arm64: vdso32: add CONFIG_THUMB2_COMPAT_VDSO

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.
Reported-by: Luis Lozano <llozano@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Manoj Gupta <manojgupta@google.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Stephen Boyd <swboyd@google.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Dave Martin <Dave.Martin@arm.com>
Link: https://static.docs.arm.com/ihi0042/i/aapcs32.pdf
Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1084372
Link: https://lore.kernel.org/r/20200608205711.109418-1-ndesaulniers@google.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/Kconfig

@@ -1297,6 +1297,14 @@ 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 32-bit vDSO for Thumb-2 mode" if EXPERT
+	depends on COMPAT_VDSO
+	default y
+	help
+	  Compile the compat vDSO with '-mthumb -fomit-frame-pointer' if y,
+	  otherwise with '-marm'.
+
 menuconfig ARMV8_DEPRECATED
 	bool "Emulate deprecated/obsolete ARMv8 instructions"
 	depends on SYSCTL

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__