1 // Copyright 2015 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 #include "courgette/rel32_finder_x86.h"
9 Rel32FinderX86::Rel32FinderX86(RVA relocs_start_rva, RVA relocs_end_rva)
10 : Rel32Finder(relocs_start_rva, relocs_end_rva) {}
12 // Scan for opcodes matching the following instructions :
14 // Jcc (excluding JPO/JPE)
15 // Falsely detected rel32 that collide with known abs32 or that point outside
16 // valid regions are discarded.
17 void Rel32FinderX86::Find(const uint8_t* start_pointer,
18 const uint8_t* end_pointer,
21 const std::vector<RVA>& abs32_locations) {
22 // Quick way to convert from Pointer to RVA within a single Section is to
23 // subtract |adjust_pointer_to_rva|.
24 const uint8_t* const adjust_pointer_to_rva = start_pointer - start_rva;
26 std::vector<RVA>::const_iterator abs32_pos = abs32_locations.begin();
28 // Find the rel32 relocations.
29 const uint8_t* p = start_pointer;
30 while (p < end_pointer) {
31 RVA current_rva = static_cast<RVA>(p - adjust_pointer_to_rva);
33 // Skip the base reloation table if we encounter it.
34 // Note: We're not bothering to handle the edge case where a Rel32 pointer
35 // collides with |relocs_start_rva_| by being {1, 2, 3}-bytes before it.
36 if (current_rva >= relocs_start_rva_ && current_rva < relocs_end_rva_) {
37 p += relocs_end_rva_ - current_rva;
41 // Heuristic discovery of rel32 locations in instruction stream: are the
42 // next few bytes the start of an instruction containing a rel32
44 const uint8_t* rel32 = nullptr;
46 if (p + 5 <= end_pointer) {
47 if (p[0] == 0xE8 || p[0] == 0xE9) { // jmp rel32 and call rel32
51 if (p + 6 <= end_pointer) {
52 if (p[0] == 0x0F && (p[1] & 0xF0) == 0x80) { // Jcc long form
53 if (p[1] != 0x8A && p[1] != 0x8B) // JPE/JPO unlikely
58 RVA rel32_rva = static_cast<RVA>(rel32 - adjust_pointer_to_rva);
60 // Is there an abs32 reloc overlapping the candidate?
61 while (abs32_pos != abs32_locations.end() && *abs32_pos < rel32_rva - 3)
63 // Now: (*abs32_pos > rel32_rva - 4) i.e. the lowest addressed 4-byte
64 // region that could overlap rel32_rva.
65 if (abs32_pos != abs32_locations.end()) {
66 if (*abs32_pos < rel32_rva + 4) {
67 // Beginning of abs32 reloc is before end of rel32 reloc so they
68 // overlap. Skip four bytes past the abs32 reloc.
69 p += (*abs32_pos + 4) - current_rva;
74 // + 4 since offset is relative to start of next instruction.
75 RVA target_rva = rel32_rva + 4 + Read32LittleEndian(rel32);
76 // Valid, rel32 target must be within image, and within this section.
77 // Subsumes |target_rva| != |kUnassignedRVA|.
78 if (start_rva <= target_rva && target_rva < end_rva) {
79 rel32_locations_.push_back(rel32_rva);
80 #if COURGETTE_HISTOGRAM_TARGETS
81 ++rel32_target_rvas_[target_rva];
91 } // namespace courgette