[PATCH v2] arm64: vdso32: add CONFIG_THUMB2_COMPAT_VDSO

[Nick Desaulniers]

Allow the compat vdso (32b) to be compiled as either THUMB2 (default) or
ARM.

For THUMB2, the register r7 is reserved for the frame pointer, but
code in arch/arm64/include/asm/vdso/compat_gettimeofday.h
uses r7. Explicitly set -fomit-frame-pointer, since unwinding through
interworked THUMB2 and ARM is unreliable anyways. See also how
CONFIG_UNWINDER_FRAME_POINTER cannot be selected for
CONFIG_THUMB2_KERNEL for ARCH=arm.

This also helps toolchains that differ in their implicit value if the
choice of -f{no-}omit-frame-pointer is left unspecified, to not error on
the use of r7.

2019 Q4 ARM AAPCS seeks to standardize the use of r11 as the reserved
frame pointer register, but no production compiler that can compile the
Linux kernel currently implements this.  We're actively discussing such
a transition with ARM toolchain developers currently.

Link: https://static.docs.arm.com/ihi0042/i/aapcs32.pdf
Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084372
Cc: Stephen Boyd <swboyd@xxxxxxxxxx>
Cc: Robin Murphy <robin.murphy@xxxxxxx>
Cc: Dave Martin <Dave.Martin@xxxxxxx>
Reported-by: Luis Lozano <llozano@xxxxxxxxxx>
Tested-by: Manoj Gupta <manojgupta@xxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
Changes V1 -> V2:
* add THUMB2_COMPAT_VDSO config, making -mthumb/-marm configurable
  rather than hard coding.
* Fixed https://reviews.llvm.org/D80828 in Clang, but still an issue.
  Not due to implicit state of -marm vs -mthumb, but actually
  -f{no-}omit-frame-pointer due to
  https://source.chromium.org/chromiumos/chromiumos/codesearch/+/master:src/third_party/toolchain-utils/compiler_wrapper/config.go;l=110,
  which prefixes -fno-omit-frame-pointer for all arches and projects.
  Projects that don't set -f{no-}omit-frame-pointer thus don't overwrite
  the prefixed -fno-omit-frame-pointer, which is an issue when inline
  asm compiled as -mthumb uses r7.
* I don't have a strong preference on the default state of this config.

 arch/arm64/Kconfig                | 8 ++++++++
 arch/arm64/kernel/vdso32/Makefile | 8 ++++++++
 2 files changed, 16 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 7f9d38444d6d..fe9e6b231cac 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1299,6 +1299,14 @@ config COMPAT_VDSO
 	  You must have a 32-bit build of glibc 2.22 or later for programs
 	  to seamlessly take advantage of this.
 
+config THUMB2_COMPAT_VDSO
+	bool "Compile the vDSO in THUMB2 mode"
+	depends on COMPAT_VDSO
+	default y
+	help
+	  Compile the compat vDSO with -mthumb -fomit-frame-pointer if y, otherwise
+	  as -marm.
+
 menuconfig ARMV8_DEPRECATED
 	bool "Emulate deprecated/obsolete ARMv8 instructions"
 	depends on SYSCTL
diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
index 3964738ebbde..7ea1e827e505 100644
--- a/arch/arm64/kernel/vdso32/Makefile
+++ b/arch/arm64/kernel/vdso32/Makefile
@@ -105,6 +105,14 @@ VDSO_CFLAGS += -D__uint128_t='void*'
 VDSO_CFLAGS += $(call cc32-disable-warning,shift-count-overflow)
 VDSO_CFLAGS += -Wno-int-to-pointer-cast
 
+# Compile as THUMB2 or ARM. Unwinding via frame-pointers in THUMB2 is
+# unreliable.
+ifeq ($(CONFIG_THUMB2_COMPAT_VDSO), y)
+VDSO_CFLAGS += -mthumb -fomit-frame-pointer
+else
+VDSO_CFLAGS += -marm
+endif
+
 VDSO_AFLAGS := $(VDSO_CAFLAGS)
 VDSO_AFLAGS += -D__ASSEMBLY__
 
-- 
2.27.0.278.ge193c7cf3a9-goog