/*
* Copyright (C) 2015 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef ART_COMPILER_LINKER_ELF_BUILDER_H_
#define ART_COMPILER_LINKER_ELF_BUILDER_H_
#include <vector>
#include "arch/instruction_set.h"
#include "arch/mips/instruction_set_features_mips.h"
#include "base/array_ref.h"
#include "base/bit_utils.h"
#include "base/casts.h"
#include "base/leb128.h"
#include "base/unix_file/fd_file.h"
#include "elf_utils.h"
#include "linker/error_delaying_output_stream.h"
namespace art {
namespace linker {
// Writes ELF file.
//
// The basic layout of the elf file:
// Elf_Ehdr - The ELF header.
// Elf_Phdr[] - Program headers for the linker.
// .note.gnu.build-id - Optional build ID section (SHA-1 digest).
// .rodata - Oat metadata.
// .text - Compiled code.
// .bss - Zero-initialized writeable section.
// .dex - Reserved NOBITS space for dex-related data.
// .MIPS.abiflags - MIPS specific section.
// .dynstr - Names for .dynsym.
// .dynsym - A few oat-specific dynamic symbols.
// .hash - Hash-table for .dynsym.
// .dynamic - Tags which let the linker locate .dynsym.
// .strtab - Names for .symtab.
// .symtab - Debug symbols.
// .eh_frame - Unwind information (CFI).
// .eh_frame_hdr - Index of .eh_frame.
// .debug_frame - Unwind information (CFI).
// .debug_frame.oat_patches - Addresses for relocation.
// .debug_info - Debug information.
// .debug_info.oat_patches - Addresses for relocation.
// .debug_abbrev - Decoding information for .debug_info.
// .debug_str - Strings for .debug_info.
// .debug_line - Line number tables.
// .debug_line.oat_patches - Addresses for relocation.
// .text.oat_patches - Addresses for relocation.
// .shstrtab - Names of ELF sections.
// Elf_Shdr[] - Section headers.
//
// Some section are optional (the debug sections in particular).
//
// We try write the section data directly into the file without much
// in-memory buffering. This means we generally write sections based on the
// dependency order (e.g. .dynamic points to .dynsym which points to .text).
//
// In the cases where we need to buffer, we write the larger section first
// and buffer the smaller one (e.g. .strtab is bigger than .symtab).
//
// The debug sections are written last for easier stripping.
//
template <typename ElfTypes>
class ElfBuilder FINAL {
public:
static constexpr size_t kMaxProgramHeaders = 16;
// SHA-1 digest. Not using SHA_DIGEST_LENGTH from openssl/sha.h to avoid
// spreading this header dependency for just this single constant.
static constexpr size_t kBuildIdLen = 20;
using Elf_Addr = typename ElfTypes::Addr;
using Elf_Off = typename ElfTypes::Off;
using Elf_Word = typename ElfTypes::Word;
using Elf_Sword = typename ElfTypes::Sword;
using Elf_Ehdr = typename ElfTypes::Ehdr;
using Elf_Shdr = typename ElfTypes::Shdr;
using Elf_Sym = typename ElfTypes::Sym;
using Elf_Phdr = typename ElfTypes::Phdr;
using Elf_Dyn = typename ElfTypes::Dyn;
// Base class of all sections.
class Section : public OutputStream {
public:
Section(ElfBuilder<ElfTypes>* owner,
const std::string& name,
Elf_Word type,
Elf_Word flags,
const Section* link,
Elf_Word info,
Elf_Word align,
Elf_Word entsize)
: OutputStream(name),
owner_(owner),
header_(),
section_index_(0),
name_(name),
link_(link),
phdr_flags_(PF_R),
phdr_type_(0) {
DCHECK_GE(align, 1u);
header_.sh_type = type;
header_.sh_flags = flags;
header_.sh_info = info;
header_.sh_addralign = align;
header_.sh_entsize = entsize;
}
// Allocate chunk of virtual memory for this section from the owning ElfBuilder.
// This must be done at the start for all SHF_ALLOC sections (i.e. mmaped by linker).
// It is fine to allocate section but never call Start/End() (e.g. the .bss section).
void AllocateVirtualMemory(Elf_Word size) {
AllocateVirtualMemory(owner_->virtual_address_, size);
}
void AllocateVirtualMemory(Elf_Addr addr, Elf_Word size) {
CHECK_NE(header_.sh_flags & SHF_ALLOC, 0u);
Elf_Word align = AddSection();
CHECK_EQ(header_.sh_addr, 0u);
header_.sh_addr = RoundUp(addr, align);
CHECK(header_.sh_size == 0u || header_.sh_size == size);
header_.sh_size = size;
CHECK_LE(owner_->virtual_address_, header_.sh_addr);
owner_->virtual_address_ = header_.sh_addr + header_.sh_size;
}
// Start writing file data of this section.
void Start() {
CHECK(owner_->current_section_ == nullptr);
Elf_Word align = AddSection();
CHECK_EQ(header_.sh_offset, 0u);
header_.sh_offset = owner_->AlignFileOffset(align);
owner_->current_section_ = this;
}
// Finish writing file data of this section.
void End() {
CHECK(owner_->current_section_ == this);
Elf_Word position = GetPosition();
CHECK(header_.sh_size == 0u || header_.sh_size == position);
header_.sh_size = position;
owner_->current_section_ = nullptr;
}
// Get the number of bytes written so far.
// Only valid while writing the section.
Elf_Word GetPosition() const {
CHECK(owner_->current_section_ == this);
off_t file_offset = owner_->stream_.Seek(0, kSeekCurrent);
DCHECK_GE(file_offset, (off_t)header_.sh_offset);
return file_offset - header_.sh_offset;
}
// Get the location of this section in virtual memory.
Elf_Addr GetAddress() const {
DCHECK_NE(header_.sh_flags & SHF_ALLOC, 0u);
DCHECK_NE(header_.sh_addr, 0u);
return header_.sh_addr;
}
// This function always succeeds to simplify code.
// Use builder's Good() to check the actual status.
bool WriteFully(const void* buffer, size_t byte_count) OVERRIDE {
CHECK(owner_->current_section_ == this);
return owner_->stream_.WriteFully(buffer, byte_count);
}
// This function always succeeds to simplify code.
// Use builder's Good() to check the actual status.
off_t Seek(off_t offset, Whence whence) OVERRIDE {
// Forward the seek as-is and trust the caller to use it reasonably.
return owner_->stream_.Seek(offset, whence);
}
// This function flushes the output and returns whether it succeeded.
// If there was a previous failure, this does nothing and returns false, i.e. failed.
bool Flush() OVERRIDE {
return owner_->stream_.Flush();
}
Elf_Word GetSectionIndex() const {
DCHECK_NE(section_index_, 0u);
return section_index_;
}
// Returns true if this section has been added.
bool Exists() const {
return section_index_ != 0;
}
protected:
// Add this section to the list of generated ELF sections (if not there already).
// It also ensures the alignment is sufficient to generate valid program headers,
// since that depends on the previous section. It returns the required alignment.
Elf_Word AddSection() {
if (section_index_ == 0) {
std::vector<Section*>& sections = owner_->sections_;
Elf_Word last = sections.empty() ? PF_R : sections.back()->phdr_flags_;
if (phdr_flags_ != last) {
header_.sh_addralign = kPageSize; // Page-align if R/W/X flags changed.
}
sections.push_back(this);
section_index_ = sections.size(); // First ELF section has index 1.
}
return owner_->write_program_headers_ ? header_.sh_addralign : 1;
}
ElfBuilder<ElfTypes>* owner_;
Elf_Shdr header_;
Elf_Word section_index_;
const std::string name_;
const Section* const link_;
Elf_Word phdr_flags_;
Elf_Word phdr_type_;
friend class ElfBuilder;
DISALLOW_COPY_AND_ASSIGN(Section);
};
class CachedSection : public Section {
public:
CachedSection(ElfBuilder<ElfTypes>* owner,
const std::string& name,
Elf_Word type,
Elf_Word flags,
const Section* link,
Elf_Word info,
Elf_Word align,
Elf_Word entsize)
: Section(owner, name, type, flags, link, info, align, entsize), cache_() { }
Elf_Word Add(const void* data, size_t length) {
Elf_Word offset = cache_.size();
const uint8_t* d = reinterpret_cast<const uint8_t*>(data);
cache_.insert(cache_.end(), d, d + length);
return offset;
}
Elf_Word GetCacheSize() {
return cache_.size();
}
void Write() {
this->WriteFully(cache_.data(), cache_.size());
cache_.clear();
cache_.shrink_to_fit();
}
void WriteCachedSection() {
this->Start();
Write();
this->End();
}
private:
std::vector<uint8_t> cache_;
};
// Writer of .dynstr section.
class CachedStringSection FINAL : public CachedSection {
public:
CachedStringSection(ElfBuilder<ElfTypes>* owner,
const std::string& name,
Elf_Word flags,
Elf_Word align)
: CachedSection(owner,
name,
SHT_STRTAB,
flags,
/* link */ nullptr,
/* info */ 0,
align,
/* entsize */ 0) { }
Elf_Word Add(const std::string& name) {
if (CachedSection::GetCacheSize() == 0u) {
DCHECK(name.empty());
}
return CachedSection::Add(name.c_str(), name.length() + 1);
}
};
// Writer of .strtab and .shstrtab sections.
class StringSection FINAL : public Section {
public:
StringSection(ElfBuilder<ElfTypes>* owner,
const std::string& name,
Elf_Word flags,
Elf_Word align)
: Section(owner,
name,
SHT_STRTAB,
flags,
/* link */ nullptr,
/* info */ 0,
align,
/* entsize */ 0),
current_offset_(0),
last_offset_(0) {
}
Elf_Word Write(const std::string& name) {
if (current_offset_ == 0) {
DCHECK(name.empty());
} else if (name == last_name_) {
return last_offset_; // Very simple string de-duplication.
}
last_name_ = name;
last_offset_ = current_offset_;
this->WriteFully(name.c_str(), name.length() + 1);
current_offset_ += name.length() + 1;
return last_offset_;
}
private:
Elf_Word current_offset_;
std::string last_name_;
Elf_Word last_offset_;
};
// Writer of .dynsym and .symtab sections.
class SymbolSection FINAL : public Section {
public:
SymbolSection(ElfBuilder<ElfTypes>* owner,
const std::string& name,
Elf_Word type,
Elf_Word flags,
Section* strtab)
: Section(owner,
name,
type,
flags,
strtab,
/* info */ 1,
sizeof(Elf_Off),
sizeof(Elf_Sym)) {
syms_.push_back(Elf_Sym()); // The symbol table always has to start with NULL symbol.
}
// Buffer symbol for this section. It will be written later.
// If the symbol's section is null, it will be considered absolute (SHN_ABS).
// (we use this in JIT to reference code which is stored outside the debug ELF file)
void Add(Elf_Word name,
const Section* section,
Elf_Addr addr,
Elf_Word size,
uint8_t binding,
uint8_t type) {
Elf_Word section_index;
if (section != nullptr) {
DCHECK_LE(section->GetAddress(), addr);
DCHECK_LE(addr, section->GetAddress() + section->header_.sh_size);
section_index = section->GetSectionIndex();
} else {
section_index = static_cast<Elf_Word>(SHN_ABS);
}
Add(name, section_index, addr, size, binding, type);
}
// Buffer symbol for this section. It will be written later.
void Add(Elf_Word name,
Elf_Word section_index,
Elf_Addr addr,
Elf_Word size,
uint8_t binding,
uint8_t type) {
Elf_Sym sym = Elf_Sym();
sym.st_name = name;
sym.st_value = addr;
sym.st_size = size;
sym.st_other = 0;
sym.st_shndx = section_index;
sym.st_info = (binding << 4) + (type & 0xf);
syms_.push_back(sym);
// The sh_info file must be set to index one-past the last local symbol.
if (binding == STB_LOCAL) {
this->header_.sh_info = syms_.size();
}
}
Elf_Word GetCacheSize() { return syms_.size() * sizeof(Elf_Sym); }
void WriteCachedSection() {
this->Start();
this->WriteFully(syms_.data(), syms_.size() * sizeof(Elf_Sym));
this->End();
}
private:
std::vector<Elf_Sym> syms_; // Buffered/cached content of the whole section.
};
class AbiflagsSection FINAL : public Section {
public:
// Section with Mips abiflag info.
static constexpr uint8_t MIPS_AFL_REG_NONE = 0; // no registers
static constexpr uint8_t MIPS_AFL_REG_32 = 1; // 32-bit registers
static constexpr uint8_t MIPS_AFL_REG_64 = 2; // 64-bit registers
static constexpr uint32_t MIPS_AFL_FLAGS1_ODDSPREG = 1; // Uses odd single-prec fp regs
static constexpr uint8_t MIPS_ABI_FP_DOUBLE = 1; // -mdouble-float
static constexpr uint8_t MIPS_ABI_FP_XX = 5; // -mfpxx
static constexpr uint8_t MIPS_ABI_FP_64A = 7; // -mips32r* -mfp64 -mno-odd-spreg
AbiflagsSection(ElfBuilder<ElfTypes>* owner,
const std::string& name,
Elf_Word type,
Elf_Word flags,
const Section* link,
Elf_Word info,
Elf_Word align,
Elf_Word entsize,
InstructionSet isa,
const InstructionSetFeatures* features)
: Section(owner, name, type, flags, link, info, align, entsize) {
if (isa == InstructionSet::kMips || isa == InstructionSet::kMips64) {
bool fpu32 = false; // assume mips64 values
uint8_t isa_rev = 6; // assume mips64 values
if (isa == InstructionSet::kMips) {
// adjust for mips32 values
fpu32 = features->AsMipsInstructionSetFeatures()->Is32BitFloatingPoint();
isa_rev = features->AsMipsInstructionSetFeatures()->IsR6()
? 6
: features->AsMipsInstructionSetFeatures()->IsMipsIsaRevGreaterThanEqual2()
? (fpu32 ? 2 : 5)
: 1;
}
abiflags_.version = 0; // version of flags structure
abiflags_.isa_level = (isa == InstructionSet::kMips) ? 32 : 64;
abiflags_.isa_rev = isa_rev;
abiflags_.gpr_size = (isa == InstructionSet::kMips) ? MIPS_AFL_REG_32 : MIPS_AFL_REG_64;
abiflags_.cpr1_size = fpu32 ? MIPS_AFL_REG_32 : MIPS_AFL_REG_64;
abiflags_.cpr2_size = MIPS_AFL_REG_NONE;
// Set the fp_abi to MIPS_ABI_FP_64A for mips32 with 64-bit FPUs (ie: mips32 R5 and R6).
// Otherwise set to MIPS_ABI_FP_DOUBLE.
abiflags_.fp_abi =
(isa == InstructionSet::kMips && !fpu32) ? MIPS_ABI_FP_64A : MIPS_ABI_FP_DOUBLE;
abiflags_.isa_ext = 0;
abiflags_.ases = 0;
// To keep the code simple, we are not using odd FP reg for single floats for both
// mips32 and mips64 ART. Therefore we are not setting the MIPS_AFL_FLAGS1_ODDSPREG bit.
abiflags_.flags1 = 0;
abiflags_.flags2 = 0;
}
}
Elf_Word GetSize() const {
return sizeof(abiflags_);
}
void Write() {
this->WriteFully(&abiflags_, sizeof(abiflags_));
}
private:
struct {
uint16_t version; // version of this structure
uint8_t isa_level, isa_rev, gpr_size, cpr1_size, cpr2_size;
uint8_t fp_abi;
uint32_t isa_ext, ases, flags1, flags2;
} abiflags_;
};
class BuildIdSection FINAL : public Section {
public:
BuildIdSection(ElfBuilder<ElfTypes>* owner,
const std::string& name,
Elf_Word type,
Elf_Word flags,
const Section* link,
Elf_Word info,
Elf_Word align,
Elf_Word entsize)
: Section(owner, name, type, flags, link, info, align, entsize),
digest_start_(-1) {
}
Elf_Word GetSize() {
return 16 + kBuildIdLen;
}
void Write() {
// The size fields are 32-bit on both 32-bit and 64-bit systems, confirmed
// with the 64-bit linker and libbfd code. The size of name and desc must
// be a multiple of 4 and it currently is.
this->WriteUint32(4); // namesz.
this->WriteUint32(kBuildIdLen); // descsz.
this->WriteUint32(3); // type = NT_GNU_BUILD_ID.
this->WriteFully("GNU", 4); // name.
digest_start_ = this->Seek(0, kSeekCurrent);
static_assert(kBuildIdLen % 4 == 0, "expecting a mutliple of 4 for build ID length");
this->WriteFully(std::string(kBuildIdLen, '\0').c_str(), kBuildIdLen); // desc.
DCHECK_EQ(this->GetPosition(), GetSize());
}
off_t GetDigestStart() {
CHECK_GT(digest_start_, 0);
return digest_start_;
}
private:
bool WriteUint32(uint32_t v) {
return this->WriteFully(&v, sizeof(v));
}
// File offset where the build ID digest starts.
// Populated with zeros first, then updated with the actual value as the
// very last thing in the output file creation.
off_t digest_start_;
};
ElfBuilder(InstructionSet isa, const InstructionSetFeatures* features, OutputStream* output)
: isa_(isa),
features_(features),
stream_(output),
rodata_(this, ".rodata", SHT_PROGBITS, SHF_ALLOC, nullptr, 0, kPageSize, 0),
text_(this, ".text", SHT_PROGBITS, SHF_ALLOC | SHF_EXECINSTR, nullptr, 0, kPageSize, 0),
bss_(this, ".bss", SHT_NOBITS, SHF_ALLOC, nullptr, 0, kPageSize, 0),
dex_(this, ".dex", SHT_NOBITS, SHF_ALLOC, nullptr, 0, kPageSize, 0),
dynstr_(this, ".dynstr", SHF_ALLOC, kPageSize),
dynsym_(this, ".dynsym", SHT_DYNSYM, SHF_ALLOC, &dynstr_),
hash_(this, ".hash", SHT_HASH, SHF_ALLOC, &dynsym_, 0, sizeof(Elf_Word), sizeof(Elf_Word)),
dynamic_(this, ".dynamic", SHT_DYNAMIC, SHF_ALLOC, &dynstr_, 0, kPageSize, sizeof(Elf_Dyn)),
eh_frame_(this, ".eh_frame", SHT_PROGBITS, SHF_ALLOC, nullptr, 0, kPageSize, 0),
eh_frame_hdr_(this, ".eh_frame_hdr", SHT_PROGBITS, SHF_ALLOC, nullptr, 0, 4, 0),
strtab_(this, ".strtab", 0, 1),
symtab_(this, ".symtab", SHT_SYMTAB, 0, &strtab_),
debug_frame_(this, ".debug_frame", SHT_PROGBITS, 0, nullptr, 0, sizeof(Elf_Addr), 0),
debug_info_(this, ".debug_info", SHT_PROGBITS, 0, nullptr, 0, 1, 0),
debug_line_(this, ".debug_line", SHT_PROGBITS, 0, nullptr, 0, 1, 0),
shstrtab_(this, ".shstrtab", 0, 1),
abiflags_(this, ".MIPS.abiflags", SHT_MIPS_ABIFLAGS, SHF_ALLOC, nullptr, 0, kPageSize, 0,
isa, features),
build_id_(this, ".note.gnu.build-id", SHT_NOTE, SHF_ALLOC, nullptr, 0, 4, 0),
current_section_(nullptr),
started_(false),
write_program_headers_(false),
loaded_size_(0u),
virtual_address_(0) {
text_.phdr_flags_ = PF_R | PF_X;
bss_.phdr_flags_ = PF_R | PF_W;
dex_.phdr_flags_ = PF_R;
dynamic_.phdr_flags_ = PF_R | PF_W;
dynamic_.phdr_type_ = PT_DYNAMIC;
eh_frame_hdr_.phdr_type_ = PT_GNU_EH_FRAME;
abiflags_.phdr_type_ = PT_MIPS_ABIFLAGS;
build_id_.phdr_type_ = PT_NOTE;
}
~ElfBuilder() {}
InstructionSet GetIsa() { return isa_; }
BuildIdSection* GetBuildId() { return &build_id_; }
Section* GetRoData() { return &rodata_; }
Section* GetText() { return &text_; }
Section* GetBss() { return &bss_; }
Section* GetDex() { return &dex_; }
StringSection* GetStrTab() { return &strtab_; }
SymbolSection* GetSymTab() { return &symtab_; }
Section* GetEhFrame() { return &eh_frame_; }
Section* GetEhFrameHdr() { return &eh_frame_hdr_; }
Section* GetDebugFrame() { return &debug_frame_; }
Section* GetDebugInfo() { return &debug_info_; }
Section* GetDebugLine() { return &debug_line_; }
// Encode patch locations as LEB128 list of deltas between consecutive addresses.
// (exposed publicly for tests)
static void EncodeOatPatches(const ArrayRef<const uintptr_t>& locations,
std::vector<uint8_t>* buffer) {
buffer->reserve(buffer->size() + locations.size() * 2); // guess 2 bytes per ULEB128.
uintptr_t address = 0; // relative to start of section.
for (uintptr_t location : locations) {
DCHECK_GE(location, address) << "Patch locations are not in sorted order";
EncodeUnsignedLeb128(buffer, dchecked_integral_cast<uint32_t>(location - address));
address = location;
}
}
void WritePatches(const char* name, const ArrayRef<const uintptr_t>& patch_locations) {
std::vector<uint8_t> buffer;
EncodeOatPatches(patch_locations, &buffer);
std::unique_ptr<Section> s(new Section(this, name, SHT_OAT_PATCH, 0, nullptr, 0, 1, 0));
s->Start();
s->WriteFully(buffer.data(), buffer.size());
s->End();
other_sections_.push_back(std::move(s));
}
void WriteSection(const char* name, const std::vector<uint8_t>* buffer) {
std::unique_ptr<Section> s(new Section(this, name, SHT_PROGBITS, 0, nullptr, 0, 1, 0));
s->Start();
s->WriteFully(buffer->data(), buffer->size());
s->End();
other_sections_.push_back(std::move(s));
}
// Reserve space for ELF header and program headers.
// We do not know the number of headers until later, so
// it is easiest to just reserve a fixed amount of space.
// Program headers are required for loading by the linker.
// It is possible to omit them for ELF files used for debugging.
void Start(bool write_program_headers = true) {
int size = sizeof(Elf_Ehdr);
if (write_program_headers) {
size += sizeof(Elf_Phdr) * kMaxProgramHeaders;
}
stream_.Seek(size, kSeekSet);
started_ = true;
virtual_address_ += size;
write_program_headers_ = write_program_headers;
}
void End() {
DCHECK(started_);
// Note: loaded_size_ == 0 for tests that don't write .rodata, .text, .bss,
// .dynstr, dynsym, .hash and .dynamic. These tests should not read loaded_size_.
// TODO: Either refactor the .eh_frame creation so that it counts towards loaded_size_,
// or remove all support for .eh_frame. (The currently unused .eh_frame counts towards
// the virtual_address_ but we don't consider it for loaded_size_.)
CHECK(loaded_size_ == 0 || loaded_size_ == RoundUp(virtual_address_, kPageSize))
<< loaded_size_ << " " << virtual_address_;
// Write section names and finish the section headers.
shstrtab_.Start();
shstrtab_.Write("");
for (auto* section : sections_) {
section->header_.sh_name = shstrtab_.Write(section->name_);
if (section->link_ != nullptr) {
section->header_.sh_link = section->link_->GetSectionIndex();
}
if (section->header_.sh_offset == 0) {
section->header_.sh_type = SHT_NOBITS;
}
}
shstrtab_.End();
// Write section headers at the end of the ELF file.
std::vector<Elf_Shdr> shdrs;
shdrs.reserve(1u + sections_.size());
shdrs.push_back(Elf_Shdr()); // NULL at index 0.
for (auto* section : sections_) {
shdrs.push_back(section->header_);
}
Elf_Off section_headers_offset;
section_headers_offset = AlignFileOffset(sizeof(Elf_Off));
stream_.WriteFully(shdrs.data(), shdrs.size() * sizeof(shdrs[0]));
// Flush everything else before writing the program headers. This should prevent
// the OS from reordering writes, so that we don't end up with valid headers
// and partially written data if we suddenly lose power, for example.
stream_.Flush();
// The main ELF header.
Elf_Ehdr elf_header = MakeElfHeader(isa_, features_);
elf_header.e_shoff = section_headers_offset;
elf_header.e_shnum = shdrs.size();
elf_header.e_shstrndx = shstrtab_.GetSectionIndex();
// Program headers (i.e. mmap instructions).
std::vector<Elf_Phdr> phdrs;
if (write_program_headers_) {
phdrs = MakeProgramHeaders();
CHECK_LE(phdrs.size(), kMaxProgramHeaders);
elf_header.e_phoff = sizeof(Elf_Ehdr);
elf_header.e_phnum = phdrs.size();
}
stream_.Seek(0, kSeekSet);
stream_.WriteFully(&elf_header, sizeof(elf_header));
stream_.WriteFully(phdrs.data(), phdrs.size() * sizeof(phdrs[0]));
stream_.Flush();
}
// The running program does not have access to section headers
// and the loader is not supposed to use them either.
// The dynamic sections therefore replicates some of the layout
// information like the address and size of .rodata and .text.
// It also contains other metadata like the SONAME.
// The .dynamic section is found using the PT_DYNAMIC program header.
void PrepareDynamicSection(const std::string& elf_file_path,
Elf_Word rodata_size,
Elf_Word text_size,
Elf_Word bss_size,
Elf_Word bss_methods_offset,
Elf_Word bss_roots_offset,
Elf_Word dex_size) {
std::string soname(elf_file_path);
size_t directory_separator_pos = soname.rfind('/');
if (directory_separator_pos != std::string::npos) {
soname = soname.substr(directory_separator_pos + 1);
}
// Allocate all pre-dynamic sections.
rodata_.AllocateVirtualMemory(rodata_size);
text_.AllocateVirtualMemory(text_size);
if (bss_size != 0) {
bss_.AllocateVirtualMemory(bss_size);
}
if (dex_size != 0) {
dex_.AllocateVirtualMemory(dex_size);
}
if (isa_ == InstructionSet::kMips || isa_ == InstructionSet::kMips64) {
abiflags_.AllocateVirtualMemory(abiflags_.GetSize());
}
// Cache .dynstr, .dynsym and .hash data.
dynstr_.Add(""); // dynstr should start with empty string.
Elf_Word oatdata = dynstr_.Add("oatdata");
dynsym_.Add(oatdata, &rodata_, rodata_.GetAddress(), rodata_size, STB_GLOBAL, STT_OBJECT);
if (text_size != 0u) {
// The runtime does not care about the size of this symbol (it uses the "lastword" symbol).
// We use size 0 (meaning "unknown size" in ELF) to prevent overlap with the debug symbols.
Elf_Word oatexec = dynstr_.Add("oatexec");
dynsym_.Add(oatexec, &text_, text_.GetAddress(), /* size */ 0, STB_GLOBAL, STT_OBJECT);
Elf_Word oatlastword = dynstr_.Add("oatlastword");
Elf_Word oatlastword_address = text_.GetAddress() + text_size - 4;
dynsym_.Add(oatlastword, &text_, oatlastword_address, 4, STB_GLOBAL, STT_OBJECT);
} else if (rodata_size != 0) {
// rodata_ can be size 0 for dwarf_test.
Elf_Word oatlastword = dynstr_.Add("oatlastword");
Elf_Word oatlastword_address = rodata_.GetAddress() + rodata_size - 4;
dynsym_.Add(oatlastword, &rodata_, oatlastword_address, 4, STB_GLOBAL, STT_OBJECT);
}
DCHECK_LE(bss_roots_offset, bss_size);
if (bss_size != 0u) {
Elf_Word oatbss = dynstr_.Add("oatbss");
dynsym_.Add(oatbss, &bss_, bss_.GetAddress(), bss_roots_offset, STB_GLOBAL, STT_OBJECT);
DCHECK_LE(bss_methods_offset, bss_roots_offset);
DCHECK_LE(bss_roots_offset, bss_size);
// Add a symbol marking the start of the methods part of the .bss, if not empty.
if (bss_methods_offset != bss_roots_offset) {
Elf_Word bss_methods_address = bss_.GetAddress() + bss_methods_offset;
Elf_Word bss_methods_size = bss_roots_offset - bss_methods_offset;
Elf_Word oatbssroots = dynstr_.Add("oatbssmethods");
dynsym_.Add(
oatbssroots, &bss_, bss_methods_address, bss_methods_size, STB_GLOBAL, STT_OBJECT);
}
// Add a symbol marking the start of the GC roots part of the .bss, if not empty.
if (bss_roots_offset != bss_size) {
Elf_Word bss_roots_address = bss_.GetAddress() + bss_roots_offset;
Elf_Word bss_roots_size = bss_size - bss_roots_offset;
Elf_Word oatbssroots = dynstr_.Add("oatbssroots");
dynsym_.Add(
oatbssroots, &bss_, bss_roots_address, bss_roots_size, STB_GLOBAL, STT_OBJECT);
}
Elf_Word oatbsslastword = dynstr_.Add("oatbsslastword");
Elf_Word bsslastword_address = bss_.GetAddress() + bss_size - 4;
dynsym_.Add(oatbsslastword, &bss_, bsslastword_address, 4, STB_GLOBAL, STT_OBJECT);
}
if (dex_size != 0u) {
Elf_Word oatdex = dynstr_.Add("oatdex");
dynsym_.Add(oatdex, &dex_, dex_.GetAddress(), /* size */ 0, STB_GLOBAL, STT_OBJECT);
Elf_Word oatdexlastword = dynstr_.Add("oatdexlastword");
Elf_Word oatdexlastword_address = dex_.GetAddress() + dex_size - 4;
dynsym_.Add(oatdexlastword, &dex_, oatdexlastword_address, 4, STB_GLOBAL, STT_OBJECT);
}
Elf_Word soname_offset = dynstr_.Add(soname);
// We do not really need a hash-table since there is so few entries.
// However, the hash-table is the only way the linker can actually
// determine the number of symbols in .dynsym so it is required.
int count = dynsym_.GetCacheSize() / sizeof(Elf_Sym); // Includes NULL.
std::vector<Elf_Word> hash;
hash.push_back(1); // Number of buckets.
hash.push_back(count); // Number of chains.
// Buckets. Having just one makes it linear search.
hash.push_back(1); // Point to first non-NULL symbol.
// Chains. This creates linked list of symbols.
hash.push_back(0); // Dummy entry for the NULL symbol.
for (int i = 1; i < count - 1; i++) {
hash.push_back(i + 1); // Each symbol points to the next one.
}
hash.push_back(0); // Last symbol terminates the chain.
hash_.Add(hash.data(), hash.size() * sizeof(hash[0]));
// Allocate all remaining sections.
dynstr_.AllocateVirtualMemory(dynstr_.GetCacheSize());
dynsym_.AllocateVirtualMemory(dynsym_.GetCacheSize());
hash_.AllocateVirtualMemory(hash_.GetCacheSize());
Elf_Dyn dyns[] = {
{ DT_HASH, { hash_.GetAddress() } },
{ DT_STRTAB, { dynstr_.GetAddress() } },
{ DT_SYMTAB, { dynsym_.GetAddress() } },
{ DT_SYMENT, { sizeof(Elf_Sym) } },
{ DT_STRSZ, { dynstr_.GetCacheSize() } },
{ DT_SONAME, { soname_offset } },
{ DT_NULL, { 0 } },
};
dynamic_.Add(&dyns, sizeof(dyns));
dynamic_.AllocateVirtualMemory(dynamic_.GetCacheSize());
loaded_size_ = RoundUp(virtual_address_, kPageSize);
}
void WriteDynamicSection() {
dynstr_.WriteCachedSection();
dynsym_.WriteCachedSection();
hash_.WriteCachedSection();
dynamic_.WriteCachedSection();
}
Elf_Word GetLoadedSize() {
CHECK_NE(loaded_size_, 0u);
return loaded_size_;
}
void WriteMIPSabiflagsSection() {
abiflags_.Start();
abiflags_.Write();
abiflags_.End();
}
void WriteBuildIdSection() {
build_id_.Start();
build_id_.Write();
build_id_.End();
}
void WriteBuildId(uint8_t build_id[kBuildIdLen]) {
stream_.Seek(build_id_.GetDigestStart(), kSeekSet);
stream_.WriteFully(build_id, kBuildIdLen);
}
// Returns true if all writes and seeks on the output stream succeeded.
bool Good() {
return stream_.Good();
}
// Returns the builder's internal stream.
OutputStream* GetStream() {
return &stream_;
}
off_t AlignFileOffset(size_t alignment) {
return stream_.Seek(RoundUp(stream_.Seek(0, kSeekCurrent), alignment), kSeekSet);
}
private:
static Elf_Ehdr MakeElfHeader(InstructionSet isa, const InstructionSetFeatures* features) {
Elf_Ehdr elf_header = Elf_Ehdr();
switch (isa) {
case InstructionSet::kArm:
// Fall through.
case InstructionSet::kThumb2: {
elf_header.e_machine = EM_ARM;
elf_header.e_flags = EF_ARM_EABI_VER5;
break;
}
case InstructionSet::kArm64: {
elf_header.e_machine = EM_AARCH64;
elf_header.e_flags = 0;
break;
}
case InstructionSet::kX86: {
elf_header.e_machine = EM_386;
elf_header.e_flags = 0;
break;
}
case InstructionSet::kX86_64: {
elf_header.e_machine = EM_X86_64;
elf_header.e_flags = 0;
break;
}
case InstructionSet::kMips: {
elf_header.e_machine = EM_MIPS;
elf_header.e_flags = (EF_MIPS_NOREORDER |
EF_MIPS_PIC |
EF_MIPS_CPIC |
EF_MIPS_ABI_O32 |
(features->AsMipsInstructionSetFeatures()->IsR6()
? EF_MIPS_ARCH_32R6
: EF_MIPS_ARCH_32R2));
break;
}
case InstructionSet::kMips64: {
elf_header.e_machine = EM_MIPS;
elf_header.e_flags = (EF_MIPS_NOREORDER |
EF_MIPS_PIC |
EF_MIPS_CPIC |
EF_MIPS_ARCH_64R6);
break;
}
case InstructionSet::kNone: {
LOG(FATAL) << "No instruction set";
break;
}
default: {
LOG(FATAL) << "Unknown instruction set " << isa;
}
}
elf_header.e_ident[EI_MAG0] = ELFMAG0;
elf_header.e_ident[EI_MAG1] = ELFMAG1;
elf_header.e_ident[EI_MAG2] = ELFMAG2;
elf_header.e_ident[EI_MAG3] = ELFMAG3;
elf_header.e_ident[EI_CLASS] = (sizeof(Elf_Addr) == sizeof(Elf32_Addr))
? ELFCLASS32 : ELFCLASS64;
elf_header.e_ident[EI_DATA] = ELFDATA2LSB;
elf_header.e_ident[EI_VERSION] = EV_CURRENT;
elf_header.e_ident[EI_OSABI] = ELFOSABI_LINUX;
elf_header.e_ident[EI_ABIVERSION] = 0;
elf_header.e_type = ET_DYN;
elf_header.e_version = 1;
elf_header.e_entry = 0;
elf_header.e_ehsize = sizeof(Elf_Ehdr);
elf_header.e_phentsize = sizeof(Elf_Phdr);
elf_header.e_shentsize = sizeof(Elf_Shdr);
return elf_header;
}
// Create program headers based on written sections.
std::vector<Elf_Phdr> MakeProgramHeaders() {
CHECK(!sections_.empty());
std::vector<Elf_Phdr> phdrs;
{
// The program headers must start with PT_PHDR which is used in
// loaded process to determine the number of program headers.
Elf_Phdr phdr = Elf_Phdr();
phdr.p_type = PT_PHDR;
phdr.p_flags = PF_R;
phdr.p_offset = phdr.p_vaddr = phdr.p_paddr = sizeof(Elf_Ehdr);
phdr.p_filesz = phdr.p_memsz = 0; // We need to fill this later.
phdr.p_align = sizeof(Elf_Off);
phdrs.push_back(phdr);
// Tell the linker to mmap the start of file to memory.
Elf_Phdr load = Elf_Phdr();
load.p_type = PT_LOAD;
load.p_flags = PF_R;
load.p_offset = load.p_vaddr = load.p_paddr = 0;
load.p_filesz = load.p_memsz = sizeof(Elf_Ehdr) + sizeof(Elf_Phdr) * kMaxProgramHeaders;
load.p_align = kPageSize;
phdrs.push_back(load);
}
// Create program headers for sections.
for (auto* section : sections_) {
const Elf_Shdr& shdr = section->header_;
if ((shdr.sh_flags & SHF_ALLOC) != 0 && shdr.sh_size != 0) {
DCHECK(shdr.sh_addr != 0u) << "Allocate virtual memory for the section";
// PT_LOAD tells the linker to mmap part of the file.
// The linker can only mmap page-aligned sections.
// Single PT_LOAD may contain several ELF sections.
Elf_Phdr& prev = phdrs.back();
Elf_Phdr load = Elf_Phdr();
load.p_type = PT_LOAD;
load.p_flags = section->phdr_flags_;
load.p_offset = shdr.sh_offset;
load.p_vaddr = load.p_paddr = shdr.sh_addr;
load.p_filesz = (shdr.sh_type != SHT_NOBITS ? shdr.sh_size : 0u);
load.p_memsz = shdr.sh_size;
load.p_align = shdr.sh_addralign;
if (prev.p_type == load.p_type &&
prev.p_flags == load.p_flags &&
prev.p_filesz == prev.p_memsz && // Do not merge .bss
load.p_filesz == load.p_memsz) { // Do not merge .bss
// Merge this PT_LOAD with the previous one.
Elf_Word size = shdr.sh_offset + shdr.sh_size - prev.p_offset;
prev.p_filesz = size;
prev.p_memsz = size;
} else {
// If we are adding new load, it must be aligned.
CHECK_EQ(shdr.sh_addralign, (Elf_Word)kPageSize);
phdrs.push_back(load);
}
}
}
for (auto* section : sections_) {
const Elf_Shdr& shdr = section->header_;
if ((shdr.sh_flags & SHF_ALLOC) != 0 && shdr.sh_size != 0) {
// Other PT_* types allow the program to locate interesting
// parts of memory at runtime. They must overlap with PT_LOAD.
if (section->phdr_type_ != 0) {
Elf_Phdr phdr = Elf_Phdr();
phdr.p_type = section->phdr_type_;
phdr.p_flags = section->phdr_flags_;
phdr.p_offset = shdr.sh_offset;
phdr.p_vaddr = phdr.p_paddr = shdr.sh_addr;
phdr.p_filesz = phdr.p_memsz = shdr.sh_size;
phdr.p_align = shdr.sh_addralign;
phdrs.push_back(phdr);
}
}
}
// Set the size of the initial PT_PHDR.
CHECK_EQ(phdrs[0].p_type, (Elf_Word)PT_PHDR);
phdrs[0].p_filesz = phdrs[0].p_memsz = phdrs.size() * sizeof(Elf_Phdr);
return phdrs;
}
InstructionSet isa_;
const InstructionSetFeatures* features_;
ErrorDelayingOutputStream stream_;
Section rodata_;
Section text_;
Section bss_;
Section dex_;
CachedStringSection dynstr_;
SymbolSection dynsym_;
CachedSection hash_;
CachedSection dynamic_;
Section eh_frame_;
Section eh_frame_hdr_;
StringSection strtab_;
SymbolSection symtab_;
Section debug_frame_;
Section debug_info_;
Section debug_line_;
StringSection shstrtab_;
AbiflagsSection abiflags_;
BuildIdSection build_id_;
std::vector<std::unique_ptr<Section>> other_sections_;
// List of used section in the order in which they were written.
std::vector<Section*> sections_;
Section* current_section_; // The section which is currently being written.
bool started_;
bool write_program_headers_;
// The size of the memory taken by the ELF file when loaded.
size_t loaded_size_;
// Used for allocation of virtual address space.
Elf_Addr virtual_address_;
DISALLOW_COPY_AND_ASSIGN(ElfBuilder);
};
} // namespace linker
} // namespace art
#endif // ART_COMPILER_LINKER_ELF_BUILDER_H_