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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.
- #include "courgette/disassembler_elf_32_x86.h"
- #include <memory>
- #include <utility>
- #include <vector>
- #include "courgette/assembly_program.h"
- #include "courgette/courgette.h"
- namespace courgette {
- CheckBool DisassemblerElf32X86::TypedRVAX86::ComputeRelativeTarget(
- const uint8_t* op_pointer) {
- set_relative_target(Read32LittleEndian(op_pointer) + 4);
- return true;
- }
- CheckBool DisassemblerElf32X86::TypedRVAX86::EmitInstruction(
- Label* label,
- InstructionReceptor* receptor) {
- return receptor->EmitRel32(label);
- }
- uint16_t DisassemblerElf32X86::TypedRVAX86::op_size() const {
- return 4;
- }
- DisassemblerElf32X86::DisassemblerElf32X86(const uint8_t* 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 =
- static_cast<elf32_rel_386_type_values>(rel.r_info & 0xFF);
- // The other 3 bytes of r_info are the symbol.
- uint32_t 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,
- InstructionReceptor* receptor) const {
- // 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.
- FileOffset file_offset = section_header->sh_offset;
- FileOffset section_end = file_offset + section_header->sh_size;
- const Elf32_Rel* section_relocs_iter = reinterpret_cast<const Elf32_Rel*>(
- FileOffsetToPointer(section_header->sh_offset));
- uint32_t section_relocs_count =
- section_header->sh_size / section_header->sh_entsize;
- if (abs32_locations_.empty())
- match = false;
- if (abs32_locations_.size() > section_relocs_count)
- match = false;
- std::vector<RVA>::const_iterator reloc_iter = abs32_locations_.begin();
- // Try to match successive reloc units with (sorted) |abs32_locations_|.
- 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) {
- // Success: Emit relocation table.
- if (!receptor->EmitElfRelocation())
- return false;
- file_offset += sizeof(Elf32_Rel) * abs32_locations_.size();
- }
- return ParseSimpleRegion(file_offset, section_end, receptor);
- }
- CheckBool DisassemblerElf32X86::ParseRel32RelocsFromSection(
- const Elf32_Shdr* section_header) {
- FileOffset start_file_offset = section_header->sh_offset;
- FileOffset end_file_offset = start_file_offset + section_header->sh_size;
- const uint8_t* start_pointer = FileOffsetToPointer(start_file_offset);
- const uint8_t* end_pointer = FileOffsetToPointer(end_file_offset);
- // Quick way to convert from Pointer to RVA within a single Section is to
- // subtract |pointer_to_rva|.
- const uint8_t* const adjust_pointer_to_rva =
- start_pointer - section_header->sh_addr;
- std::vector<RVA>::iterator abs32_pos = abs32_locations_.begin();
- // Find the rel32 relocations.
- const uint8_t* p = start_pointer;
- while (p < end_pointer) {
- // 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_t* rel32 = nullptr;
- 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);
- // Is there an abs32 reloc overlapping the candidate?
- while (abs32_pos != abs32_locations_.end() && *abs32_pos < rel32_rva - 3)
- ++abs32_pos;
- // Now: (*abs32_pos > rel32_rva - 4) i.e. the lowest addressed 4-byte
- // region that could overlap rel32_rva.
- if (abs32_pos != abs32_locations_.end()) {
- if (*abs32_pos < rel32_rva + 4) {
- // Beginning of abs32 reloc is before end of rel32 reloc so they
- // overlap. Skip four bytes past the abs32 reloc.
- RVA current_rva = static_cast<RVA>(p - adjust_pointer_to_rva);
- p += (*abs32_pos + 4) - current_rva;
- continue;
- }
- }
- std::unique_ptr<TypedRVAX86> typed_rel32_rva(new TypedRVAX86(rel32_rva));
- if (!typed_rel32_rva->ComputeRelativeTarget(rel32))
- return false;
- RVA target_rva = typed_rel32_rva->rva() +
- typed_rel32_rva->relative_target();
- if (IsValidTargetRVA(target_rva)) {
- rel32_locations_.push_back(std::move(typed_rel32_rva));
- #if COURGETTE_HISTOGRAM_TARGETS
- ++rel32_target_rvas_[target_rva];
- #endif
- p = rel32 + 4;
- continue;
- }
- }
- p += 1;
- }
- return true;
- }
- } // namespace courgette
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