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

On Fri, Nov 04, 2016 at 10:30:08AM -0600, Kevin Brodsky wrote:

On 04/11/2016 09:50, Catalin Marinas wrote:

quoted

On Thu, Oct 27, 2016 at 05:30:58PM +0100, Kevin Brodsky wrote:

quoted

* 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.

-- 
Catalin