ARM: kprobes: Use conditional breakpoints for ARM probes

Now we no longer trigger probes on conditional instructions when the
condition is false, we can make use of conditional instructions as
breakpoints in ARM code to avoid taking unnecessary exceptions.

Note, we can't rely on not getting an exception when the condition check
fails, as that is Implementation Defined on newer ARM architectures. We
therefore still need to perform manual condition checks as well.
Signed-off-by: Jon Medhurst <tixy@yxit.co.uk>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>

arch/arm/kernel/kprobes.c

@@ -138,7 +138,13 @@ void __kprobes arch_arm_kprobe(struct kprobe *p)
 
 void __kprobes arch_arm_kprobe(struct kprobe *p)
 {
-	*p->addr = KPROBE_ARM_BREAKPOINT_INSTRUCTION;
+	kprobe_opcode_t insn = p->opcode;
+	kprobe_opcode_t brkp = KPROBE_ARM_BREAKPOINT_INSTRUCTION;
+	if (insn >= 0xe0000000)
+		brkp |= 0xe0000000;  /* Unconditional instruction */
+	else
+		brkp |= insn & 0xf0000000;  /* Copy condition from insn */
+	*p->addr = brkp;
 	flush_insns(p->addr, sizeof(p->addr[0]));
 }
 
@@ -625,7 +631,7 @@ static struct undef_hook kprobes_thumb32_break_hook = {
 #else  /* !CONFIG_THUMB2_KERNEL */
 
 static struct undef_hook kprobes_arm_break_hook = {
-	.instr_mask	= 0xffffffff,
+	.instr_mask	= 0x0fffffff,
 	.instr_val	= KPROBE_ARM_BREAKPOINT_INSTRUCTION,
 	.cpsr_mask	= MODE_MASK,
 	.cpsr_val	= SVC_MODE,

arch/arm/kernel/kprobes.h

@@ -21,7 +21,7 @@
  * These undefined instructions must be unique and
  * reserved solely for kprobes' use.
  */
-#define KPROBE_ARM_BREAKPOINT_INSTRUCTION	0xe7f001f8
+#define KPROBE_ARM_BREAKPOINT_INSTRUCTION	0x07f001f8
 #define KPROBE_THUMB16_BREAKPOINT_INSTRUCTION	0xde18
 #define KPROBE_THUMB32_BREAKPOINT_INSTRUCTION	0xf7f0a018