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flutter_flutter/tools/relocation_packer/src/elf_file.cc

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// Copyright 2014 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 notes:
//
// We need to remove a piece from the ELF shared library. However, we also
// want to ensure that code and data loads at the same addresses as before
// packing, so that tools like breakpad can still match up addresses found
// in any crash dumps with data extracted from the pre-packed version of
// the shared library.
//
// Arranging this means that we have to split one of the LOAD segments into
// two. Unfortunately, the program headers are located at the very start
// of the shared library file, so expanding the program header section
// would cause a lot of consequent changes to files offsets that we don't
// really want to have to handle.
//
// Luckily, though, there is a segment that is always present and always
// unused on Android; the GNU_STACK segment. What we do is to steal that
// and repurpose it to be one of the split LOAD segments. We then have to
// sort LOAD segments by offset to keep the crazy linker happy.
//
// All of this takes place in SplitProgramHeadersForHole(), used on packing,
// and is unraveled on unpacking in CoalesceProgramHeadersForHole(). See
// commentary on those functions for an example of this segment stealing
// in action.
#include "elf_file.h"
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <algorithm>
#include <string>
#include <vector>
#include "debug.h"
#include "elf_traits.h"
#include "libelf.h"
#include "packer.h"
namespace relocation_packer {
// Stub identifier written to 'null out' packed data, "NULL".
static const uint32_t kStubIdentifier = 0x4c4c554eu;
// Out-of-band dynamic tags used to indicate the offset and size of the
// android packed relocations section.
static const ELF::Sword DT_ANDROID_REL_OFFSET = DT_LOOS;
static const ELF::Sword DT_ANDROID_REL_SIZE = DT_LOOS + 1;
// Alignment to preserve, in bytes. This must be at least as large as the
// largest d_align and sh_addralign values found in the loaded file.
// Out of caution for RELRO page alignment, we preserve to a complete target
// page. See http://www.airs.com/blog/archives/189.
static const size_t kPreserveAlignment = 4096;
// Alignment values used by ld and gold for the GNU_STACK segment. Different
// linkers write different values; the actual value is immaterial on Android
// because it ignores GNU_STACK segments. However, it is useful for binary
// comparison and unit test purposes if packing and unpacking can preserve
// them through a round-trip.
static const size_t kLdGnuStackSegmentAlignment = 16;
static const size_t kGoldGnuStackSegmentAlignment = 0;
namespace {
// Get section data. Checks that the section has exactly one data entry,
// so that the section size and the data size are the same. True in
// practice for all sections we resize when packing or unpacking. Done
// by ensuring that a call to elf_getdata(section, data) returns NULL as
// the next data entry.
Elf_Data* GetSectionData(Elf_Scn* section) {
Elf_Data* data = elf_getdata(section, NULL);
CHECK(data && elf_getdata(section, data) == NULL);
return data;
}
// Rewrite section data. Allocates new data and makes it the data element's
// buffer. Relies on program exit to free allocated data.
void RewriteSectionData(Elf_Scn* section,
const void* section_data,
size_t size) {
Elf_Data* data = GetSectionData(section);
CHECK(size == data->d_size);
uint8_t* area = new uint8_t[size];
memcpy(area, section_data, size);
data->d_buf = area;
}
// Verbose ELF header logging.
void VerboseLogElfHeader(const ELF::Ehdr* elf_header) {
VLOG(1) << "e_phoff = " << elf_header->e_phoff;
VLOG(1) << "e_shoff = " << elf_header->e_shoff;
VLOG(1) << "e_ehsize = " << elf_header->e_ehsize;
VLOG(1) << "e_phentsize = " << elf_header->e_phentsize;
VLOG(1) << "e_phnum = " << elf_header->e_phnum;
VLOG(1) << "e_shnum = " << elf_header->e_shnum;
VLOG(1) << "e_shstrndx = " << elf_header->e_shstrndx;
}
// Verbose ELF program header logging.
void VerboseLogProgramHeader(size_t program_header_index,
const ELF::Phdr* program_header) {
std::string type;
switch (program_header->p_type) {
case PT_NULL: type = "NULL"; break;
case PT_LOAD: type = "LOAD"; break;
case PT_DYNAMIC: type = "DYNAMIC"; break;
case PT_INTERP: type = "INTERP"; break;
case PT_PHDR: type = "PHDR"; break;
case PT_GNU_RELRO: type = "GNU_RELRO"; break;
case PT_GNU_STACK: type = "GNU_STACK"; break;
case PT_ARM_EXIDX: type = "EXIDX"; break;
default: type = "(OTHER)"; break;
}
VLOG(1) << "phdr[" << program_header_index << "] : " << type;
VLOG(1) << " p_offset = " << program_header->p_offset;
VLOG(1) << " p_vaddr = " << program_header->p_vaddr;
VLOG(1) << " p_paddr = " << program_header->p_paddr;
VLOG(1) << " p_filesz = " << program_header->p_filesz;
VLOG(1) << " p_memsz = " << program_header->p_memsz;
VLOG(1) << " p_flags = " << program_header->p_flags;
VLOG(1) << " p_align = " << program_header->p_align;
}
// Verbose ELF section header logging.
void VerboseLogSectionHeader(const std::string& section_name,
const ELF::Shdr* section_header) {
VLOG(1) << "section " << section_name;
VLOG(1) << " sh_addr = " << section_header->sh_addr;
VLOG(1) << " sh_offset = " << section_header->sh_offset;
VLOG(1) << " sh_size = " << section_header->sh_size;
VLOG(1) << " sh_addralign = " << section_header->sh_addralign;
}
// Verbose ELF section data logging.
void VerboseLogSectionData(const Elf_Data* data) {
VLOG(1) << " data";
VLOG(1) << " d_buf = " << data->d_buf;
VLOG(1) << " d_off = " << data->d_off;
VLOG(1) << " d_size = " << data->d_size;
VLOG(1) << " d_align = " << data->d_align;
}
} // namespace
// Load the complete ELF file into a memory image in libelf, and identify
// the .rel.dyn or .rela.dyn, .dynamic, and .android.rel.dyn or
// .android.rela.dyn sections. No-op if the ELF file has already been loaded.
bool ElfFile::Load() {
if (elf_)
return true;
Elf* elf = elf_begin(fd_, ELF_C_RDWR, NULL);
CHECK(elf);
if (elf_kind(elf) != ELF_K_ELF) {
LOG(ERROR) << "File not in ELF format";
return false;
}
ELF::Ehdr* elf_header = ELF::getehdr(elf);
if (!elf_header) {
LOG(ERROR) << "Failed to load ELF header: " << elf_errmsg(elf_errno());
return false;
}
if (elf_header->e_machine != ELF::kMachine) {
LOG(ERROR) << "ELF file architecture is not " << ELF::Machine();
return false;
}
if (elf_header->e_type != ET_DYN) {
LOG(ERROR) << "ELF file is not a shared object";
return false;
}
// Require that our endianness matches that of the target, and that both
// are little-endian. Safe for all current build/target combinations.
const int endian = elf_header->e_ident[EI_DATA];
CHECK(endian == ELFDATA2LSB);
CHECK(__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__);
// Also require that the file class is as expected.
const int file_class = elf_header->e_ident[EI_CLASS];
CHECK(file_class == ELF::kFileClass);
VLOG(1) << "endian = " << endian << ", file class = " << file_class;
VerboseLogElfHeader(elf_header);
const ELF::Phdr* elf_program_header = ELF::getphdr(elf);
CHECK(elf_program_header);
const ELF::Phdr* dynamic_program_header = NULL;
for (size_t i = 0; i < elf_header->e_phnum; ++i) {
const ELF::Phdr* program_header = &elf_program_header[i];
VerboseLogProgramHeader(i, program_header);
if (program_header->p_type == PT_DYNAMIC) {
CHECK(dynamic_program_header == NULL);
dynamic_program_header = program_header;
}
}
CHECK(dynamic_program_header != NULL);
size_t string_index;
elf_getshdrstrndx(elf, &string_index);
// Notes of the dynamic relocations, packed relocations, and .dynamic
// sections. Found while iterating sections, and later stored in class
// attributes.
Elf_Scn* found_relocations_section = NULL;
Elf_Scn* found_android_relocations_section = NULL;
Elf_Scn* found_dynamic_section = NULL;
// Notes of relocation section types seen. We require one or the other of
// these; both is unsupported.
bool has_rel_relocations = false;
bool has_rela_relocations = false;
Elf_Scn* section = NULL;
while ((section = elf_nextscn(elf, section)) != NULL) {
const ELF::Shdr* section_header = ELF::getshdr(section);
std::string name = elf_strptr(elf, string_index, section_header->sh_name);
VerboseLogSectionHeader(name, section_header);
// Note relocation section types.
if (section_header->sh_type == SHT_REL) {
has_rel_relocations = true;
}
if (section_header->sh_type == SHT_RELA) {
has_rela_relocations = true;
}
// Note special sections as we encounter them.
if ((name == ".rel.dyn" || name == ".rela.dyn") &&
section_header->sh_size > 0) {
found_relocations_section = section;
}
if ((name == ".android.rel.dyn" || name == ".android.rela.dyn") &&
section_header->sh_size > 0) {
found_android_relocations_section = section;
}
if (section_header->sh_offset == dynamic_program_header->p_offset) {
found_dynamic_section = section;
}
// Ensure we preserve alignment, repeated later for the data block(s).
CHECK(section_header->sh_addralign <= kPreserveAlignment);
Elf_Data* data = NULL;
while ((data = elf_getdata(section, data)) != NULL) {
CHECK(data->d_align <= kPreserveAlignment);
VerboseLogSectionData(data);
}
}
// Loading failed if we did not find the required special sections.
if (!found_relocations_section) {
LOG(ERROR) << "Missing or empty .rel.dyn or .rela.dyn section";
return false;
}
if (!found_android_relocations_section) {
LOG(ERROR) << "Missing or empty .android.rel.dyn or .android.rela.dyn "
<< "section (to fix, run with --help and follow the "
<< "pre-packing instructions)";
return false;
}
if (!found_dynamic_section) {
LOG(ERROR) << "Missing .dynamic section";
return false;
}
// Loading failed if we could not identify the relocations type.
if (!has_rel_relocations && !has_rela_relocations) {
LOG(ERROR) << "No relocations sections found";
return false;
}
if (has_rel_relocations && has_rela_relocations) {
LOG(ERROR) << "Multiple relocations sections with different types found, "
<< "not currently supported";
return false;
}
elf_ = elf;
relocations_section_ = found_relocations_section;
dynamic_section_ = found_dynamic_section;
android_relocations_section_ = found_android_relocations_section;
relocations_type_ = has_rel_relocations ? REL : RELA;
return true;
}
namespace {
// Helper for ResizeSection(). Adjust the main ELF header for the hole.
void AdjustElfHeaderForHole(ELF::Ehdr* elf_header,
ELF::Off hole_start,
ssize_t hole_size) {
if (elf_header->e_phoff > hole_start) {
elf_header->e_phoff += hole_size;
VLOG(1) << "e_phoff adjusted to " << elf_header->e_phoff;
}
if (elf_header->e_shoff > hole_start) {
elf_header->e_shoff += hole_size;
VLOG(1) << "e_shoff adjusted to " << elf_header->e_shoff;
}
}
// Helper for ResizeSection(). Adjust all section headers for the hole.
void AdjustSectionHeadersForHole(Elf* elf,
ELF::Off hole_start,
ssize_t hole_size) {
size_t string_index;
elf_getshdrstrndx(elf, &string_index);
Elf_Scn* section = NULL;
while ((section = elf_nextscn(elf, section)) != NULL) {
ELF::Shdr* section_header = ELF::getshdr(section);
std::string name = elf_strptr(elf, string_index, section_header->sh_name);
if (section_header->sh_offset > hole_start) {
section_header->sh_offset += hole_size;
VLOG(1) << "section " << name
<< " sh_offset adjusted to " << section_header->sh_offset;
}
}
}
// Helper for ResizeSection(). Adjust the offsets of any program headers
// that have offsets currently beyond the hole start.
void AdjustProgramHeaderOffsets(ELF::Phdr* program_headers,
size_t count,
ELF::Phdr* ignored_1,
ELF::Phdr* ignored_2,
ELF::Off hole_start,
ssize_t hole_size) {
for (size_t i = 0; i < count; ++i) {
ELF::Phdr* program_header = &program_headers[i];
if (program_header == ignored_1 || program_header == ignored_2)
continue;
if (program_header->p_offset > hole_start) {
// The hole start is past this segment, so adjust offset.
program_header->p_offset += hole_size;
VLOG(1) << "phdr[" << i
<< "] p_offset adjusted to "<< program_header->p_offset;
}
}
}
// Helper for ResizeSection(). Find the first loadable segment in the
// file. We expect it to map from file offset zero.
ELF::Phdr* FindFirstLoadSegment(ELF::Phdr* program_headers,
size_t count) {
ELF::Phdr* first_loadable_segment = NULL;
for (size_t i = 0; i < count; ++i) {
ELF::Phdr* program_header = &program_headers[i];
if (program_header->p_type == PT_LOAD &&
program_header->p_offset == 0 &&
program_header->p_vaddr == 0 &&
program_header->p_paddr == 0) {
first_loadable_segment = program_header;
}
}
LOG_IF(FATAL, !first_loadable_segment)
<< "Cannot locate a LOAD segment with address and offset zero";
return first_loadable_segment;
}
// Helper for ResizeSection(). Deduce the alignment that the PT_GNU_STACK
// segment will use. Determined by sensing the linker that was used to
// create the shared library.
size_t DeduceGnuStackSegmentAlignment(Elf* elf) {
size_t string_index;
elf_getshdrstrndx(elf, &string_index);
Elf_Scn* section = NULL;
size_t gnu_stack_segment_alignment = kLdGnuStackSegmentAlignment;
while ((section = elf_nextscn(elf, section)) != NULL) {
const ELF::Shdr* section_header = ELF::getshdr(section);
std::string name = elf_strptr(elf, string_index, section_header->sh_name);
if (name == ".note.gnu.gold-version") {
gnu_stack_segment_alignment = kGoldGnuStackSegmentAlignment;
break;
}
}
return gnu_stack_segment_alignment;
}
// Helper for ResizeSection(). Find the PT_GNU_STACK segment, and check
// that it contains what we expect so we can restore it on unpack if needed.
ELF::Phdr* FindUnusedGnuStackSegment(Elf* elf,
ELF::Phdr* program_headers,
size_t count) {
ELF::Phdr* unused_segment = NULL;
const size_t stack_alignment = DeduceGnuStackSegmentAlignment(elf);
for (size_t i = 0; i < count; ++i) {
ELF::Phdr* program_header = &program_headers[i];
if (program_header->p_type == PT_GNU_STACK &&
program_header->p_offset == 0 &&
program_header->p_vaddr == 0 &&
program_header->p_paddr == 0 &&
program_header->p_filesz == 0 &&
program_header->p_memsz == 0 &&
program_header->p_flags == (PF_R | PF_W) &&
program_header->p_align == stack_alignment) {
unused_segment = program_header;
}
}
LOG_IF(FATAL, !unused_segment)
<< "Cannot locate the expected GNU_STACK segment";
return unused_segment;
}
// Helper for ResizeSection(). Find the segment that was the first loadable
// one before we split it into two. This is the one into which we coalesce
// the split segments on unpacking.
ELF::Phdr* FindOriginalFirstLoadSegment(ELF::Phdr* program_headers,
size_t count) {
const ELF::Phdr* first_loadable_segment =
FindFirstLoadSegment(program_headers, count);
ELF::Phdr* original_first_loadable_segment = NULL;
for (size_t i = 0; i < count; ++i) {
ELF::Phdr* program_header = &program_headers[i];
// The original first loadable segment is the one that follows on from
// the one we wrote on split to be the current first loadable segment.
if (program_header->p_type == PT_LOAD &&
program_header->p_offset == first_loadable_segment->p_filesz) {
original_first_loadable_segment = program_header;
}
}
LOG_IF(FATAL, !original_first_loadable_segment)
<< "Cannot locate the LOAD segment that follows a LOAD at offset zero";
return original_first_loadable_segment;
}
// Helper for ResizeSection(). Find the segment that contains the hole.
Elf_Scn* FindSectionContainingHole(Elf* elf,
ELF::Off hole_start,
ssize_t hole_size) {
Elf_Scn* section = NULL;
Elf_Scn* last_unholed_section = NULL;
while ((section = elf_nextscn(elf, section)) != NULL) {
const ELF::Shdr* section_header = ELF::getshdr(section);
// Because we get here after section headers have been adjusted for the
// hole, we need to 'undo' that adjustment to give a view of the original
// sections layout.
ELF::Off offset = section_header->sh_offset;
if (section_header->sh_offset >= hole_start) {
offset -= hole_size;
}
if (offset <= hole_start) {
last_unholed_section = section;
}
}
LOG_IF(FATAL, !last_unholed_section)
<< "Cannot identify the section before the one containing the hole";
// The section containing the hole is the one after the last one found
// by the loop above.
Elf_Scn* holed_section = elf_nextscn(elf, last_unholed_section);
LOG_IF(FATAL, !holed_section)
<< "Cannot identify the section containing the hole";
return holed_section;
}
// Helper for ResizeSection(). Find the last section contained in a segment.
Elf_Scn* FindLastSectionInSegment(Elf* elf,
ELF::Phdr* program_header,
ELF::Off hole_start,
ssize_t hole_size) {
const ELF::Off segment_end =
program_header->p_offset + program_header->p_filesz;
Elf_Scn* section = NULL;
Elf_Scn* last_section = NULL;
while ((section = elf_nextscn(elf, section)) != NULL) {
const ELF::Shdr* section_header = ELF::getshdr(section);
// As above, 'undo' any section offset adjustment to give a view of the
// original sections layout.
ELF::Off offset = section_header->sh_offset;
if (section_header->sh_offset >= hole_start) {
offset -= hole_size;
}
if (offset < segment_end) {
last_section = section;
}
}
LOG_IF(FATAL, !last_section)
<< "Cannot identify the last section in the given segment";
return last_section;
}
// Helper for ResizeSection(). Order loadable segments by their offsets.
// The crazy linker contains assumptions about loadable segment ordering,
// and it is better if we do not break them.
void SortOrderSensitiveProgramHeaders(ELF::Phdr* program_headers,
size_t count) {
std::vector<ELF::Phdr*> orderable;
// Collect together orderable program headers. These are all the LOAD
// segments, and any GNU_STACK that may be present (removed on packing,
// but replaced on unpacking).
for (size_t i = 0; i < count; ++i) {
ELF::Phdr* program_header = &program_headers[i];
if (program_header->p_type == PT_LOAD ||
program_header->p_type == PT_GNU_STACK) {
orderable.push_back(program_header);
}
}
// Order these program headers so that any PT_GNU_STACK is last, and
// the LOAD segments that precede it appear in offset order. Uses
// insertion sort.
for (size_t i = 1; i < orderable.size(); ++i) {
for (size_t j = i; j > 0; --j) {
ELF::Phdr* first = orderable[j - 1];
ELF::Phdr* second = orderable[j];
if (!(first->p_type == PT_GNU_STACK ||
first->p_offset > second->p_offset)) {
break;
}
std::swap(*first, *second);
}
}
}
// Helper for ResizeSection(). The GNU_STACK program header is unused in
// Android, so we can repurpose it here. Before packing, the program header
// table contains something like:
//
// Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
// LOAD 0x000000 0x00000000 0x00000000 0x1efc818 0x1efc818 R E 0x1000
// LOAD 0x1efd008 0x01efe008 0x01efe008 0x17ec3c 0x1a0324 RW 0x1000
// DYNAMIC 0x205ec50 0x0205fc50 0x0205fc50 0x00108 0x00108 RW 0x4
// GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RW 0
//
// The hole in the file is in the first of these. In order to preserve all
// load addresses, what we do is to turn the GNU_STACK into a new LOAD entry
// that maps segments up to where we created the hole, adjust the first LOAD
// entry so that it maps segments after that, adjust any other program
// headers whose offset is after the hole start, and finally order the LOAD
// segments by offset, to give:
//
// Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
// LOAD 0x000000 0x00000000 0x00000000 0x14ea4 0x14ea4 R E 0x1000
// LOAD 0x014ea4 0x00212ea4 0x00212ea4 0x1cea164 0x1cea164 R E 0x1000
// DYNAMIC 0x1e60c50 0x0205fc50 0x0205fc50 0x00108 0x00108 RW 0x4
// LOAD 0x1cff008 0x01efe008 0x01efe008 0x17ec3c 0x1a0324 RW 0x1000
//
// We work out the split points by finding the .rel.dyn or .rela.dyn section
// that contains the hole, and by finding the last section in a given segment.
//
// To unpack, we reverse the above to leave the file as it was originally.
void SplitProgramHeadersForHole(Elf* elf,
ELF::Off hole_start,
ssize_t hole_size) {
CHECK(hole_size < 0);
const ELF::Ehdr* elf_header = ELF::getehdr(elf);
CHECK(elf_header);
ELF::Phdr* elf_program_header = ELF::getphdr(elf);
CHECK(elf_program_header);
const size_t program_header_count = elf_header->e_phnum;
// Locate the segment that we can overwrite to form the new LOAD entry,
// and the segment that we are going to split into two parts.
ELF::Phdr* spliced_header =
FindUnusedGnuStackSegment(elf, elf_program_header, program_header_count);
ELF::Phdr* split_header =
FindFirstLoadSegment(elf_program_header, program_header_count);
VLOG(1) << "phdr[" << split_header - elf_program_header << "] split";
VLOG(1) << "phdr[" << spliced_header - elf_program_header << "] new LOAD";
// Find the section that contains the hole. We split on the section that
// follows it.
Elf_Scn* holed_section =
FindSectionContainingHole(elf, hole_start, hole_size);
size_t string_index;
elf_getshdrstrndx(elf, &string_index);
ELF::Shdr* section_header = ELF::getshdr(holed_section);
std::string name = elf_strptr(elf, string_index, section_header->sh_name);
VLOG(1) << "section " << name << " split after";
// Find the last section in the segment we are splitting.
Elf_Scn* last_section =
FindLastSectionInSegment(elf, split_header, hole_start, hole_size);
section_header = ELF::getshdr(last_section);
name = elf_strptr(elf, string_index, section_header->sh_name);
VLOG(1) << "section " << name << " split end";
// Split on the section following the holed one, and up to (but not
// including) the section following the last one in the split segment.
Elf_Scn* split_section = elf_nextscn(elf, holed_section);
LOG_IF(FATAL, !split_section)
<< "No section follows the section that contains the hole";
Elf_Scn* end_section = elf_nextscn(elf, last_section);
LOG_IF(FATAL, !end_section)
<< "No section follows the last section in the segment being split";
// Split the first portion of split_header into spliced_header.
const ELF::Shdr* split_section_header = ELF::getshdr(split_section);
spliced_header->p_type = split_header->p_type;
spliced_header->p_offset = split_header->p_offset;
spliced_header->p_vaddr = split_header->p_vaddr;
spliced_header->p_paddr = split_header->p_paddr;
CHECK(split_header->p_filesz == split_header->p_memsz);
spliced_header->p_filesz = split_section_header->sh_offset;
spliced_header->p_memsz = split_section_header->sh_offset;
spliced_header->p_flags = split_header->p_flags;
spliced_header->p_align = split_header->p_align;
// Now rewrite split_header to remove the part we spliced from it.
const ELF::Shdr* end_section_header = ELF::getshdr(end_section);
split_header->p_offset = spliced_header->p_filesz;
CHECK(split_header->p_vaddr == split_header->p_paddr);
split_header->p_vaddr = split_section_header->sh_addr;
split_header->p_paddr = split_section_header->sh_addr;
CHECK(split_header->p_filesz == split_header->p_memsz);
split_header->p_filesz =
end_section_header->sh_offset - spliced_header->p_filesz;
split_header->p_memsz =
end_section_header->sh_offset - spliced_header->p_filesz;
// Adjust the offsets of all program headers that are not one of the pair
// we just created by splitting.
AdjustProgramHeaderOffsets(elf_program_header,
program_header_count,
spliced_header,
split_header,
hole_start,
hole_size);
// Finally, order loadable segments by offset/address. The crazy linker
// contains assumptions about loadable segment ordering.
SortOrderSensitiveProgramHeaders(elf_program_header,
program_header_count);
}
// Helper for ResizeSection(). Undo the work of SplitProgramHeadersForHole().
void CoalesceProgramHeadersForHole(Elf* elf,
ELF::Off hole_start,
ssize_t hole_size) {
CHECK(hole_size > 0);
const ELF::Ehdr* elf_header = ELF::getehdr(elf);
CHECK(elf_header);
ELF::Phdr* elf_program_header = ELF::getphdr(elf);
CHECK(elf_program_header);
const size_t program_header_count = elf_header->e_phnum;
// Locate the segment that we overwrote to form the new LOAD entry, and
// the segment that we split into two parts on packing.
ELF::Phdr* spliced_header =
FindFirstLoadSegment(elf_program_header, program_header_count);
ELF::Phdr* split_header =
FindOriginalFirstLoadSegment(elf_program_header, program_header_count);
VLOG(1) << "phdr[" << spliced_header - elf_program_header << "] stack";
VLOG(1) << "phdr[" << split_header - elf_program_header << "] coalesce";
// Find the last section in the second segment we are coalescing.
Elf_Scn* last_section =
FindLastSectionInSegment(elf, split_header, hole_start, hole_size);
size_t string_index;
elf_getshdrstrndx(elf, &string_index);
const ELF::Shdr* section_header = ELF::getshdr(last_section);
std::string name = elf_strptr(elf, string_index, section_header->sh_name);
VLOG(1) << "section " << name << " coalesced";
// Rewrite the coalesced segment into split_header.
const ELF::Shdr* last_section_header = ELF::getshdr(last_section);
split_header->p_offset = spliced_header->p_offset;
CHECK(split_header->p_vaddr == split_header->p_paddr);
split_header->p_vaddr = spliced_header->p_vaddr;
split_header->p_paddr = spliced_header->p_vaddr;
CHECK(split_header->p_filesz == split_header->p_memsz);
split_header->p_filesz =
last_section_header->sh_offset + last_section_header->sh_size;
split_header->p_memsz =
last_section_header->sh_offset + last_section_header->sh_size;
// Reconstruct the original GNU_STACK segment into spliced_header.
const size_t stack_alignment = DeduceGnuStackSegmentAlignment(elf);
spliced_header->p_type = PT_GNU_STACK;
spliced_header->p_offset = 0;
spliced_header->p_vaddr = 0;
spliced_header->p_paddr = 0;
spliced_header->p_filesz = 0;
spliced_header->p_memsz = 0;
spliced_header->p_flags = PF_R | PF_W;
spliced_header->p_align = stack_alignment;
// Adjust the offsets of all program headers that are not one of the pair
// we just coalesced.
AdjustProgramHeaderOffsets(elf_program_header,
program_header_count,
spliced_header,
split_header,
hole_start,
hole_size);
// Finally, order loadable segments by offset/address. The crazy linker
// contains assumptions about loadable segment ordering.
SortOrderSensitiveProgramHeaders(elf_program_header,
program_header_count);
}
// Helper for ResizeSection(). Rewrite program headers.
void RewriteProgramHeadersForHole(Elf* elf,
ELF::Off hole_start,
ssize_t hole_size) {
// If hole_size is negative then we are removing a piece of the file, and
// we want to split program headers so that we keep the same addresses
// for text and data. If positive, then we are putting that piece of the
// file back in, so we coalesce the previously split program headers.
if (hole_size < 0)
SplitProgramHeadersForHole(elf, hole_start, hole_size);
else if (hole_size > 0)
CoalesceProgramHeadersForHole(elf, hole_start, hole_size);
}
// Helper for ResizeSection(). Locate and return the dynamic section.
Elf_Scn* GetDynamicSection(Elf* elf) {
const ELF::Ehdr* elf_header = ELF::getehdr(elf);
CHECK(elf_header);
const ELF::Phdr* elf_program_header = ELF::getphdr(elf);
CHECK(elf_program_header);
// Find the program header that describes the dynamic section.
const ELF::Phdr* dynamic_program_header = NULL;
for (size_t i = 0; i < elf_header->e_phnum; ++i) {
const ELF::Phdr* program_header = &elf_program_header[i];
if (program_header->p_type == PT_DYNAMIC) {
dynamic_program_header = program_header;
}
}
CHECK(dynamic_program_header);
// Now find the section with the same offset as this program header.
Elf_Scn* dynamic_section = NULL;
Elf_Scn* section = NULL;
while ((section = elf_nextscn(elf, section)) != NULL) {
ELF::Shdr* section_header = ELF::getshdr(section);
if (section_header->sh_offset == dynamic_program_header->p_offset) {
dynamic_section = section;
}
}
CHECK(dynamic_section != NULL);
return dynamic_section;
}
// Helper for ResizeSection(). Adjust the .dynamic section for the hole.
template <typename Rel>
void AdjustDynamicSectionForHole(Elf_Scn* dynamic_section,
ELF::Off hole_start,
ssize_t hole_size) {
Elf_Data* data = GetSectionData(dynamic_section);
const ELF::Dyn* dynamic_base = reinterpret_cast<ELF::Dyn*>(data->d_buf);
std::vector<ELF::Dyn> dynamics(
dynamic_base,
dynamic_base + data->d_size / sizeof(dynamics[0]));
for (size_t i = 0; i < dynamics.size(); ++i) {
ELF::Dyn* dynamic = &dynamics[i];
const ELF::Sword tag = dynamic->d_tag;
// DT_RELSZ or DT_RELASZ indicate the overall size of relocations.
// Only one will be present. Adjust by hole size.
if (tag == DT_RELSZ || tag == DT_RELASZ) {
dynamic->d_un.d_val += hole_size;
VLOG(1) << "dynamic[" << i << "] " << dynamic->d_tag
<< " d_val adjusted to " << dynamic->d_un.d_val;
}
// DT_RELCOUNT or DT_RELACOUNT hold the count of relative relocations.
// Only one will be present. Packing reduces it to the alignment
// padding, if any; unpacking restores it to its former value. The
// crazy linker does not use it, but we update it anyway.
if (tag == DT_RELCOUNT || tag == DT_RELACOUNT) {
// Cast sizeof to a signed type to avoid the division result being
// promoted into an unsigned size_t.
const ssize_t sizeof_rel = static_cast<ssize_t>(sizeof(Rel));
dynamic->d_un.d_val += hole_size / sizeof_rel;
VLOG(1) << "dynamic[" << i << "] " << dynamic->d_tag
<< " d_val adjusted to " << dynamic->d_un.d_val;
}
// DT_RELENT and DT_RELAENT do not change, but make sure they are what
// we expect. Only one will be present.
if (tag == DT_RELENT || tag == DT_RELAENT) {
CHECK(dynamic->d_un.d_val == sizeof(Rel));
}
}
void* section_data = &dynamics[0];
size_t bytes = dynamics.size() * sizeof(dynamics[0]);
RewriteSectionData(dynamic_section, section_data, bytes);
}
// Resize a section. If the new size is larger than the current size, open
// up a hole by increasing file offsets that come after the hole. If smaller
// than the current size, remove the hole by decreasing those offsets.
template <typename Rel>
void ResizeSection(Elf* elf, Elf_Scn* section, size_t new_size) {
ELF::Shdr* section_header = ELF::getshdr(section);
if (section_header->sh_size == new_size)
return;
// Note if we are resizing the real dyn relocations.
size_t string_index;
elf_getshdrstrndx(elf, &string_index);
const std::string section_name =
elf_strptr(elf, string_index, section_header->sh_name);
const bool is_relocations_resize =
(section_name == ".rel.dyn" || section_name == ".rela.dyn");
// Require that the section size and the data size are the same. True
// in practice for all sections we resize when packing or unpacking.
Elf_Data* data = GetSectionData(section);
CHECK(data->d_off == 0 && data->d_size == section_header->sh_size);
// Require that the section is not zero-length (that is, has allocated
// data that we can validly expand).
CHECK(data->d_size && data->d_buf);
const ELF::Off hole_start = section_header->sh_offset;
const ssize_t hole_size = new_size - data->d_size;
VLOG_IF(1, (hole_size > 0)) << "expand section size = " << data->d_size;
VLOG_IF(1, (hole_size < 0)) << "shrink section size = " << data->d_size;
// Resize the data and the section header.
data->d_size += hole_size;
section_header->sh_size += hole_size;
// Add the hole size to all offsets in the ELF file that are after the
// start of the hole. If the hole size is positive we are expanding the
// section to create a new hole; if negative, we are closing up a hole.
// Start with the main ELF header.
ELF::Ehdr* elf_header = ELF::getehdr(elf);
AdjustElfHeaderForHole(elf_header, hole_start, hole_size);
// Adjust all section headers.
AdjustSectionHeadersForHole(elf, hole_start, hole_size);
// If resizing the dynamic relocations, rewrite the program headers to
// either split or coalesce segments, and adjust dynamic entries to match.
if (is_relocations_resize) {
RewriteProgramHeadersForHole(elf, hole_start, hole_size);
Elf_Scn* dynamic_section = GetDynamicSection(elf);
AdjustDynamicSectionForHole<Rel>(dynamic_section, hole_start, hole_size);
}
}
// Find the first slot in a dynamics array with the given tag. The array
// always ends with a free (unused) element, and which we exclude from the
// search. Returns dynamics->size() if not found.
size_t FindDynamicEntry(ELF::Sword tag,
std::vector<ELF::Dyn>* dynamics) {
// Loop until the penultimate entry. We exclude the end sentinel.
for (size_t i = 0; i < dynamics->size() - 1; ++i) {
if (dynamics->at(i).d_tag == tag)
return i;
}
// The tag was not found.
return dynamics->size();
}
// Replace the first free (unused) slot in a dynamics vector with the given
// value. The vector always ends with a free (unused) element, so the slot
// found cannot be the last one in the vector.
void AddDynamicEntry(const ELF::Dyn& dyn,
std::vector<ELF::Dyn>* dynamics) {
const size_t slot = FindDynamicEntry(DT_NULL, dynamics);
if (slot == dynamics->size()) {
LOG(FATAL) << "No spare dynamic array slots found "
<< "(to fix, increase gold's --spare-dynamic-tags value)";
}
// Replace this entry with the one supplied.
dynamics->at(slot) = dyn;
VLOG(1) << "dynamic[" << slot << "] overwritten with " << dyn.d_tag;
}
// Remove the element in the dynamics vector that matches the given tag with
// unused slot data. Shuffle the following elements up, and ensure that the
// last is the null sentinel.
void RemoveDynamicEntry(ELF::Sword tag,
std::vector<ELF::Dyn>* dynamics) {
const size_t slot = FindDynamicEntry(tag, dynamics);
CHECK(slot != dynamics->size());
// Remove this entry by shuffling up everything that follows.
for (size_t i = slot; i < dynamics->size() - 1; ++i) {
dynamics->at(i) = dynamics->at(i + 1);
VLOG(1) << "dynamic[" << i
<< "] overwritten with dynamic[" << i + 1 << "]";
}
// Ensure that the end sentinel is still present.
CHECK(dynamics->at(dynamics->size() - 1).d_tag == DT_NULL);
}
// Construct a null relocation without addend.
void NullRelocation(ELF::Rel* relocation) {
relocation->r_offset = 0;
relocation->r_info = ELF_R_INFO(0, ELF::kNoRelocationCode);
}
// Construct a null relocation with addend.
void NullRelocation(ELF::Rela* relocation) {
relocation->r_offset = 0;
relocation->r_info = ELF_R_INFO(0, ELF::kNoRelocationCode);
relocation->r_addend = 0;
}
// Pad relocations with the given number of null entries. Generates its
// null entry with the appropriate NullRelocation() invocation.
template <typename Rel>
void PadRelocations(size_t count, std::vector<Rel>* relocations) {
Rel null_relocation;
NullRelocation(&null_relocation);
std::vector<Rel> padding(count, null_relocation);
relocations->insert(relocations->end(), padding.begin(), padding.end());
}
} // namespace
// Remove relative entries from dynamic relocations and write as packed
// data into android packed relocations.
bool ElfFile::PackRelocations() {
// Load the ELF file into libelf.
if (!Load()) {
LOG(ERROR) << "Failed to load as ELF";
return false;
}
// Retrieve the current dynamic relocations section data.
Elf_Data* data = GetSectionData(relocations_section_);
if (relocations_type_ == REL) {
// Convert data to a vector of relocations.
const ELF::Rel* relocations_base = reinterpret_cast<ELF::Rel*>(data->d_buf);
std::vector<ELF::Rel> relocations(
relocations_base,
relocations_base + data->d_size / sizeof(relocations[0]));
LOG(INFO) << "Relocations : REL";
return PackTypedRelocations<ELF::Rel>(relocations);
}
if (relocations_type_ == RELA) {
// Convert data to a vector of relocations with addends.
const ELF::Rela* relocations_base =
reinterpret_cast<ELF::Rela*>(data->d_buf);
std::vector<ELF::Rela> relocations(
relocations_base,
relocations_base + data->d_size / sizeof(relocations[0]));
LOG(INFO) << "Relocations : RELA";
return PackTypedRelocations<ELF::Rela>(relocations);
}
NOTREACHED();
return false;
}
// Helper for PackRelocations(). Rel type is one of ELF::Rel or ELF::Rela.
template <typename Rel>
bool ElfFile::PackTypedRelocations(const std::vector<Rel>& relocations) {
// Filter relocations into those that are relative and others.
std::vector<Rel> relative_relocations;
std::vector<Rel> other_relocations;
for (size_t i = 0; i < relocations.size(); ++i) {
const Rel& relocation = relocations[i];
if (ELF_R_TYPE(relocation.r_info) == ELF::kRelativeRelocationCode) {
CHECK(ELF_R_SYM(relocation.r_info) == 0);
relative_relocations.push_back(relocation);
} else {
other_relocations.push_back(relocation);
}
}
LOG(INFO) << "Relative : " << relative_relocations.size() << " entries";
LOG(INFO) << "Other : " << other_relocations.size() << " entries";
LOG(INFO) << "Total : " << relocations.size() << " entries";
// If no relative relocations then we have nothing packable. Perhaps
// the shared object has already been packed?
if (relative_relocations.empty()) {
LOG(ERROR) << "No relative relocations found (already packed?)";
return false;
}
// If not padding fully, apply only enough padding to preserve alignment.
// Otherwise, pad so that we do not shrink the relocations section at all.
if (!is_padding_relocations_) {
// Calculate the size of the hole we will close up when we rewrite
// dynamic relocations.
ssize_t hole_size =
relative_relocations.size() * sizeof(relative_relocations[0]);
const ssize_t unaligned_hole_size = hole_size;
// Adjust the actual hole size to preserve alignment. We always adjust
// by a whole number of NONE-type relocations.
while (hole_size % kPreserveAlignment)
hole_size -= sizeof(relative_relocations[0]);
LOG(INFO) << "Compaction : " << hole_size << " bytes";
// Adjusting for alignment may have removed any packing benefit.
if (hole_size == 0) {
LOG(INFO) << "Too few relative relocations to pack after alignment";
return false;
}
// Find the padding needed in other_relocations to preserve alignment.
// Ensure that we never completely empty the real relocations section.
size_t padding_bytes = unaligned_hole_size - hole_size;
if (padding_bytes == 0 && other_relocations.size() == 0) {
do {
padding_bytes += sizeof(relative_relocations[0]);
} while (padding_bytes % kPreserveAlignment);
}
CHECK(padding_bytes % sizeof(other_relocations[0]) == 0);
const size_t padding = padding_bytes / sizeof(other_relocations[0]);
// Padding may have removed any packing benefit.
if (padding >= relative_relocations.size()) {
LOG(INFO) << "Too few relative relocations to pack after padding";
return false;
}
// Add null relocations to other_relocations to preserve alignment.
PadRelocations<Rel>(padding, &other_relocations);
LOG(INFO) << "Alignment pad : " << padding << " relocations";
} else {
// If padding, add NONE-type relocations to other_relocations to make it
// the same size as the the original relocations we read in. This makes
// the ResizeSection() below a no-op.
const size_t padding = relocations.size() - other_relocations.size();
PadRelocations<Rel>(padding, &other_relocations);
}
// Pack relative relocations.
const size_t initial_bytes =
relative_relocations.size() * sizeof(relative_relocations[0]);
LOG(INFO) << "Unpacked relative: " << initial_bytes << " bytes";
std::vector<uint8_t> packed;
RelocationPacker packer;
packer.PackRelativeRelocations(relative_relocations, &packed);
const void* packed_data = &packed[0];
const size_t packed_bytes = packed.size() * sizeof(packed[0]);
LOG(INFO) << "Packed relative: " << packed_bytes << " bytes";
// If we have insufficient relative relocations to form a run then
// packing fails.
if (packed.empty()) {
LOG(INFO) << "Too few relative relocations to pack";
return false;
}
// Run a loopback self-test as a check that packing is lossless.
std::vector<Rel> unpacked;
packer.UnpackRelativeRelocations(packed, &unpacked);
CHECK(unpacked.size() == relative_relocations.size());
CHECK(!memcmp(&unpacked[0],
&relative_relocations[0],
unpacked.size() * sizeof(unpacked[0])));
// Make sure packing saved some space.
if (packed_bytes >= initial_bytes) {
LOG(INFO) << "Packing relative relocations saves no space";
return false;
}
// Rewrite the current dynamic relocations section to be only the ARM
// non-relative relocations, then shrink it to size.
const void* section_data = &other_relocations[0];
const size_t bytes = other_relocations.size() * sizeof(other_relocations[0]);
ResizeSection<Rel>(elf_, relocations_section_, bytes);
RewriteSectionData(relocations_section_, section_data, bytes);
// Rewrite the current packed android relocations section to hold the packed
// relative relocations.
ResizeSection<Rel>(elf_, android_relocations_section_, packed_bytes);
RewriteSectionData(android_relocations_section_, packed_data, packed_bytes);
// Rewrite .dynamic to include two new tags describing the packed android
// relocations.
Elf_Data* data = GetSectionData(dynamic_section_);
const ELF::Dyn* dynamic_base = reinterpret_cast<ELF::Dyn*>(data->d_buf);
std::vector<ELF::Dyn> dynamics(
dynamic_base,
dynamic_base + data->d_size / sizeof(dynamics[0]));
// Use two of the spare slots to describe the packed section.
ELF::Shdr* section_header = ELF::getshdr(android_relocations_section_);
{
ELF::Dyn dyn;
dyn.d_tag = DT_ANDROID_REL_OFFSET;
dyn.d_un.d_ptr = section_header->sh_offset;
AddDynamicEntry(dyn, &dynamics);
}
{
ELF::Dyn dyn;
dyn.d_tag = DT_ANDROID_REL_SIZE;
dyn.d_un.d_val = section_header->sh_size;
AddDynamicEntry(dyn, &dynamics);
}
const void* dynamics_data = &dynamics[0];
const size_t dynamics_bytes = dynamics.size() * sizeof(dynamics[0]);
RewriteSectionData(dynamic_section_, dynamics_data, dynamics_bytes);
Flush();
return true;
}
// Find packed relative relocations in the packed android relocations
// section, unpack them, and rewrite the dynamic relocations section to
// contain unpacked data.
bool ElfFile::UnpackRelocations() {
// Load the ELF file into libelf.
if (!Load()) {
LOG(ERROR) << "Failed to load as ELF";
return false;
}
// Retrieve the current packed android relocations section data.
Elf_Data* data = GetSectionData(android_relocations_section_);
// Convert data to a vector of bytes.
const uint8_t* packed_base = reinterpret_cast<uint8_t*>(data->d_buf);
std::vector<uint8_t> packed(
packed_base,
packed_base + data->d_size / sizeof(packed[0]));
if (packed.size() > 3 &&
packed[0] == 'A' &&
packed[1] == 'P' &&
packed[2] == 'R' &&
packed[3] == '1') {
// Signature is APR1, unpack relocations.
CHECK(relocations_type_ == REL);
LOG(INFO) << "Relocations : REL";
return UnpackTypedRelocations<ELF::Rel>(packed);
}
if (packed.size() > 3 &&
packed[0] == 'A' &&
packed[1] == 'P' &&
packed[2] == 'A' &&
packed[3] == '1') {
// Signature is APA1, unpack relocations with addends.
CHECK(relocations_type_ == RELA);
LOG(INFO) << "Relocations : RELA";
return UnpackTypedRelocations<ELF::Rela>(packed);
}
LOG(ERROR) << "Packed relative relocations not found (not packed?)";
return false;
}
// Helper for UnpackRelocations(). Rel type is one of ELF::Rel or ELF::Rela.
template <typename Rel>
bool ElfFile::UnpackTypedRelocations(const std::vector<uint8_t>& packed) {
// Unpack the data to re-materialize the relative relocations.
const size_t packed_bytes = packed.size() * sizeof(packed[0]);
LOG(INFO) << "Packed relative: " << packed_bytes << " bytes";
std::vector<Rel> relative_relocations;
RelocationPacker packer;
packer.UnpackRelativeRelocations(packed, &relative_relocations);
const size_t unpacked_bytes =
relative_relocations.size() * sizeof(relative_relocations[0]);
LOG(INFO) << "Unpacked relative: " << unpacked_bytes << " bytes";
// Retrieve the current dynamic relocations section data.
Elf_Data* data = GetSectionData(relocations_section_);
// Interpret data as relocations.
const Rel* relocations_base = reinterpret_cast<Rel*>(data->d_buf);
std::vector<Rel> relocations(
relocations_base,
relocations_base + data->d_size / sizeof(relocations[0]));
std::vector<Rel> other_relocations;
size_t padding = 0;
// Filter relocations to locate any that are NONE-type. These will occur
// if padding was turned on for packing.
for (size_t i = 0; i < relocations.size(); ++i) {
const Rel& relocation = relocations[i];
if (ELF_R_TYPE(relocation.r_info) != ELF::kNoRelocationCode) {
other_relocations.push_back(relocation);
} else {
++padding;
}
}
LOG(INFO) << "Relative : " << relative_relocations.size() << " entries";
LOG(INFO) << "Other : " << other_relocations.size() << " entries";
// If we found the same number of null relocation entries in the dynamic
// relocations section as we hold as unpacked relative relocations, then
// this is a padded file.
const bool is_padded = padding == relative_relocations.size();
// Unless padded, report by how much we expand the file.
if (!is_padded) {
// Calculate the size of the hole we will open up when we rewrite
// dynamic relocations.
ssize_t hole_size =
relative_relocations.size() * sizeof(relative_relocations[0]);
// Adjust the hole size for the padding added to preserve alignment.
hole_size -= padding * sizeof(other_relocations[0]);
LOG(INFO) << "Expansion : " << hole_size << " bytes";
}
// Rewrite the current dynamic relocations section to be the relative
// relocations followed by other relocations. This is the usual order in
// which we find them after linking, so this action will normally put the
// entire dynamic relocations section back to its pre-split-and-packed state.
relocations.assign(relative_relocations.begin(), relative_relocations.end());
relocations.insert(relocations.end(),
other_relocations.begin(), other_relocations.end());
const void* section_data = &relocations[0];
const size_t bytes = relocations.size() * sizeof(relocations[0]);
LOG(INFO) << "Total : " << relocations.size() << " entries";
ResizeSection<Rel>(elf_, relocations_section_, bytes);
RewriteSectionData(relocations_section_, section_data, bytes);
// Nearly empty the current packed android relocations section. Leaves a
// four-byte stub so that some data remains allocated to the section.
// This is a convenience which allows us to re-pack this file again without
// having to remove the section and then add a new small one with objcopy.
// The way we resize sections relies on there being some data in a section.
ResizeSection<Rel>(
elf_, android_relocations_section_, sizeof(kStubIdentifier));
RewriteSectionData(
android_relocations_section_, &kStubIdentifier, sizeof(kStubIdentifier));
// Rewrite .dynamic to remove two tags describing packed android relocations.
data = GetSectionData(dynamic_section_);
const ELF::Dyn* dynamic_base = reinterpret_cast<ELF::Dyn*>(data->d_buf);
std::vector<ELF::Dyn> dynamics(
dynamic_base,
dynamic_base + data->d_size / sizeof(dynamics[0]));
RemoveDynamicEntry(DT_ANDROID_REL_OFFSET, &dynamics);
RemoveDynamicEntry(DT_ANDROID_REL_SIZE, &dynamics);
const void* dynamics_data = &dynamics[0];
const size_t dynamics_bytes = dynamics.size() * sizeof(dynamics[0]);
RewriteSectionData(dynamic_section_, dynamics_data, dynamics_bytes);
Flush();
return true;
}
// Flush rewritten shared object file data.
void ElfFile::Flush() {
// Flag all ELF data held in memory as needing to be written back to the
// file, and tell libelf that we have controlled the file layout.
elf_flagelf(elf_, ELF_C_SET, ELF_F_DIRTY);
elf_flagelf(elf_, ELF_C_SET, ELF_F_LAYOUT);
// Write ELF data back to disk.
const off_t file_bytes = elf_update(elf_, ELF_C_WRITE);
CHECK(file_bytes > 0);
VLOG(1) << "elf_update returned: " << file_bytes;
// Clean up libelf, and truncate the output file to the number of bytes
// written by elf_update().
elf_end(elf_);
elf_ = NULL;
const int truncate = ftruncate(fd_, file_bytes);
CHECK(truncate == 0);
}
} // namespace relocation_packer
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