[PATCH v2 2/2] ARM: Thumb2: use "bx reg" not "mov pc, reg" for all registers

[ard.biesheuvel@linaro.org (Ard Biesheuvel)]

Commit 6ebbf2ce437b (ARM: convert all "mov.* pc, reg" to "bx reg"
for ARMv6+) replaced all occurrences of 'mov pc, <reg>' with the
'ret' macro. However, this macro only emits the 'bx' instruction
when used with the 'lr' register, but still uses 'mov pc, <reg>'
for everything else.

Since ARM/Thumb2 interworking is allowed in the static kernel
(i.e., inside vmlinux), this is potentially unsafe, since the
Thumb mov instruction T1 variant that allows PC as the target will
not switch modes based on the Thumb bit

So when building a Thumb2 kernel, emit the 'bx' instruction for
all registers, not just the 'lr' register.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 arch/arm/include/asm/assembler.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
index 186270b3e194..1c057f923258 100644
--- a/arch/arm/include/asm/assembler.h
+++ b/arch/arm/include/asm/assembler.h
@@ -434,6 +434,8 @@ THUMB(	orr	\reg , \reg , #PSR_T_BIT	)
 	.macro	ret\c, reg
 #if __LINUX_ARM_ARCH__ < 6
 	mov\c	pc, \reg
+#elif defined(CONFIG_THUMB2_KERNEL)
+	bx\c	\reg
 #else
 	.ifeqs	"\reg", "lr"
 	bx\c	\reg
-- 
1.8.3.2