[PATCH] kprobes: only test 'sub pc, pc, #1b-2b+8-2' on ARMv6

Rabin Vincent rabin.vincent at stericsson.com
Tue Jun 12 08:07:54 EDT 2012

'sub pc, pc, #1b-2b+8-2' results in address<1:0> == '10'.

sub pc, pc, #const (== ADR pc, #const) performs an interworking branch
(BXWritePC()) on ARMv7+ and a simple branch (BranchWritePC()) on earlier

In ARM state, BXWritePC() is UNPREDICTABLE when address<1:0> == '10'.

In ARM state on ARMv6+, BranchWritePC() ignores address<1:0>.  Before
ARMv6, BranchWritePC() is UNPREDICTABLE if address<1:0> != '00'

So the instruction is UNPREDICTABLE both before and after v6.

Signed-off-by: Rabin Vincent <rabin.vincent at stericsson.com>
 arch/arm/kernel/kprobes-test-arm.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kernel/kprobes-test-arm.c b/arch/arm/kernel/kprobes-test-arm.c
index ba32b39..6c926d5 100644
--- a/arch/arm/kernel/kprobes-test-arm.c
+++ b/arch/arm/kernel/kprobes-test-arm.c
@@ -187,8 +187,8 @@ void kprobe_arm_test_cases(void)
 	TEST_BF_R ("mov	pc, r",0,2f,"")
 	TEST_BF_RR("mov	pc, r",0,2f,", asl r",1,0,"")
 	TEST_BB(   "sub	pc, pc, #1b-2b+8")
-#if __LINUX_ARM_ARCH__ >= 6
-	TEST_BB(   "sub	pc, pc, #1b-2b+8-2") /* UNPREDICTABLE before ARMv6 */
+#if __LINUX_ARM_ARCH__ == 6
+	TEST_BB(   "sub	pc, pc, #1b-2b+8-2") /* UNPREDICTABLE before and after ARMv6 */
 	TEST_BB_R( "sub	pc, pc, r",14, 1f-2f+8,"")
 	TEST_BB_R( "rsb	pc, r",14,1f-2f+8,", pc")

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