ARM: kprobes: Make str_pc_offset a constant on ARMv7

The str_pc_offset value is architecturally defined on ARMv7 onwards so
we can make it a compile time constant. This means on Thumb kernels the
runtime checking code isn't needed, which saves us from having to fix it
to work for Thumb.

Signed-off-by: Jon Medhurst <tixy@yxit.co.uk>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>

arch/arm/kernel/kprobes-common.c

@@ -17,6 +17,8 @@
 #include "kprobes.h"
 
 
+#ifndef find_str_pc_offset
+
 /*
  * For STR and STM instructions, an ARM core may choose to use either
  * a +8 or a +12 displacement from the current instruction's address.
@@ -40,6 +42,8 @@ void __init find_str_pc_offset(void)
 	str_pc_offset = ret;
 }
 
+#endif /* !find_str_pc_offset */
+
 
 void __init arm_kprobe_decode_init(void)
 {

arch/arm/kernel/kprobes.h

@@ -36,7 +36,21 @@ void __init arm_kprobe_decode_init(void);
 
 extern kprobe_check_cc * const kprobe_condition_checks[16];
 
+
+#if __LINUX_ARM_ARCH__ >= 7
+
+/* str_pc_offset is architecturally defined from ARMv7 onwards */
+#define str_pc_offset 8
+#define find_str_pc_offset()
+
+#else /* __LINUX_ARM_ARCH__ < 7 */
+
+/* We need a run-time check to determine str_pc_offset */
 extern int str_pc_offset;
+void __init find_str_pc_offset(void);
+
+#endif
+
 
 /*
  * Test if load/store instructions writeback the address register.