[RFC PATCH v3 00/11] arm64: Add a compat vDSO

On Tue, Dec 06, 2016 at 04:03:42PM +0000, Kevin Brodsky wrote:

Hi,

Hi Kevin,

This series adds support for a compat (AArch32) vDSO, providing two
userspace functionalities to compat processes:

* "Virtual" time syscalls (gettimeofday and clock_gettime). The
  implementation is an adaptation of the arm vDSO (vgettimeofday.c),
  sharing the data page with the 64-bit vDSO.

* sigreturn trampolines, following the example of the 64-bit vDSO
  (sigreturn.S), but slightly more complicated because we provide A32
  and T32 variants for both sigreturn and rt_sigreturn, and appropriate
  arm-specific unwinding directives.

The first point brings the performance improvement expected of a vDSO,
by implementing time syscalls directly in userspace. The second point
allows to provide unwinding information for sigreturn trampolines,
achieving feature parity with the trampolines provided by glibc.

Unfortunately, this time we cannot escape using a 32-bit toolchain. To
build the compat VDSO, CONFIG_COMPAT_VDSO must be set *and*
CROSS_COMPILE_ARM32 must be defined to the prefix of a 32-bit compiler.
Failure to do so will not prevent building the kernel, but a warning
will be printed and the compat vDSO will not be built.

v3 is a major refactor of the series. The main change is that the kuser
helpers are no longer mutually exclusive with the 32-bit vDSO, and can
be disabled independently of the 32-bit vDSO (they are kept enabled by
default). To this end, the "old" sigreturn trampolines have been moved
out of the [vectors] page into an independent [sigreturn] page, similar
to [sigpage] on arm. The [vectors] page is now [kuserhelpers], and its
presence is controlled by CONFIG_KUSER_HELPERS. The [sigreturn] page is
only present when the 32-bit vDSO is not included (the latter provides
its own sigreturn trampolines with unwinding information). The following
table summarises which pages are now added in 32-bit processes:

+----------------+----------------+----------------+
|     CONFIG     | !VDSO32        | VDSO32         |
+----------------+----------------+----------------+
| !KUSER_HELPERS | [sigreturn]    | [vvar]         |
|                |                | [vdso]         |
+----------------+----------------+----------------+
| KUSER_HELPERS  | [sigreturn]    | [vvar]         |
|                | [kuserhelpers] | [vdso]         |
|                |                | [kuserhelpers] |
+----------------+----------------+----------------+

Why is this different to arch/arm/? The idea of the COMPAT layer is to
do the same thing as arch/arm, so we should use [sigpage] and [vectors]
instead of [sigreturn] and [kuserhelpers]. I also think that the /proc/maps
code assumes the "gate vma" is at the end, but that's more of a niggle
than anything else.

The big issue with this series is that I share Nathan's concerns that we're
needlessly forking code from arch/arm/, without a commitment to follow up
with the refactoring effort.  So, whilst I can see that technically this is
looking pretty solid, it's a big ask to merge this into mainline as-is.

Will