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// Copyright (c) 2012 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.
* Implementation of PreamblePatcher
#include "preamble_patcher.h"
#include "mini_disassembler.h"
// Definitions of assembly statements we need
#define ASM_JMP32REL 0xE9
#define ASM_INT3 0xCC
namespace sidestep {
SideStepError PreamblePatcher::RawPatchWithStub(
void* target_function,
void *replacement_function,
unsigned char* preamble_stub,
unsigned long stub_size,
unsigned long* bytes_needed) {
if ((NULL == target_function) ||
(NULL == replacement_function) ||
(NULL == preamble_stub)) {
ASSERT(false, "Invalid parameters - either pTargetFunction or "
"pReplacementFunction or pPreambleStub were NULL.");
// TODO(V7:joi) Siggi and I just had a discussion and decided that both
// patching and unpatching are actually unsafe. We also discussed a
// method of making it safe, which is to freeze all other threads in the
// process, check their thread context to see if their eip is currently
// inside the block of instructions we need to copy to the stub, and if so
// wait a bit and try again, then unfreeze all threads once we've patched.
// Not implementing this for now since we're only using SideStep for unit
// testing, but if we ever use it for production code this is what we
// should do.
// NOTE: Stoyan suggests we can write 8 or even 10 bytes atomically using
// FPU instructions, and on newer processors we could use cmpxchg8b or
// cmpxchg16b. So it might be possible to do the patching/unpatching
// atomically and avoid having to freeze other threads. Note though, that
// doing it atomically does not help if one of the other threads happens
// to have its eip in the middle of the bytes you change while you change
// them.
unsigned char* target = reinterpret_cast<unsigned char*>(target_function);
// First, deal with a special case that we see with functions that
// point into an IAT table (including functions linked statically
// into the application): these function already starts with
// ASM_JMP32REL. For instance, malloc() might be implemented as a
// JMP to __malloc(). In that case, we replace the destination of
// the JMP (__malloc), rather than the JMP itself (malloc). This
// way we get the correct behavior no matter how malloc gets called.
if (target[0] == ASM_JMP32REL) {
// target[1-4] holds the place the jmp goes to, but it's
// relative to the next instruction.
int relative_offset; // Windows guarantees int is 4 bytes
ASSERT1(sizeof(relative_offset) == 4);
reinterpret_cast<void*>(target + 1), 4);
// I'd like to just say "target = target + 5 + relative_offset" here, but
// I can't, because the new target will need to have its protections set.
return RawPatchWithStubAndProtections(target + 5 + relative_offset,
replacement_function, preamble_stub,
stub_size, bytes_needed);
// Let's disassemble the preamble of the target function to see if we can
// patch, and to see how much of the preamble we need to take. We need 5
// bytes for our jmp instruction, so let's find the minimum number of
// instructions to get 5 bytes.
MiniDisassembler disassembler;
unsigned int preamble_bytes = 0;
while (preamble_bytes < 5) {
InstructionType instruction_type =
disassembler.Disassemble(target + preamble_bytes, preamble_bytes);
if (IT_JUMP == instruction_type) {
ASSERT(false, "Unable to patch because there is a jump instruction "
"in the first 5 bytes.");
} else if (IT_RETURN == instruction_type) {
ASSERT(false, "Unable to patch because function is too short");
} else if (IT_GENERIC != instruction_type) {
ASSERT(false, "Disassembler encountered unsupported instruction "
"(either unused or unknown)");
if (NULL != bytes_needed)
*bytes_needed = preamble_bytes + 5;
// Inv: cbPreamble is the number of bytes (at least 5) that we need to take
// from the preamble to have whole instructions that are 5 bytes or more
// in size total. The size of the stub required is cbPreamble + size of
// jmp (5)
if (preamble_bytes + 5 > stub_size) {
// First, copy the preamble that we will overwrite.
reinterpret_cast<void*>(target), preamble_bytes);
// Now, make a jmp instruction to the rest of the target function (minus the
// preamble bytes we moved into the stub) and copy it into our preamble-stub.
// find address to jump to, relative to next address after jmp instruction
#ifdef _MSC_VER
#pragma warning(push)
#pragma warning(disable:4244)
int relative_offset_to_target_rest
= ((reinterpret_cast<unsigned char*>(target) + preamble_bytes) -
(preamble_stub + preamble_bytes + 5));
#ifdef _MSC_VER
#pragma warning(pop)
// jmp (Jump near, relative, displacement relative to next instruction)
preamble_stub[preamble_bytes] = ASM_JMP32REL;
// copy the address
memcpy(reinterpret_cast<void*>(preamble_stub + preamble_bytes + 1),
reinterpret_cast<void*>(&relative_offset_to_target_rest), 4);
// Inv: preamble_stub points to assembly code that will execute the
// original function by first executing the first cbPreamble bytes of the
// preamble, then jumping to the rest of the function.
// Overwrite the first 5 bytes of the target function with a jump to our
// replacement function.
// (Jump near, relative, displacement relative to next instruction)
target[0] = ASM_JMP32REL;
// Find offset from instruction after jmp, to the replacement function.
#ifdef _MSC_VER
#pragma warning(push)
#pragma warning(disable:4244)
int offset_to_replacement_function =
reinterpret_cast<unsigned char*>(replacement_function) -
reinterpret_cast<unsigned char*>(target) - 5;
#ifdef _MSC_VER
#pragma warning(pop)
// complete the jmp instruction
memcpy(reinterpret_cast<void*>(target + 1),
reinterpret_cast<void*>(&offset_to_replacement_function), 4);
// Set any remaining bytes that were moved to the preamble-stub to INT3 so
// as not to cause confusion (otherwise you might see some strange
// instructions if you look at the disassembly, or even invalid
// instructions). Also, by doing this, we will break into the debugger if
// some code calls into this portion of the code. If this happens, it
// means that this function cannot be patched using this patcher without
// further thought.
if (preamble_bytes > 5) {
memset(reinterpret_cast<void*>(target + 5), ASM_INT3, preamble_bytes - 5);
// Inv: The memory pointed to by target_function now points to a relative
// jump instruction that jumps over to the preamble_stub. The preamble
// stub contains the first stub_size bytes of the original target
// function's preamble code, followed by a relative jump back to the next
// instruction after the first cbPreamble bytes.
}; // namespace sidestep