FROMLIST: [PATCH v3 2/3] arm64: compat: Split the sigreturn trampolines and kuser...

FROMLIST: [PATCH v3 2/3] arm64: compat: Split the sigreturn trampolines and kuser helpers (assembler sources)

(cherry picked from url http://lkml.iu.edu/hypermail/linux/kernel/1709.1/01902.html)

AArch32 processes are currently installed a special [vectors] page that
contains the sigreturn trampolines and the kuser helpers, at the fixed
address mandated by the kuser helpers ABI.

Having both functionalities in the same page has become problematic,
because:

* It makes it impossible to disable the kuser helpers (the sigreturn
  trampolines cannot be removed), which is possible on arm.

* A future 32-bit vDSO would provide the sigreturn trampolines itself,
  making those in [vectors] redundant.

This patch addresses the problem by moving the sigreturn trampolines
sources to its own file.  Wrapped the comments to reduce the wrath of
checkpatch.pl.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Bug: 9674955
Bug: 63737556
Bug: 20045882
Change-Id: I1d7b96e7cfbe979ecf4cb4996befd1f3ae0e64fd