MIPS Kprobes: Deny probes on ll/sc instructions

As ll/sc instruction are for atomic read-modify-write operations, allowing
probes on top of these insturctions is a bad idea.
Signed-off-by: Victor Kamensky <kamensky@cisco.com>
Signed-off-by: Maneesh Soni <manesoni@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/2912/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/kprobes.c

@@ -113,6 +113,30 @@ static int __kprobes insn_has_delayslot(union mips_instruction insn)
 	return 0;
 }
 
+/*
+ * insn_has_ll_or_sc function checks whether instruction is ll or sc
+ * one; putting breakpoint on top of atomic ll/sc pair is bad idea;
+ * so we need to prevent it and refuse kprobes insertion for such
+ * instructions; cannot do much about breakpoint in the middle of
+ * ll/sc pair; it is upto user to avoid those places
+ */
+static int __kprobes insn_has_ll_or_sc(union mips_instruction insn)
+{
+	int ret = 0;
+
+	switch (insn.i_format.opcode) {
+	case ll_op:
+	case lld_op:
+	case sc_op:
+	case scd_op:
+		ret = 1;
+		break;
+	default:
+		break;
+	}
+	return ret;
+}
+
 int __kprobes arch_prepare_kprobe(struct kprobe *p)
 {
 	union mips_instruction insn;
@@ -121,6 +145,13 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
 
 	insn = p->addr[0];
 
+	if (insn_has_ll_or_sc(insn)) {
+		pr_notice("Kprobes for ll and sc instructions are not"
+			  "supported\n");
+		ret = -EINVAL;
+		goto out;
+	}
+
 	if (insn_has_delayslot(insn)) {
 		pr_notice("Kprobes for branch and jump instructions are not"
 			  "supported\n");