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- // Copyright 2018 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 "base/debug/elf_reader.h"
- #include <arpa/inet.h>
- #include <elf.h>
- #include <string.h>
- #include "base/bits.h"
- #include "base/containers/span.h"
- #include "base/hash/sha1.h"
- #include "base/strings/safe_sprintf.h"
- #include "build/build_config.h"
- #include "third_party/abseil-cpp/absl/types/optional.h"
- // NOTE: This code may be used in crash handling code, so the implementation
- // must avoid dynamic memory allocation or using data structures which rely on
- // dynamic allocation.
- namespace base {
- namespace debug {
- namespace {
- // See https://refspecs.linuxbase.org/elf/elf.pdf for the ELF specification.
- #if __SIZEOF_POINTER__ == 4
- using Ehdr = Elf32_Ehdr;
- using Dyn = Elf32_Dyn;
- using Half = Elf32_Half;
- using Nhdr = Elf32_Nhdr;
- using Word = Elf32_Word;
- using Xword = Elf32_Word;
- #else
- using Ehdr = Elf64_Ehdr;
- using Dyn = Elf64_Dyn;
- using Half = Elf64_Half;
- using Nhdr = Elf64_Nhdr;
- using Word = Elf64_Word;
- using Xword = Elf64_Xword;
- #endif
- constexpr char kGnuNoteName[] = "GNU";
- // Returns a pointer to the header of the ELF binary mapped into memory, or a
- // null pointer if the header is invalid. Here and below |elf_mapped_base| is a
- // pointer to the start of the ELF image.
- const Ehdr* GetElfHeader(const void* elf_mapped_base) {
- if (strncmp(reinterpret_cast<const char*>(elf_mapped_base), ELFMAG,
- SELFMAG) != 0)
- return nullptr;
- return reinterpret_cast<const Ehdr*>(elf_mapped_base);
- }
- } // namespace
- size_t ReadElfBuildId(const void* elf_mapped_base,
- bool uppercase,
- ElfBuildIdBuffer build_id) {
- // NOTE: Function should use async signal safe calls only.
- const Ehdr* elf_header = GetElfHeader(elf_mapped_base);
- if (!elf_header)
- return 0;
- const size_t relocation_offset = GetRelocationOffset(elf_mapped_base);
- for (const Phdr& header : GetElfProgramHeaders(elf_mapped_base)) {
- if (header.p_type != PT_NOTE)
- continue;
- // Look for a NT_GNU_BUILD_ID note with name == "GNU".
- const char* current_section =
- reinterpret_cast<const char*>(header.p_vaddr + relocation_offset);
- const char* section_end = current_section + header.p_memsz;
- const Nhdr* current_note = nullptr;
- bool found = false;
- while (current_section < section_end) {
- current_note = reinterpret_cast<const Nhdr*>(current_section);
- if (current_note->n_type == NT_GNU_BUILD_ID) {
- StringPiece note_name(current_section + sizeof(Nhdr),
- current_note->n_namesz);
- // Explicit constructor is used to include the '\0' character.
- if (note_name == StringPiece(kGnuNoteName, sizeof(kGnuNoteName))) {
- found = true;
- break;
- }
- }
- size_t section_size = bits::AlignUp(current_note->n_namesz, 4u) +
- bits::AlignUp(current_note->n_descsz, 4u) +
- sizeof(Nhdr);
- if (section_size > static_cast<size_t>(section_end - current_section))
- return 0;
- current_section += section_size;
- }
- if (!found)
- continue;
- // Validate that the serialized build ID will fit inside |build_id|.
- size_t note_size = current_note->n_descsz;
- if ((note_size * 2) > kMaxBuildIdStringLength)
- continue;
- // Write out the build ID as a null-terminated hex string.
- const uint8_t* build_id_raw =
- reinterpret_cast<const uint8_t*>(current_note) + sizeof(Nhdr) +
- bits::AlignUp(current_note->n_namesz, 4u);
- size_t i = 0;
- for (i = 0; i < current_note->n_descsz; ++i) {
- strings::SafeSNPrintf(&build_id[i * 2], 3, (uppercase ? "%02X" : "%02x"),
- build_id_raw[i]);
- }
- build_id[i * 2] = '\0';
- // Return the length of the string.
- return i * 2;
- }
- return 0;
- }
- absl::optional<StringPiece> ReadElfLibraryName(const void* elf_mapped_base) {
- // NOTE: Function should use async signal safe calls only.
- const Ehdr* elf_header = GetElfHeader(elf_mapped_base);
- if (!elf_header)
- return {};
- const size_t relocation_offset = GetRelocationOffset(elf_mapped_base);
- for (const Phdr& header : GetElfProgramHeaders(elf_mapped_base)) {
- if (header.p_type != PT_DYNAMIC)
- continue;
- // Read through the ELF dynamic sections to find the string table and
- // SONAME offsets, which are used to compute the offset of the library
- // name string.
- const Dyn* dynamic_start =
- reinterpret_cast<const Dyn*>(header.p_vaddr + relocation_offset);
- const Dyn* dynamic_end = reinterpret_cast<const Dyn*>(
- header.p_vaddr + relocation_offset + header.p_memsz);
- Xword soname_strtab_offset = 0;
- const char* strtab_addr = 0;
- for (const Dyn* dynamic_iter = dynamic_start; dynamic_iter < dynamic_end;
- ++dynamic_iter) {
- if (dynamic_iter->d_tag == DT_STRTAB) {
- #if BUILDFLAG(IS_FUCHSIA) || BUILDFLAG(IS_ANDROID)
- // Fuchsia and Android do not relocate the symtab pointer on ELF load.
- strtab_addr = static_cast<size_t>(dynamic_iter->d_un.d_ptr) +
- reinterpret_cast<const char*>(relocation_offset);
- #else
- strtab_addr = reinterpret_cast<const char*>(dynamic_iter->d_un.d_ptr);
- #endif
- } else if (dynamic_iter->d_tag == DT_SONAME) {
- // The Android NDK wrongly defines `d_val` as an Elf32_Sword for 32 bits
- // and thus needs this cast.
- soname_strtab_offset = static_cast<Xword>(dynamic_iter->d_un.d_val);
- }
- }
- if (soname_strtab_offset && strtab_addr)
- return StringPiece(strtab_addr + soname_strtab_offset);
- }
- return absl::nullopt;
- }
- span<const Phdr> GetElfProgramHeaders(const void* elf_mapped_base) {
- // NOTE: Function should use async signal safe calls only.
- const Ehdr* elf_header = GetElfHeader(elf_mapped_base);
- if (!elf_header)
- return {};
- const char* phdr_start =
- reinterpret_cast<const char*>(elf_header) + elf_header->e_phoff;
- return span<const Phdr>(reinterpret_cast<const Phdr*>(phdr_start),
- elf_header->e_phnum);
- }
- // Returns the offset to add to virtual addresses in the image to compute the
- // mapped virtual address.
- size_t GetRelocationOffset(const void* elf_mapped_base) {
- span<const Phdr> headers = GetElfProgramHeaders(elf_mapped_base);
- for (const Phdr& header : headers) {
- if (header.p_type == PT_LOAD) {
- // |elf_mapped_base| + |header.p_offset| is the mapped address of this
- // segment. |header.p_vaddr| is the specified virtual address within the
- // ELF image.
- const char* const mapped_address =
- reinterpret_cast<const char*>(elf_mapped_base) + header.p_offset;
- return reinterpret_cast<uintptr_t>(mapped_address) - header.p_vaddr;
- }
- }
- // Assume the virtual addresses in the image start at 0, so the offset is
- // from 0 to the actual mapped base address.
- return static_cast<size_t>(reinterpret_cast<uintptr_t>(elf_mapped_base) -
- reinterpret_cast<uintptr_t>(nullptr));
- }
- } // namespace debug
- } // namespace base
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