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/uprobes.c b/arch/mips/kernel/uprobes.c
index 1161c93a74cb..dbb917403131 100644
--- a/arch/mips/kernel/uprobes.c
+++ b/arch/mips/kernel/uprobes.c
@@ -36,6 +36,12 @@ int arch_uprobe_analyze_insn(struct arch_uprobe *aup,
 		return -EINVAL;
 
 	inst.word = aup->insn[0];
+
+	if (__insn_is_compact_branch(inst)) {
+		pr_notice("Uprobes for compact branches are not supported\n");
+		return -EINVAL;
+	}
+
 	aup->ixol[0] = aup->insn[insn_has_delay_slot(inst)];
 	aup->ixol[1] = UPROBE_BRK_UPROBE_XOL;		/* NOP  */