summaryrefslogtreecommitdiffstats
path: root/courgette/disassembler_elf_32_x86.cc
blob: 00bb650f205b29113514baa64306136a9301aec0 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
// 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.

#include "courgette/disassembler_elf_32_x86.h"

#include <algorithm>
#include <string>
#include <vector>

#include "base/basictypes.h"
#include "base/logging.h"

#include "courgette/assembly_program.h"
#include "courgette/courgette.h"
#include "courgette/encoded_program.h"

namespace courgette {

DisassemblerElf32X86::DisassemblerElf32X86(const void* start, size_t length)
  : DisassemblerElf32(start, length) {
}

// Convert an ELF relocation struction into an RVA
CheckBool DisassemblerElf32X86::RelToRVA(Elf32_Rel rel, RVA* result) const {

  // The rightmost byte of r_info is the type...
  elf32_rel_386_type_values type =
      (elf32_rel_386_type_values)(unsigned char)rel.r_info;

  // The other 3 bytes of r_info are the symbol
  uint32 symbol =  rel.r_info >> 8;

  switch(type)
  {
    case R_386_NONE:
    case R_386_32:
    case R_386_PC32:
    case R_386_GOT32:
    case R_386_PLT32:
    case R_386_COPY:
    case R_386_GLOB_DAT:
    case R_386_JMP_SLOT:
      return false;

    case R_386_RELATIVE:
      if (symbol != 0)
        return false;

      // This is a basic ABS32 relocation address
      *result = rel.r_offset;
      return true;

    case R_386_GOTOFF:
    case R_386_GOTPC:
    case R_386_TLS_TPOFF:
      return false;
  }

  return false;
}

CheckBool DisassemblerElf32X86::ParseRelocationSection(
    const Elf32_Shdr *section_header,
      AssemblyProgram* program) {
  // We can reproduce the R_386_RELATIVE entries in one of the relocation
  // table based on other information in the patch, given these
  // conditions....
  //
  // All R_386_RELATIVE entries are:
  //   1) In the same relocation table
  //   2) Are consecutive
  //   3) Are sorted in memory address order
  //
  // Happily, this is normally the case, but it's not required by spec
  // so we check, and just don't do it if we don't match up.

  // The expectation is that one relocation section will contain
  // all of our R_386_RELATIVE entries in the expected order followed
  // by assorted other entries we can't use special handling for.

  bool match = true;

  // Walk all the bytes in the section, matching relocation table or not
  size_t file_offset = section_header->sh_offset;
  size_t section_end = section_header->sh_offset + section_header->sh_size;

  Elf32_Rel *section_relocs_iter =
      (Elf32_Rel *)OffsetToPointer(section_header->sh_offset);

  uint32 section_relocs_count = section_header->sh_size /
                                section_header->sh_entsize;

  if (abs32_locations_.size() > section_relocs_count)
    match = false;

  std::vector<RVA>::iterator reloc_iter = abs32_locations_.begin();

  while (match && (reloc_iter !=  abs32_locations_.end())) {
    if (section_relocs_iter->r_info != R_386_RELATIVE ||
        section_relocs_iter->r_offset != *reloc_iter)
      match = false;
    section_relocs_iter++;
    reloc_iter++;
  }

  if (match) {
    // Skip over relocation tables
    if (!program->EmitElfRelocationInstruction())
      return false;
    file_offset += sizeof(Elf32_Rel) * abs32_locations_.size();
  }

  return ParseSimpleRegion(file_offset, section_end, program);
}

CheckBool DisassemblerElf32X86::ParseRel32RelocsFromSection(
    const Elf32_Shdr* section_header) {

  uint32 start_file_offset = section_header->sh_offset;
  uint32 end_file_offset = start_file_offset + section_header->sh_size;

  const uint8* start_pointer = OffsetToPointer(start_file_offset);
  const uint8* end_pointer = OffsetToPointer(end_file_offset);

  // Quick way to convert from Pointer to RVA within a single Section is to
  // subtract 'pointer_to_rva'.
  const uint8* const adjust_pointer_to_rva = start_pointer -
                                             section_header->sh_addr;

  // Find the rel32 relocations.
  const uint8* p = start_pointer;
  while (p < end_pointer) {
    //RVA current_rva = static_cast<RVA>(p - adjust_pointer_to_rva);

    // Heuristic discovery of rel32 locations in instruction stream: are the
    // next few bytes the start of an instruction containing a rel32
    // addressing mode?
    const uint8* rel32 = NULL;

    if (p + 5 <= end_pointer) {
      if (*p == 0xE8 || *p == 0xE9) {  // jmp rel32 and call rel32
        rel32 = p + 1;
      }
    }
    if (p + 6 <= end_pointer) {
      if (*p == 0x0F  &&  (*(p+1) & 0xF0) == 0x80) {  // Jcc long form
        if (p[1] != 0x8A && p[1] != 0x8B)  // JPE/JPO unlikely
          rel32 = p + 2;
      }
    }
    if (rel32) {
      RVA rel32_rva = static_cast<RVA>(rel32 - adjust_pointer_to_rva);

      RVA target_rva = rel32_rva + 4 + Read32LittleEndian(rel32);
      // To be valid, rel32 target must be within image, and within this
      // section.
      if (IsValidRVA(target_rva)) {
        rel32_locations_.push_back(rel32_rva);
#if COURGETTE_HISTOGRAM_TARGETS
        ++rel32_target_rvas_[target_rva];
#endif
        p = rel32 + 4;
        continue;
      }
    }
    p += 1;
  }

  return true;
}

}  // namespace courgette