Change BKPT and UDF encodings on ARM.

src/trusted/validator_arm/model.h

 /*
  * Copyright (c) 2012 The Native Client Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef NATIVE_CLIENT_SRC_TRUSTED_VALIDATOR_ARM_V2_MODEL_H
 #define NATIVE_CLIENT_SRC_TRUSTED_VALIDATOR_ARM_V2_MODEL_H
 
 /*
@@ skipping 238 matching lines @@
   RegisterList& operator=(const RegisterList& r);  // Disallow assignment.
 };
 
 
 // The number of bits in an ARM instruction.
 static const int kArm32InstSize = 32;
 
 // The number of bits in a word of a THUMB instruction.
 static const int kThumbWordSize = 16;
 
-// BKPT #0x7777 is used as literal pool head.
-static const uint32_t kLiteralPoolHeadInstruction = 0xE1277777;
+// Specially chosen BKPT and UDF instructions that also correspond to
+// BKPT and UDF when decoded as Thumb instructions.
+//  - BKPT #0x5BE0 is used as literal pool head.
+//  - BKPT #0x5BEF is used as our generic breakpoint.
+//  - UDF #0xEDEF is used as halt-fill.
+//  - UDF #0xEDE0 is used as abort-now (such as __builtin_trap).
+//  - UDF #0xEDE1 is used to always fail validation.
+// All other values are disallowed by the validator out of paranoia.
+static const uint32_t kLiteralPoolHeadInstruction = 0xE125BE70;
+static const uint32_t kBreakpoint = 0xE125BE7F;
+static const uint32_t kHaltFill = 0xE7FEDEFF;
+static const uint32_t kAbortNow = 0xE7FEDEF0;
+static const uint32_t kFailValidation = 0xE7FEDEF1;
 
 // Models an instruction, either a 32-bit ARM instruction of unspecified type,
 // or one word (16-bit) and two word (32-bit) THUMB instructions.
 //
 // This class is designed for efficiency:
 //  - Its public methods for bitfield extraction are short and inline.
 //  - It has no vtable, so on 32-bit platforms it's exactly the size of the
 //    instruction it models.
 //  - API's exist for accessing both ARM (32-bit) instructions and
 //    THUMB instructions (which are 1 or two (16-bit) words).
@@ skipping 219 matching lines @@
   }
 
  private:
   uint32_t bits_;
   Instruction& operator=(const Instruction& insn);  // Disallow assignment.
 };
 
 }  // namespace nacl_arm_dec
 
 #endif  // NATIVE_CLIENT_SRC_TRUSTED_VALIDATOR_ARM_V2_MODEL_H