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// Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// Definition of PreamblePatcher
namespace sidestep {
// Maximum size of the preamble stub. We overwrite at least the first 5
// bytes of the function. Considering the worst case scenario, we need 4
// bytes + the max instruction size + 5 more bytes for our jump back to
// the original code. With that in mind, 32 is a good number :)
const size_t kMaxPreambleStubSize = 32;
// Possible results of patching/unpatching
enum SideStepError {
// Implements a patching mechanism that overwrites the first few bytes of
// a function preamble with a jump to our hook function, which is then
// able to call the original function via a specially-made preamble-stub
// that imitates the action of the original preamble.
// Note that there are a number of ways that this method of patching can
// fail. The most common are:
// - If there is a jump (jxx) instruction in the first 5 bytes of
// the function being patched, we cannot patch it because in the
// current implementation we do not know how to rewrite relative
// jumps after relocating them to the preamble-stub. Note that
// if you really really need to patch a function like this, it
// would be possible to add this functionality (but at some cost).
// - If there is a return (ret) instruction in the first 5 bytes
// we cannot patch the function because it may not be long enough
// for the jmp instruction we use to inject our patch.
// - If there is another thread currently executing within the bytes
// that are copied to the preamble stub, it will crash in an undefined
// way.
// If you get any other error than the above, you're either pointing the
// patcher at an invalid instruction (e.g. into the middle of a multi-
// byte instruction, or not at memory containing executable instructions)
// or, there may be a bug in the disassembler we use to find
// instruction boundaries.
class PreamblePatcher {
// Patches target_function to point to replacement_function using a provided
// preamble_stub of stub_size bytes.
// Returns An error code indicating the result of patching.
template <class T>
static SideStepError Patch(T target_function, T replacement_function,
void* preamble_stub, size_t stub_size) {
return RawPatchWithStub(target_function, replacement_function,
reinterpret_cast<unsigned char*>(preamble_stub),
stub_size, NULL);
// Patches a function by overwriting its first few bytes with
// a jump to a different function. This is similar to the RawPatch
// function except that it uses the stub allocated by the caller
// instead of allocating it.
// To use this function, you first have to call VirtualProtect to make the
// target function writable at least for the duration of the call.
// target_function: A pointer to the function that should be
// patched.
// replacement_function: A pointer to the function that should
// replace the target function. The replacement function must have
// exactly the same calling convention and parameters as the original
// function.
// preamble_stub: A pointer to a buffer where the preamble stub
// should be copied. The size of the buffer should be sufficient to
// hold the preamble bytes.
// stub_size: Size in bytes of the buffer allocated for the
// preamble_stub
// bytes_needed: Pointer to a variable that receives the minimum
// number of bytes required for the stub. Can be set to NULL if you're
// not interested.
// Returns An error code indicating the result of patching.
static SideStepError RawPatchWithStub(void* target_function,
void *replacement_function,
unsigned char* preamble_stub,
size_t stub_size,
size_t* bytes_needed);
}; // namespace sidestep