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, 6 insertions, 0 deletions

diff --git a/arch/mips/kernel/kprobes.c b/arch/mips/kernel/kprobes.c
index 747e3bf7bd9f..f5c8bce70db2 100644
--- a/arch/mips/kernel/kprobes.c
+++ b/arch/mips/kernel/kprobes.c
@@ -106,6 +106,12 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
 		goto out;
 	}
 
+	if (__insn_is_compact_branch(insn)) {
+		pr_notice("Kprobes for compact branches are not supported\n");
+		ret = -EINVAL;
+		goto out;
+	}
+
 	/* insn: must be on special executable page on mips. */
 	p->ainsn.insn = get_insn_slot();
 	if (!p->ainsn.insn) {