MIPS Kprobes: Support branch instructions probing

This patch provides support for kprobes on branch instructions. The branch
instruction at the probed address is actually emulated and not executed
out-of-line like other normal instructions. Instead the delay-slot instruction
is copied and single stepped out of line.

At the time of probe hit, the original branch instruction is evaluated
and the target cp0_epc is computed similar to compute_retrun_epc(). It
is also checked if the delay slot instruction can be skipped, which is
true if there is a NOP in delay slot or branch is taken in case of
branch likely instructions. Once the delay slot instruction is single
stepped the normal execution resume with the cp0_epc updated the earlier
computed cp0_epc as per the branch instructions.

Signed-off-by: Maneesh Soni <manesoni@cisco.com>
Signed-off-by: Victor Kamensky <kamensky@cisco.com>
Cc: David Daney <david.daney@cavium.com>
Cc: ananth@in.ibm.com
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2914/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/arch/mips/include/asm/kprobes.h b/arch/mips/include/asm/kprobes.h
index e6ea4d4d7205..1fbbca01e681 100644
--- a/arch/mips/include/asm/kprobes.h
+++ b/arch/mips/include/asm/kprobes.h
@@ -74,6 +74,8 @@ struct prev_kprobe {
 		: MAX_JPROBES_STACK_SIZE)
 
 
+#define SKIP_DELAYSLOT 0x0001
+
 /* per-cpu kprobe control block */
 struct kprobe_ctlblk {
 	unsigned long kprobe_status;
@@ -82,6 +84,9 @@ struct kprobe_ctlblk {
 	unsigned long kprobe_saved_epc;
 	unsigned long jprobe_saved_sp;
 	struct pt_regs jprobe_saved_regs;
+	/* Per-thread fields, used while emulating branches */
+	unsigned long flags;
+	unsigned long target_epc;
 	u8 jprobes_stack[MAX_JPROBES_STACK_SIZE];
 	struct prev_kprobe prev_kprobe;
 };