[RFC PATCH v2 4/8] arm64: compat: Add a 32-bit vDSO

Provide the files necessary for building a compat (AArch32) vDSO in
kernel/vdso32.

This is mostly an adaptation of the arm vDSO. The most significant
change in vgettimeofday.c is the use of the arm64 vdso_data struct,
allowing the vDSO data page to be shared between the 32 and 64-bit
vDSOs.

In addition to the time functions, sigreturn trampolines are also
provided, aiming at replacing those in the vector page. To improve
debugging, CFI and unwinding directives are used, based on glibc's
implementation. Symbol offsets are made available to the kernel using
the same method as the 64-bit vDSO.

There is unfortunately an important caveat to all this: we cannot get
away with hand-coding 32-bit instructions like in kernel/kuser32.S,
this time we really need a 32-bit compiler. The compat vDSO Makefile
relies on CROSS_COMPILE_ARM32 to provide a 32-bit compiler,
appropriate logic will be added to the arm64 Makefile later on to
ensure that an attempt to build the compat vDSO is made only if this
variable has been set properly.

Signed-off-by: Kevin Brodsky <redacted>
---
 arch/arm64/kernel/vdso32/Makefile        | 121 +++++++++++++
 arch/arm64/kernel/vdso32/sigreturn.S     |  86 +++++++++
 arch/arm64/kernel/vdso32/vdso.S          |  32 ++++
 arch/arm64/kernel/vdso32/vdso.lds.S      |  98 +++++++++++
 arch/arm64/kernel/vdso32/vgettimeofday.c | 294 +++++++++++++++++++++++++++++++
 5 files changed, 631 insertions(+)
 create mode 100644 arch/arm64/kernel/vdso32/Makefile
 create mode 100644 arch/arm64/kernel/vdso32/sigreturn.S
 create mode 100644 arch/arm64/kernel/vdso32/vdso.S
 create mode 100644 arch/arm64/kernel/vdso32/vdso.lds.S
 create mode 100644 arch/arm64/kernel/vdso32/vgettimeofday.c

diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
new file mode 100644
index 000000000000..38facc870f6e
--- /dev/null
+++ b/arch/arm64/kernel/vdso32/Makefile

@@ -0,0 +1,121 @@
+#
+# Building a vDSO image for AArch32.
+#
+# Author: Kevin Brodsky <kevin.brodsky@arm.com>
+# A mix between the arm64 and arm vDSO Makefiles.
+
+CC_ARM32 := $(CROSS_COMPILE_ARM32)gcc
+
+# Same as cc-ldoption, but using CC_ARM32 instead of CC
+cc32-ldoption = $(call try-run,\
+        $(CC_ARM32) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
+
+# Borrow vdsomunge.c from the arm vDSO
+munge := arch/arm/vdso/vdsomunge
+hostprogs-y := $(srctree)/$(munge)
+
+c-obj-vdso := vgettimeofday.o
+asm-obj-vdso := sigreturn.o
+
+# Build rules
+targets := $(c-obj-vdso) $(asm-obj-vdso) vdso.so vdso.so.dbg vdso.so.raw
+c-obj-vdso := $(addprefix $(obj)/, $(c-obj-vdso))
+asm-obj-vdso := $(addprefix $(obj)/, $(asm-obj-vdso))
+obj-vdso := $(c-obj-vdso) $(asm-obj-vdso)
+
+ccflags-y := -fPIC -fno-common -fno-builtin -fno-stack-protector
+ccflags-y += -DDISABLE_BRANCH_PROFILING
+
+# Force -O2 to avoid libgcc dependencies
+VDSO_CFLAGS := -march=armv8-a -O2