[RFC PATCH v2 8/8] arm64: Wire up and expose the new compat vDSO

[kevin.brodsky@arm.com (Kevin Brodsky)]

On 04/11/2016 10:47, Catalin Marinas wrote:
> On Fri, Nov 04, 2016 at 10:30:08AM -0600, Kevin Brodsky wrote:
>> On 04/11/2016 09:50, Catalin Marinas wrote:
>>> On Thu, Oct 27, 2016 at 05:30:58PM +0100, Kevin Brodsky wrote:
>>>> * The vDSO page replaces the vector page. The vDSO provides its own
>>>>     sigreturn trampolines, replacing those in the vector page, but the
>>>>     kuser helpers are gone. As a result enabling the compat vDSO will
>>>>     break userspace programs relying on the kuser helpers.
>>> I think vDSO and vectors page should not exclude each other. If you want
>>> to disable the vectors page, let's make it an independent config option
>>> like the KUSER_HELPERS in arch/arm64/mm/Kconfig. But I would very much
>>> like to be able to have both the vDSO and the vectors page at the same
>>> time.
>> Indeed, I've had exactly the same feedback from Google yesterday (apparently
>> many Android apps with native libs still target ARMv6....). I'll add the
>> option to keep the kuser helpers.
>>
>> There's a small problem though: how to ensure that the kuser helpers +
>> sigreturn trampolines are always included if the compat vDSO is not built? I
>> can enforce CONFIG_KUSER_HELPERS if !CONFIG_VDSO32 (directly in the
>> code/Makefiles), but the dependency cannot be expressed in Kconfig.
> Or you could insert a separate "sigpage" as arm32 does. This could leave
> independently of vDSO or vectors page.

Yeah I thought about this too. It's a bit more work but probably cleaner and more 
flexible, that would also allow to disable the kuser helpers independently of the 
compat vDSO.

Thanks,
Kevin