary code
>> in the kernel. Jeremy also found this approach less than ideal:
>
>> https://lore.kernel.org/r/59fc8a58-5013-606b-f544-8277cda18e50@arm.com
>
> I'm not sure there was a fundamental concern with the approach there but
> rather some pushback on the instance on turning it off by default.

Right, this one seems to have the smallest impact on systemd as it
exists today. I would have expected the default to be on, because IMHO
this set corrects what at first glance just looks like a small
oversight. I find the ABI questions a bit theoretical, given that this
should only affect environments that don't exist outside of
labs/development orgs at this point (aka systemd services on HW that
implements BTI).

The other approach works, and if the systemd folks are on board with it
also should solve the underlying problem, but it creates a bit of a
compatibility problem with existing containers/etc that might exist
today (although running systemd/services in a container is itself a
discussion).

So, frankly, I don't see why they aren't complementary. This fixes a bug
we have today, the other set creates a generic mechanism for the future.

Thanks,