1    :  // Copyright 2015 Google Inc. All Rights Reserved.
   2    :  //
   3    :  // Licensed under the Apache License, Version 2.0 (the "License");
   4    :  // you may not use this file except in compliance with the License.
   5    :  // You may obtain a copy of the License at
   6    :  //
   7    :  //     http://www.apache.org/licenses/LICENSE-2.0
   8    :  //
   9    :  // Unless required by applicable law or agreed to in writing, software
  10    :  // distributed under the License is distributed on an "AS IS" BASIS,
  11    :  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  12    :  // See the License for the specific language governing permissions and
  13    :  // limitations under the License.
  14    :  //
  15    :  // This class hot patches a function in memory: all calls to a function will
  16    :  // redirected to a new function using JMP instructions.
  17    :  //
  18    :  // The function must have been prepared for hot patching: it must start with an
  19    :  // instruction that is at least two bytes long and must have at least 5-byte
  20    :  // padding before it and must have at least 2-byte alignment. The alignment
  21    :  // precondition is checked using the CHECK macro. The rest of the preconditions
  22    :  // are not checked.
  23    :  //
  24    :  // The hot patching does the following:
  25    :  // - Removes write protection from the pages where it needs to write.
  26    :  // - Writes the a PC-relative JMP instruction to the 5-byte padding before
  27    :  //   the function. (Opcode: 0xE9 [32-bit PC-relative address])
  28    :  // - Overwrites the first two bytes of the function with a JMP -5 short jump
  29    :  //   instruction. (Opcode: 0xEB 0xF9)
  30    :  // - Restores the old protection.
  31    :  //
  32    :  // We also DCHECK that the bytes in the padding that we overwrite are all 0xCC
  33    :  // bytes. These are used by the instrumenter in the paddings. These DCHECKs
  34    :  // need to be removed to support hot patching a function more than once.
  35    :  
  36    :  #ifndef SYZYGY_AGENT_COMMON_HOT_PATCHER_H_
  37    :  #define SYZYGY_AGENT_COMMON_HOT_PATCHER_H_
  38    :  
  39    :  #include <base/macros.h>
  40    :  
  41    :  namespace agent {
  42    :  namespace common {
  43    :  
  44    :  class HotPatcher {
  45    :   public:
  46    :    typedef void* FunctionPointer;
  47    :  
  48  E :    HotPatcher() { }
  49  E :    ~HotPatcher() { }
  50    :  
  51    :    // Applies hot patching to a function.
  52    :    // @param function_entry_point The start address of the function to be hot
  53    :    //     patched.
  54    :    // @param new_entry_point A new function with the same signature that should
  55    :    //     be called instead of the old one.
  56    :    // @pre The function must have been prepared for hot patching: it must start
  57    :    //     with an instruction that is at least two bytes long and must have
  58    :    //     at least 5-byte padding before it and must have at least 2-byte
  59    :    //     alignment.
  60    :    bool Patch(FunctionPointer function_entry_point,
  61    :               FunctionPointer new_entry_point);
  62    :  
  63    :   private:
  64    :    DISALLOW_COPY_AND_ASSIGN(HotPatcher);
  65    :  };
  66    :  
  67    :  }  // namespace common
  68    :  }  // namespace agent
  69    :  
  70    :  #endif  // SYZYGY_AGENT_COMMON_HOT_PATCHER_H_