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>

2 files changed, 9 insertions, 3 deletions

diff --git a/arch/arm/kernel/kprobes.c b/arch/arm/kernel/kprobes.c
index b6e9a1cc1c55..0003dfd3b854 100644
--- a/arch/arm/kernel/kprobes.c
+++ b/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,
diff --git a/arch/arm/kernel/kprobes.h b/arch/arm/kernel/kprobes.h
index 5e2485c4cacd..e3803c65c4be 100644
--- a/arch/arm/kernel/kprobes.h
+++ b/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