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>

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 */

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) {

arch/mips/kernel/probes-common.h

@@ -13,6 +13,8 @@
 
 #include <asm/inst.h>
 
+int __insn_is_compact_branch(union mips_instruction insn);
+
 static inline int __insn_has_delay_slot(const union mips_instruction insn)
 {
 	switch (insn.i_format.opcode) {

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  */