MIPS: tracing: disable uprobe/kprobe on compact branch instructions

Current instruction decoder for uprobe/kprobe handler only handles
branches with delay slots. For compact branches the behaviour is rather
unpredictable - and depending on the encoding of a compact branch
instruction may result in one (or more) of:
- executing an instruction that follows a branch which wasn't in a delay
  slot and shouldn't have been executed
- incorrectly emulating a branch leading to a jump to a wrong location
- unexpected branching out of the single-stepped code and never reaching
  the breakpoint that should terminate the probe handler

Results of these actions are generally unpredictable, but can end up
with a probed application or kernel crash, so disable placing probes on
compact branches until they are handled properly.

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14336/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 34 insertions, 0 deletions

diff --git a/arch/mips/kernel/branch.c b/arch/mips/kernel/branch.c
index f5c68483c98e..12c718181e5e 100644
--- a/arch/mips/kernel/branch.c
+++ b/arch/mips/kernel/branch.c
@@ -866,3 +866,37 @@ unaligned:
 	force_sig(SIGBUS, current);
 	return -EFAULT;
 }
+
+#if (defined CONFIG_KPROBES) || (defined CONFIG_UPROBES)
+
+int __insn_is_compact_branch(union mips_instruction insn)
+{
+	if (!cpu_has_mips_r6)
+		return 0;
+
+	switch (insn.i_format.opcode) {
+	case blezl_op:
+	case bgtzl_op:
+	case blez_op:
+	case bgtz_op:
+		/*
+		 * blez[l] and bgtz[l] opcodes with non-zero rt
+		 * are MIPS R6 compact branches
+		 */
+		if (insn.i_format.rt)
+			return 1;
+		break;
+	case bc6_op:
+	case balc6_op:
+	case pop10_op:
+	case pop30_op:
+	case pop66_op:
+	case pop76_op:
+		return 1;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(__insn_is_compact_branch);
+
+#endif  /* CONFIG_KPROBES || CONFIG_UPROBES */