Change BKPT and UDF encodings on ARM.

src/trusted/validator_arm/baseline_classes.cc

Index: src/trusted/validator_arm/baseline_classes.cc
diff --git a/src/trusted/validator_arm/baseline_classes.cc b/src/trusted/validator_arm/baseline_classes.cc
index 253013932af31c3026c147ef2350d841d6041ca0..001ad76d1dd66acb525764b1d65b88b3c63732d9 100644
--- a/src/trusted/validator_arm/baseline_classes.cc
+++ b/src/trusted/validator_arm/baseline_classes.cc
@@ -90,10 +90,15 @@ int32_t BranchImmediate24::branch_target_offset(Instruction i) const {
 
 // BreakPointAndConstantPoolHead
 SafetyLevel BreakPointAndConstantPoolHead::safety(const Instruction i) const {
-  return i.GetCondition() == Instruction::AL
-      ? MAY_BE_SAFE
-      : UNPREDICTABLE;
+  if (i.GetCondition() != Instruction::AL)
+    return UNPREDICTABLE;
+  // Restrict BKPT's encoding to values we've chosen as safe.
+  if ((i.Bits(31, 0) == kLiteralPoolHeadInstruction) ||
+      (i.Bits(31, 0) == kBreakpoint))
+    return MAY_BE_SAFE;
+  return FORBIDDEN_OPERANDS;
 }
+
 bool BreakPointAndConstantPoolHead::
 is_literal_pool_head(const Instruction i) const {
   return i.Bits(31, 0) == kLiteralPoolHeadInstruction;