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dart / sdk / refs/tags/3.6.0-113.0.dev / . / runtime / vm / elf.cc
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// Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
#include "vm/elf.h"
#include "platform/elf.h"
#include "platform/unwinding_records.h"
#include "vm/cpu.h"
#include "vm/dwarf.h"
#include "vm/hash_map.h"
#include "vm/image_snapshot.h"
#include "vm/stack_frame.h"
#include "vm/thread.h"
#include "vm/unwinding_records.h"
#include "vm/zone_text_buffer.h"
namespace dart {
#if defined(DART_PRECOMPILER)
// A wrapper around BaseWriteStream that provides methods useful for
// writing ELF files (e.g., using ELF definitions of data sizes).
class ElfWriteStream : public ValueObject {
public:
explicit ElfWriteStream(BaseWriteStream* stream, const Elf& elf)
: stream_(ASSERT_NOTNULL(stream)),
elf_(elf),
start_(stream_->Position()) {
// So that we can use the underlying stream's Align, as all alignments
// will be less than or equal to this alignment.
ASSERT(Utils::IsAligned(start_, Elf::kPageSize));
}
// Subclasses of Section may need to query the Elf object during Write(),
// so we store it in the ElfWriteStream for easy access.
const Elf& elf() const { return elf_; }
// We return positions in terms of the ELF content that has been written,
// ignoring any previous content on the stream.
intptr_t Position() const { return stream_->Position() - start_; }
void Align(const intptr_t alignment) {
ASSERT(Utils::IsPowerOfTwo(alignment));
ASSERT(alignment <= Elf::kPageSize);
stream_->Align(alignment);
}
void WriteBytes(const uint8_t* b, intptr_t size) {
stream_->WriteBytes(b, size);
}
void WriteByte(uint8_t value) { stream_->WriteByte(value); }
void WriteHalf(uint16_t value) { stream_->WriteFixed(value); }
void WriteWord(uint32_t value) { stream_->WriteFixed(value); }
void WriteAddr(compiler::target::uword value) { stream_->WriteFixed(value); }
void WriteOff(compiler::target::uword value) { stream_->WriteFixed(value); }
#if defined(TARGET_ARCH_IS_64_BIT)
void WriteXWord(uint64_t value) { stream_->WriteFixed(value); }
#endif
private:
BaseWriteStream* const stream_;
const Elf& elf_;
const intptr_t start_;
};
static constexpr intptr_t kLinearInitValue = -1;
#define DEFINE_LINEAR_FIELD_METHODS(name) \
intptr_t name() const { \
ASSERT(name##_ != kLinearInitValue); \
return name##_; \
} \
bool name##_is_set() const { \
return name##_ != kLinearInitValue; \
} \
void set_##name(intptr_t value) { \
ASSERT(value != kLinearInitValue); \
ASSERT_EQUAL(name##_, kLinearInitValue); \
name##_ = value; \
}
#define DEFINE_LINEAR_FIELD(name) intptr_t name##_ = kLinearInitValue;
// We only allow for dynamic casting to a subset of section types, since
// these are the only ones we need to distinguish at runtime.
#define FOR_EACH_SECTION_TYPE(V) \
V(ReservedSection) \
V(SymbolTable) \
V(DynamicTable) \
V(BitsContainer) \
V(TextSection) V(DataSection) V(BssSection) V(PseudoSection) V(SectionTable)
#define DEFINE_TYPE_CHECK_FOR(Type) \
bool Is##Type() const { \
return true; \
}
#define DECLARE_SECTION_TYPE_CLASS(Type) class Type;
FOR_EACH_SECTION_TYPE(DECLARE_SECTION_TYPE_CLASS)
#undef DECLARE_SECTION_TYPE_CLASS
class BitsContainer;
class Segment;
// Align note sections and segments to 4 byte boundaries.
static constexpr intptr_t kNoteAlignment = 4;
class Section : public ZoneAllocated {
public:
Section(elf::SectionHeaderType t,
bool allocate,
bool executable,
bool writable,
intptr_t align = compiler::target::kWordSize)
: type(t),
flags(EncodeFlags(allocate, executable, writable)),
alignment(align),
// Non-segments will never have a memory offset, here represented by 0.
memory_offset_(allocate ? kLinearInitValue : 0) {
// Only SHT_NULL sections (namely, the reserved section) are allowed to have
// an alignment of 0 (as the written section header entry for the reserved
// section must be all 0s).
ASSERT(alignment > 0 || type == elf::SectionHeaderType::SHT_NULL);
// Non-zero alignments must be a power of 2.
ASSERT(alignment == 0 || Utils::IsPowerOfTwo(alignment));
}
virtual ~Section() {}
// Linker view.
const elf::SectionHeaderType type;
const intptr_t flags;
const intptr_t alignment;
// These are fields that only are not set for most kinds of sections and so we
// set them to a reasonable default.
intptr_t link = elf::SHN_UNDEF;
intptr_t info = 0;
intptr_t entry_size = 0;
// This field is set for all sections, but due to reordering, we may set it
// more than once.
intptr_t index = elf::SHN_UNDEF;
#define FOR_EACH_SECTION_LINEAR_FIELD(M) \
M(name) \
M(file_offset)
FOR_EACH_SECTION_LINEAR_FIELD(DEFINE_LINEAR_FIELD_METHODS);
// Only needs to be overridden for sections that may not be allocated or
// for allocated sections where MemorySize() and FileSize() may differ.
virtual intptr_t FileSize() const {
if (!IsAllocated()) {
UNREACHABLE();
}
return MemorySize();
}
// Loader view.
#define FOR_EACH_SEGMENT_LINEAR_FIELD(M) M(memory_offset)
FOR_EACH_SEGMENT_LINEAR_FIELD(DEFINE_LINEAR_FIELD_METHODS);
// Only needs to be overridden for sections that may be allocated.
virtual intptr_t MemorySize() const {
if (IsAllocated()) {
UNREACHABLE();
}
return 0;
}
// Other methods.
bool IsAllocated() const {
return (flags & elf::SHF_ALLOC) == elf::SHF_ALLOC;
}
bool IsExecutable() const {
return (flags & elf::SHF_EXECINSTR) == elf::SHF_EXECINSTR;
}
bool IsWritable() const { return (flags & elf::SHF_WRITE) == elf::SHF_WRITE; }
bool HasBits() const { return type != elf::SectionHeaderType::SHT_NOBITS; }
// Returns whether the size of a section can change.
bool HasBeenFinalized() const {
// Sections can grow or shrink up until Elf::ComputeOffsets has been run,
// which sets the file (and memory, if applicable) offsets.
return file_offset_is_set();
}
#define DEFINE_BASE_TYPE_CHECKS(Type) \
Type* As##Type() { \
return Is##Type() ? reinterpret_cast<Type*>(this) : nullptr; \
} \
const Type* As##Type() const { \
return const_cast<Type*>(const_cast<Section*>(this)->As##Type()); \
} \
virtual bool Is##Type() const { return false; }
FOR_EACH_SECTION_TYPE(DEFINE_BASE_TYPE_CHECKS)
#undef DEFINE_BASE_TYPE_CHECKS
// Only some sections support merging.
virtual bool CanMergeWith(const Section& other) const { return false; }
virtual void Merge(const Section& other) { UNREACHABLE(); }
// Writes the file contents of the section.
virtual void Write(ElfWriteStream* stream) const { UNREACHABLE(); }
virtual void WriteSectionHeader(ElfWriteStream* stream) const {
#if defined(TARGET_ARCH_IS_32_BIT)
stream->WriteWord(name());
stream->WriteWord(static_cast<uint32_t>(type));
stream->WriteWord(flags);
stream->WriteAddr(memory_offset());
stream->WriteOff(file_offset());
stream->WriteWord(FileSize());
stream->WriteWord(link);
stream->WriteWord(info);
stream->WriteWord(alignment);
stream->WriteWord(entry_size);
#else
stream->WriteWord(name());
stream->WriteWord(static_cast<uint32_t>(type));
stream->WriteXWord(flags);
stream->WriteAddr(memory_offset());
stream->WriteOff(file_offset());
stream->WriteXWord(FileSize());
stream->WriteWord(link);
stream->WriteWord(info);
stream->WriteXWord(alignment);
stream->WriteXWord(entry_size);
#endif
}
private:
static intptr_t EncodeFlags(bool allocate, bool executable, bool writable) {
// We currently don't allow sections that are both executable and writable.
ASSERT(!executable || !writable);
intptr_t flags = 0;
if (allocate) flags |= elf::SHF_ALLOC;
if (executable) flags |= elf::SHF_EXECINSTR;
if (writable) flags |= elf::SHF_WRITE;
return flags;
}
FOR_EACH_SECTION_LINEAR_FIELD(DEFINE_LINEAR_FIELD);
FOR_EACH_SEGMENT_LINEAR_FIELD(DEFINE_LINEAR_FIELD);
#undef FOR_EACH_SECTION_LINEAR_FIELD
#undef FOR_EACH_SEGMENT_LINEAR_FIELD
};
#undef DEFINE_LINEAR_FIELD
#undef DEFINE_LINEAR_FIELD_METHODS
class Segment : public ZoneAllocated {
public:
Segment(Zone* zone,
Section* initial_section,
elf::ProgramHeaderType segment_type)
: type(segment_type),
// Flags for the segment are the same as the initial section.
flags(EncodeFlags(ASSERT_NOTNULL(initial_section)->IsExecutable(),
ASSERT_NOTNULL(initial_section)->IsWritable())),
sections_(zone, 0) {
// Unlike sections, we don't have a reserved segment with the null type,
// so we never should pass this value.
ASSERT(segment_type != elf::ProgramHeaderType::PT_NULL);
// All segments should have at least one section.
ASSERT(initial_section != nullptr);
sections_.Add(initial_section);
}
virtual ~Segment() {}
const GrowableArray<Section*>& sections() const { return sections_; }
intptr_t Alignment() const {
switch (type) {
case elf::ProgramHeaderType::PT_LOAD:
return Elf::kPageSize;
case elf::ProgramHeaderType::PT_PHDR:
case elf::ProgramHeaderType::PT_DYNAMIC:
return compiler::target::kWordSize;
case elf::ProgramHeaderType::PT_NOTE:
return kNoteAlignment;
case elf::ProgramHeaderType::PT_GNU_STACK:
return 1;
default:
UNREACHABLE();
return 0;
}
}
bool IsExecutable() const { return (flags & elf::PF_X) == elf::PF_X; }
bool IsWritable() const { return (flags & elf::PF_W) == elf::PF_W; }
void WriteProgramHeader(ElfWriteStream* stream) const {
#if defined(TARGET_ARCH_IS_32_BIT)
stream->WriteWord(static_cast<uint32_t>(type));
stream->WriteOff(FileOffset());
stream->WriteAddr(MemoryOffset()); // Virtual address.
stream->WriteAddr(MemoryOffset()); // Physical address.
stream->WriteWord(FileSize());
stream->WriteWord(MemorySize());
stream->WriteWord(flags);
stream->WriteWord(Alignment());
#else
stream->WriteWord(static_cast<uint32_t>(type));
stream->WriteWord(flags);
stream->WriteOff(FileOffset());
stream->WriteAddr(MemoryOffset()); // Virtual address.
stream->WriteAddr(MemoryOffset()); // Physical address.
stream->WriteXWord(FileSize());
stream->WriteXWord(MemorySize());
stream->WriteXWord(Alignment());
#endif
}
// Adds a given section to the end of this segment. Returns whether the
// section was successfully added.
bool Add(Section* section) {
ASSERT(section != nullptr);
// We can't add if memory offsets have already been calculated.
ASSERT(!section->memory_offset_is_set());
// We only add additional sections to load segments.
ASSERT(type == elf::ProgramHeaderType::PT_LOAD);
// We only add sections with the same executable and writable bits.
if (IsExecutable() != section->IsExecutable() ||
IsWritable() != section->IsWritable()) {
return false;
}
sections_.Add(section);
return true;
}
intptr_t FileOffset() const { return sections_[0]->file_offset(); }
intptr_t FileSize() const {
auto const last = sections_.Last();
const intptr_t end = last->file_offset() + last->FileSize();
return end - FileOffset();
}
intptr_t MemoryOffset() const { return sections_[0]->memory_offset(); }
intptr_t MemorySize() const {
auto const last = sections_.Last();
const intptr_t end = last->memory_offset() + last->MemorySize();
return end - MemoryOffset();
}
intptr_t MemoryEnd() const { return MemoryOffset() + MemorySize(); }
const elf::ProgramHeaderType type;
const intptr_t flags;
private:
static intptr_t EncodeFlags(bool executable, bool writable) {
intptr_t flags = elf::PF_R;
if (executable) flags |= elf::PF_X;
if (writable) flags |= elf::PF_W;
return flags;
}
GrowableArray<Section*> sections_;
};
// Represents the first entry in the section table, which should only contain
// zero values and does not correspond to a memory segment.
class ReservedSection : public Section {
public:
ReservedSection()
: Section(elf::SectionHeaderType::SHT_NULL,
/*allocate=*/false,
/*executable=*/false,
/*writable=*/false,
/*alignment=*/0) {
set_file_offset(0);
}
DEFINE_TYPE_CHECK_FOR(ReservedSection);
intptr_t FileSize() const { return 0; }
};
// Specifies the permissions used for the stack, notably whether the stack
// should be executable. If absent, the stack will be executable.
class GnuStackSection : public Section {
public:
GnuStackSection()
: Section(elf::SectionHeaderType::SHT_NULL,
/*allocate=*/false,
/*executable=*/false,
/*writable=*/true) {
set_file_offset(0);
}
intptr_t FileSize() const { return 0; }
};
class StringTable : public Section {
public:
explicit StringTable(Zone* zone, bool allocate)
: Section(elf::SectionHeaderType::SHT_STRTAB,
allocate,
/*executable=*/false,
/*writable=*/false),
dynamic_(allocate),
text_(zone, 128),
text_indices_(zone) {
Add("");
}
intptr_t FileSize() const { return text_.length(); }
intptr_t MemorySize() const { return dynamic_ ? FileSize() : 0; }
void Write(ElfWriteStream* stream) const {
stream->WriteBytes(reinterpret_cast<const uint8_t*>(text_.buffer()),
text_.length());
}
intptr_t Add(const char* str) {
ASSERT(str != nullptr);
if (auto const kv = text_indices_.Lookup(str)) {
return kv->value;
}
intptr_t offset = text_.length();
text_.AddString(str);
text_.AddChar('\0');
text_indices_.Insert({str, offset});
return offset;
}
const char* At(intptr_t index) const {
if (index >= text_.length()) return nullptr;
return text_.buffer() + index;
}
static constexpr intptr_t kNotIndexed = CStringIntMapKeyValueTrait::kNoValue;
// Returns the index of |str| if it is present in the string table
// and |kNotIndexed| otherwise.
intptr_t Lookup(const char* str) const {
return text_indices_.LookupValue(str);
}
const bool dynamic_;
ZoneTextBuffer text_;
CStringIntMap text_indices_;
};
class SymbolTable : public Section {
public:
SymbolTable(Zone* zone, StringTable* table, bool dynamic)
: Section(dynamic ? elf::SectionHeaderType::SHT_DYNSYM
: elf::SectionHeaderType::SHT_SYMTAB,
dynamic,
/*executable=*/false,
/*writable=*/false),
zone_(zone),
table_(table),
dynamic_(dynamic),
symbols_(zone, 1),
by_label_index_(zone) {
link = table_->index;
entry_size = sizeof(elf::Symbol);
// The first symbol table entry is reserved and must be all zeros.
// (String tables always have the empty string at the 0th index.)
ASSERT_EQUAL(table_->Lookup(""), 0);
symbols_.Add({/*name_index=*/0, elf::STB_LOCAL, elf::STT_NOTYPE, /*size=*/0,
elf::SHN_UNDEF, /*offset=*/0, /*label =*/0});
// The info field on a symbol table section holds the index of the first
// non-local symbol, so since there are none yet, it points past the single
// symbol we do have.
info = 1;
}
DEFINE_TYPE_CHECK_FOR(SymbolTable)
const StringTable& strtab() const { return *table_; }
intptr_t FileSize() const { return symbols_.length() * entry_size; }
intptr_t MemorySize() const { return dynamic_ ? FileSize() : 0; }
struct Symbol {
void Write(ElfWriteStream* stream) const {
const intptr_t start = stream->Position();
ASSERT(section_index == elf::SHN_UNDEF || offset > 0);
stream->WriteWord(name_index);
#if defined(TARGET_ARCH_IS_32_BIT)
stream->WriteAddr(offset);
stream->WriteWord(size);
stream->WriteByte(elf::SymbolInfo(binding, type));
stream->WriteByte(0);
stream->WriteHalf(section_index);
#else
stream->WriteByte(elf::SymbolInfo(binding, type));
stream->WriteByte(0);
stream->WriteHalf(section_index);
stream->WriteAddr(offset);
stream->WriteXWord(size);
#endif
ASSERT_EQUAL(stream->Position() - start, sizeof(elf::Symbol));
}
intptr_t name_index;
intptr_t binding;
intptr_t type;
intptr_t size;
// Must be updated whenever sections are reordered.
intptr_t section_index;
// Initialized to the section-relative offset, must be updated to the
// snapshot-relative offset before writing.
intptr_t offset;
// Only used within the VM and not written as part of the ELF file. If 0,
// this symbol cannot be looked up via label.
intptr_t label;
private:
DISALLOW_ALLOCATION();
};
const GrowableArray<Symbol>& symbols() const { return symbols_; }
void Initialize(const GrowableArray<Section*>& sections);
void Write(ElfWriteStream* stream) const {
for (const auto& symbol : symbols_) {
const intptr_t start = stream->Position();
symbol.Write(stream);
ASSERT_EQUAL(stream->Position() - start, entry_size);
}
}
void AddSymbol(const char* name,
intptr_t binding,
intptr_t type,
intptr_t size,
intptr_t index,
intptr_t offset,
intptr_t label) {
ASSERT(label > 0);
ASSERT(!table_->HasBeenFinalized());
auto const name_index = table_->Add(name);
ASSERT(name_index != 0);
const intptr_t new_index = symbols_.length();
symbols_.Add({name_index, binding, type, size, index, offset, label});
by_label_index_.Insert(label, new_index);
// The info field on a symbol table section holds the index of the first
// non-local symbol, so that local symbols can be skipped if desired. Thus,
// we need to make sure local symbols are before any non-local ones.
if (binding == elf::STB_LOCAL) {
if (info != new_index) {
// There are non-local symbols, as otherwise [info] would be the
// index of the new symbol. Since the order doesn't otherwise matter,
// swap the new local symbol with the value at index [info], so when
// [info] is incremented it will point just past the new local symbol.
ASSERT(symbols_[info].binding != elf::STB_LOCAL);
symbols_.Swap(info, new_index);
// Since by_label_index has indices into symbols_, we need to update it.
by_label_index_.Update({symbols_[info].label, info});
by_label_index_.Update({symbols_[new_index].label, new_index});
}
info += 1;
}
}
void UpdateSectionIndices(const GrowableArray<intptr_t>& index_map) {
#if defined(DEBUG)
const intptr_t map_size = index_map.length();
// The first entry must be 0 so that symbols with index SHN_UNDEF, like
// the initial reserved symbol, are unchanged.
ASSERT_EQUAL(index_map[0], 0);
for (intptr_t i = 1; i < map_size; i++) {
ASSERT(index_map[i] != 0);
ASSERT(index_map[i] < map_size);
}
#endif
for (auto& symbol : symbols_) {
DEBUG_ASSERT(symbol.section_index < map_size);
symbol.section_index = index_map[symbol.section_index];
}
}
void Finalize(const GrowableArray<intptr_t>& address_map) {
#if defined(DEBUG)
const intptr_t map_size = address_map.length();
// The first entry must be 0 so that symbols with index SHN_UNDEF, like
// the initial reserved symbol, are unchanged.
ASSERT_EQUAL(address_map[0], 0);
for (intptr_t i = 1; i < map_size; i++) {
// No section begins at the start of the snapshot.
ASSERT(address_map[i] != 0);
}
#endif
for (auto& symbol : symbols_) {
DEBUG_ASSERT(symbol.section_index < map_size);
symbol.offset += address_map[symbol.section_index];
}
}
const Symbol* FindUid(intptr_t label) const {
ASSERT(label > 0);
const intptr_t symbols_index = by_label_index_.Lookup(label);
if (symbols_index == 0) return nullptr; // Not found.
return &symbols_[symbols_index];
}
private:
Zone* const zone_;
StringTable* const table_;
const bool dynamic_;
GrowableArray<Symbol> symbols_;
// Maps positive symbol labels to indexes in symbols_. No entry for the
// reserved symbol, which has index 0, the same as the IntMap's kNoValue.
IntMap<intptr_t> by_label_index_;
};
class SymbolHashTable : public Section {
public:
SymbolHashTable(Zone* zone, SymbolTable* symtab)
: Section(elf::SectionHeaderType::SHT_HASH,
/*allocate=*/true,
/*executable=*/false,
/*writable=*/false),
buckets_(zone, 0),
chains_(zone, 0) {
link = symtab->index;
entry_size = sizeof(int32_t);
const auto& symbols = symtab->symbols();
const intptr_t num_symbols = symbols.length();
buckets_.FillWith(elf::STN_UNDEF, 0, num_symbols);
chains_.FillWith(elf::STN_UNDEF, 0, num_symbols);
for (intptr_t i = 1; i < num_symbols; i++) {
const auto& symbol = symbols[i];
uint32_t hash = HashSymbolName(symtab->strtab().At(symbol.name_index));
uint32_t probe = hash % num_symbols;
chains_[i] = buckets_[probe]; // next = head
buckets_[probe] = i; // head = symbol
}
}
intptr_t MemorySize() const {
return entry_size * (buckets_.length() + chains_.length() + 2);
}
void Write(ElfWriteStream* stream) const {
stream->WriteWord(buckets_.length());
stream->WriteWord(chains_.length());
for (const int32_t bucket : buckets_) {
stream->WriteWord(bucket);
}
for (const int32_t chain : chains_) {
stream->WriteWord(chain);
}
}
static uint32_t HashSymbolName(const void* p) {
auto* name = reinterpret_cast<const uint8_t*>(p);
uint32_t h = 0;
while (*name != '\0') {
h = (h << 4) + *name++;
uint32_t g = h & 0xf0000000;
h ^= g;
h ^= g >> 24;
}
return h;
}
private:
GrowableArray<int32_t> buckets_; // "Head"
GrowableArray<int32_t> chains_; // "Next"
};
class DynamicTable : public Section {
public:
// .dynamic section is expected to be writable on most Linux systems
// unless dynamic linker is explicitly built with support for an read-only
// .dynamic section (DL_RO_DYN_SECTION).
DynamicTable(Zone* zone, SymbolTable* symtab, SymbolHashTable* hash)
: Section(elf::SectionHeaderType::SHT_DYNAMIC,
/*allocate=*/true,
/*executable=*/false,
/*writable=*/true),
symtab_(symtab),
hash_(hash) {
link = strtab().index;
entry_size = sizeof(elf::DynamicEntry);
AddEntry(zone, elf::DynamicEntryType::DT_HASH, kInvalidEntry);
AddEntry(zone, elf::DynamicEntryType::DT_STRTAB, kInvalidEntry);
AddEntry(zone, elf::DynamicEntryType::DT_STRSZ, kInvalidEntry);
AddEntry(zone, elf::DynamicEntryType::DT_SYMTAB, kInvalidEntry);
AddEntry(zone, elf::DynamicEntryType::DT_SYMENT, sizeof(elf::Symbol));
AddEntry(zone, elf::DynamicEntryType::DT_NULL, 0);
}
static constexpr intptr_t kInvalidEntry = -1;
DEFINE_TYPE_CHECK_FOR(DynamicTable)
const SymbolHashTable& hash() const { return *hash_; }
const SymbolTable& symtab() const { return *symtab_; }
const StringTable& strtab() const { return symtab().strtab(); }
intptr_t MemorySize() const { return entries_.length() * entry_size; }
void Write(ElfWriteStream* stream) const {
for (intptr_t i = 0; i < entries_.length(); i++) {
entries_[i]->Write(stream);
}
}
void Finalize() {
FinalizeEntry(elf::DynamicEntryType::DT_HASH, hash().memory_offset());
FinalizeEntry(elf::DynamicEntryType::DT_STRTAB, strtab().memory_offset());
FinalizeEntry(elf::DynamicEntryType::DT_STRSZ, strtab().MemorySize());
FinalizeEntry(elf::DynamicEntryType::DT_SYMTAB, symtab().memory_offset());
}
private:
struct Entry : public ZoneAllocated {
Entry(elf::DynamicEntryType tag, intptr_t value) : tag(tag), value(value) {}
void Write(ElfWriteStream* stream) const {
ASSERT(value != kInvalidEntry);
const intptr_t start = stream->Position();
#if defined(TARGET_ARCH_IS_32_BIT)
stream->WriteWord(static_cast<uint32_t>(tag));
stream->WriteAddr(value);
#else
stream->WriteXWord(static_cast<uint64_t>(tag));
stream->WriteAddr(value);
#endif
ASSERT_EQUAL(stream->Position() - start, sizeof(elf::DynamicEntry));
}
elf::DynamicEntryType tag;
intptr_t value;
};
void AddEntry(Zone* zone, elf::DynamicEntryType tag, intptr_t value) {
auto const entry = new (zone) Entry(tag, value);
entries_.Add(entry);
}
void FinalizeEntry(elf::DynamicEntryType tag, intptr_t value) {
for (auto* entry : entries_) {
if (entry->tag == tag) {
entry->value = value;
break;
}
}
}
SymbolTable* const symtab_;
SymbolHashTable* const hash_;
GrowableArray<Entry*> entries_;
};
class BitsContainer : public Section {
public:
// Fully specified BitsContainer information. Unless otherwise specified,
// BitContainers are aligned on byte boundaries (i.e., no padding is used).
BitsContainer(elf::SectionHeaderType type,
bool allocate,
bool executable,
bool writable,
int alignment = 1)
: Section(type, allocate, executable, writable, alignment) {}
// For BitsContainers used only as unallocated sections.
explicit BitsContainer(elf::SectionHeaderType type, intptr_t alignment = 1)
: BitsContainer(type,
/*allocate=*/false,
/*executable=*/false,
/*writable=*/false,
alignment) {}
// For BitsContainers used as segments whose type differ on the type of the
// ELF file. Creates an elf::SHT_PROGBITS section if type is Snapshot,
// otherwise creates an elf::SHT_NOBITS section.
BitsContainer(Elf::Type t,
bool executable,
bool writable,
intptr_t alignment = 1)
: BitsContainer(t == Elf::Type::Snapshot
? elf::SectionHeaderType::SHT_PROGBITS
: elf::SectionHeaderType::SHT_NOBITS,
/*allocate=*/true,
executable,
writable,
alignment) {}
DEFINE_TYPE_CHECK_FOR(BitsContainer)
bool IsNoBits() const { return type == elf::SectionHeaderType::SHT_NOBITS; }
bool HasBytes() const {
return portions_.length() != 0 && portions_[0].bytes != nullptr;
}
struct Portion {
void Write(ElfWriteStream* stream, intptr_t section_start) const {
ASSERT(bytes != nullptr);
if (relocations == nullptr) {
stream->WriteBytes(bytes, size);
return;
}
const SymbolTable& symtab = stream->elf().symtab();
// Resolve relocations as we write.
intptr_t current_pos = 0;
for (const auto& reloc : *relocations) {
// We assume here that the relocations are sorted in increasing order,
// with unique section offsets.
ASSERT(current_pos <= reloc.section_offset);
if (current_pos < reloc.section_offset) {
stream->WriteBytes(bytes + current_pos,
reloc.section_offset - current_pos);
}
intptr_t source_address = reloc.source_offset;
if (reloc.source_label > 0) {
auto* const source_symbol = symtab.FindUid(reloc.source_label);
ASSERT(source_symbol != nullptr);
source_address += source_symbol->offset;
} else if (reloc.source_label == Elf::Relocation::kSelfRelative) {
source_address += section_start + offset + reloc.section_offset;
} else {
ASSERT_EQUAL(reloc.source_label, Elf::Relocation::kSnapshotRelative);
// No change to source_address.
}
ASSERT(reloc.size_in_bytes <= kWordSize);
word to_write = reloc.target_offset - source_address;
if (reloc.target_label > 0) {
if (auto* const target_symbol = symtab.FindUid(reloc.target_label)) {
to_write += target_symbol->offset;
} else {
ASSERT_EQUAL(reloc.target_label, Elf::kBuildIdLabel);
ASSERT_EQUAL(reloc.target_offset, 0);
ASSERT_EQUAL(reloc.source_offset, 0);
ASSERT_EQUAL(reloc.size_in_bytes, compiler::target::kWordSize);
// TODO(dartbug.com/43516): Special case for snapshots with deferred
// sections that handles the build ID relocation in an
// InstructionsSection when there is no build ID.
to_write = Image::kNoRelocatedAddress;
}
} else if (reloc.target_label == Elf::Relocation::kSelfRelative) {
to_write += section_start + offset + reloc.section_offset;
} else {
ASSERT_EQUAL(reloc.target_label, Elf::Relocation::kSnapshotRelative);
// No change to source_address.
}
ASSERT(Utils::IsInt(reloc.size_in_bytes * kBitsPerByte, to_write));
stream->WriteBytes(reinterpret_cast<const uint8_t*>(&to_write),
reloc.size_in_bytes);
current_pos = reloc.section_offset + reloc.size_in_bytes;
}
stream->WriteBytes(bytes + current_pos, size - current_pos);
}
intptr_t offset;
const char* symbol_name;
intptr_t label;
const uint8_t* bytes;
intptr_t size;
const ZoneGrowableArray<Elf::Relocation>* relocations;
const ZoneGrowableArray<Elf::SymbolData>* symbols;
private:
DISALLOW_ALLOCATION();
};
const GrowableArray<Portion>& portions() const { return portions_; }
const Portion& AddPortion(
const uint8_t* bytes,
intptr_t size,
const ZoneGrowableArray<Elf::Relocation>* relocations = nullptr,
const ZoneGrowableArray<Elf::SymbolData>* symbols = nullptr,
const char* symbol_name = nullptr,
intptr_t label = 0) {
// Any named portion should also have a valid symbol label.
ASSERT(symbol_name == nullptr || label > 0);
ASSERT(IsNoBits() || bytes != nullptr);
ASSERT(bytes != nullptr || relocations == nullptr);
// Make sure all portions are consistent in containing bytes.
ASSERT(portions_.is_empty() || HasBytes() == (bytes != nullptr));
const intptr_t offset = Utils::RoundUp(total_size_, alignment);
portions_.Add(
{offset, symbol_name, label, bytes, size, relocations, symbols});
const Portion& portion = portions_.Last();
total_size_ = offset + size;
return portion;
}
void Write(ElfWriteStream* stream) const {
if (type == elf::SectionHeaderType::SHT_NOBITS) return;
intptr_t start_position = stream->Position(); // Used for checks.
for (const auto& portion : portions_) {
stream->Align(alignment);
ASSERT_EQUAL(stream->Position(), start_position + portion.offset);
portion.Write(stream, memory_offset());
}
ASSERT_EQUAL(stream->Position(), start_position + total_size_);
}
// Returns the hash for the portion corresponding to symbol_name.
// Returns 0 if the portion has no bytes or no portions have that name.
uint32_t Hash(const char* symbol_name) const {
for (const auto& portion : portions_) {
if (strcmp(symbol_name, portion.symbol_name) == 0) {
if (portion.bytes == nullptr) return 0;
const uint32_t hash = Utils::StringHash(portion.bytes, portion.size);
// Ensure a non-zero return.
return hash == 0 ? 1 : hash;
}
}
return 0;
}
intptr_t FileSize() const { return IsNoBits() ? 0 : total_size_; }
intptr_t MemorySize() const { return IsAllocated() ? total_size_ : 0; }
private:
GrowableArray<Portion> portions_;
intptr_t total_size_ = 0;
};
class NoteSection : public BitsContainer {
public:
NoteSection()
: BitsContainer(elf::SectionHeaderType::SHT_NOTE,
/*allocate=*/true,
/*executable=*/false,
/*writable=*/false,
kNoteAlignment) {}
};
// Abstract bits container that allows merging by just appending the portion
// information (with properly adjusted offsets) of the other to this one.
class ConcatenableBitsContainer : public BitsContainer {
public:
ConcatenableBitsContainer(Elf::Type type,
bool executable,
bool writable,
intptr_t alignment)
: BitsContainer(type, executable, writable, alignment) {}
virtual bool CanMergeWith(const Section& other) const = 0;
virtual void Merge(const Section& other) {
ASSERT(other.IsBitsContainer());
ASSERT(CanMergeWith(other));
for (const auto& portion : other.AsBitsContainer()->portions()) {
AddPortion(portion.bytes, portion.size, portion.relocations,
portion.symbols, portion.symbol_name, portion.label);
}
}
};
class TextSection : public ConcatenableBitsContainer {
public:
explicit TextSection(Elf::Type t)
: ConcatenableBitsContainer(t,
/*executable=*/true,
/*writable=*/false,
ImageWriter::kTextAlignment) {}
DEFINE_TYPE_CHECK_FOR(TextSection);
virtual bool CanMergeWith(const Section& other) const {
return other.IsTextSection();
}
};
class DataSection : public ConcatenableBitsContainer {
public:
explicit DataSection(Elf::Type t)
: ConcatenableBitsContainer(t,
/*executable=*/false,
/*writable=*/false,
ImageWriter::kRODataAlignment) {}
DEFINE_TYPE_CHECK_FOR(DataSection);
virtual bool CanMergeWith(const Section& other) const {
return other.IsDataSection();
}
};
class BssSection : public ConcatenableBitsContainer {
public:
explicit BssSection(Elf::Type t)
: ConcatenableBitsContainer(t,
/*executable=*/false,
/*writable=*/true,
ImageWriter::kBssAlignment) {}
DEFINE_TYPE_CHECK_FOR(BssSection);
virtual bool CanMergeWith(const Section& other) const {
return other.IsBssSection();
}
};
// Represents portions of the file/memory space which do not correspond to
// sections from the section header. Should never be added to the section table,
// but may be added to segments.
class PseudoSection : public Section {
public:
// All PseudoSections are aligned to target word size.
static constexpr intptr_t kAlignment = compiler::target::kWordSize;
PseudoSection(bool allocate, bool executable, bool writable)
: Section(elf::SectionHeaderType::SHT_NULL,
allocate,
executable,
writable,
kAlignment) {}
DEFINE_TYPE_CHECK_FOR(PseudoSection)
void Write(ElfWriteStream* stream) const = 0;
};
class ProgramTable : public PseudoSection {
public:
explicit ProgramTable(Zone* zone)
: PseudoSection(/*allocate=*/true,
/*executable=*/false,
/*writable=*/false),
segments_(zone, 0) {
entry_size = sizeof(elf::ProgramHeader);
}
const GrowableArray<Segment*>& segments() const { return segments_; }
intptr_t SegmentCount() const { return segments_.length(); }
intptr_t MemorySize() const {
return segments_.length() * sizeof(elf::ProgramHeader);
}
void Add(Segment* segment) {
ASSERT(segment != nullptr);
segments_.Add(segment);
}
void Write(ElfWriteStream* stream) const;
private:
GrowableArray<Segment*> segments_;
};
// This particular PseudoSection should not appear in segments either (hence
// being marked non-allocated), but is directly held by the Elf object.
class SectionTable : public PseudoSection {
public:
explicit SectionTable(Zone* zone)
: PseudoSection(/*allocate=*/false,
/*executable=*/false,
/*writable=*/false),
zone_(zone),
sections_(zone_, 2),
shstrtab_(zone_, /*allocate=*/false) {
entry_size = sizeof(elf::SectionHeader);
// The section at index 0 (elf::SHN_UNDEF) must be all 0s.
ASSERT_EQUAL(shstrtab_.Lookup(""), 0);
Add(new (zone_) ReservedSection(), "");
Add(&shstrtab_, ".shstrtab");
}
const GrowableArray<Section*>& sections() const { return sections_; }
intptr_t SectionCount() const { return sections_.length(); }
intptr_t StringTableIndex() const { return shstrtab_.index; }
bool HasSectionNamed(const char* name) {
return shstrtab_.Lookup(name) != StringTable::kNotIndexed;
}
void Add(Section* section, const char* name = nullptr) {
ASSERT(!section->IsPseudoSection());
ASSERT(name != nullptr || section->name_is_set());
if (name != nullptr) {
// First, check for an existing section with the same table name.
if (auto* const old_section = Find(name)) {
ASSERT(old_section->CanMergeWith(*section));
old_section->Merge(*section);
return;
}
// No existing section with this name.
const intptr_t name_index = shstrtab_.Add(name);
section->set_name(name_index);
}
section->index = sections_.length();
sections_.Add(section);
}
Section* Find(const char* name) {
const intptr_t name_index = shstrtab_.Lookup(name);
if (name_index == StringTable::kNotIndexed) {
// We're guaranteed that no section with this name has been added yet.
return nullptr;
}
// We check walk all sections to check for uniqueness in DEBUG mode.
Section* result = nullptr;
for (Section* const section : sections_) {
if (section->name() == name_index) {
#if defined(DEBUG)
ASSERT(result == nullptr);
result = section;
#else
return section;
#endif
}
}
return result;
}
intptr_t FileSize() const {
return sections_.length() * sizeof(elf::SectionHeader);
}
void Write(ElfWriteStream* stream) const;
// Reorders the sections for creating a minimal amount of segments and
// creates and returns an appropriate program table.
//
// Also takes and adjusts section indices in the static symbol table, since it
// is not recorded in sections_ for stripped outputs.
ProgramTable* CreateProgramTable(SymbolTable* symtab);
private:
Zone* const zone_;
GrowableArray<Section*> sections_;
StringTable shstrtab_;
};
class ElfHeader : public PseudoSection {
public:
ElfHeader(const ProgramTable& program_table,
const SectionTable& section_table)
: PseudoSection(/*allocate=*/true,
/*executable=*/false,
/*writable=*/false),
program_table_(program_table),
section_table_(section_table) {}
intptr_t MemorySize() const { return sizeof(elf::ElfHeader); }
void Write(ElfWriteStream* stream) const;
private:
const ProgramTable& program_table_;
const SectionTable& section_table_;
};
#undef DEFINE_TYPE_CHECK_FOR
#undef FOR_EACH_SECTION_TYPE
Elf::Elf(Zone* zone, BaseWriteStream* stream, Type type, Dwarf* dwarf)
: zone_(zone),
unwrapped_stream_(stream),
type_(type),
dwarf_(dwarf),
section_table_(new(zone) SectionTable(zone)) {
// Separate debugging information should always have a Dwarf object.
ASSERT(type_ == Type::Snapshot || dwarf_ != nullptr);
// Assumed by various offset logic in this file.
ASSERT_EQUAL(unwrapped_stream_->Position(), 0);
}
void Elf::AddText(const char* name,
intptr_t label,
const uint8_t* bytes,
intptr_t size,
const ZoneGrowableArray<Relocation>* relocations,
const ZoneGrowableArray<SymbolData>* symbols) {
auto* const container = new (zone_) TextSection(type_);
container->AddPortion(bytes, size, relocations, symbols, name, label);
section_table_->Add(container, kTextName);
}
void Elf::CreateBSS() {
// Not idempotent.
ASSERT(section_table_->Find(kBssName) == nullptr);
// No text section means no BSS section.
auto* const text_section = section_table_->Find(kTextName);
if (text_section == nullptr) return;
ASSERT(text_section->IsTextSection());
auto* const bss_container = new (zone_) BssSection(type_);
for (const auto& portion : text_section->AsBitsContainer()->portions()) {
size_t size;
const char* symbol_name;
intptr_t label;
// First determine whether this is the VM's text portion or the isolate's.
if (strcmp(portion.symbol_name, kVmSnapshotInstructionsAsmSymbol) == 0) {
size = BSS::kVmEntryCount * compiler::target::kWordSize;
symbol_name = kVmSnapshotBssAsmSymbol;
label = kVmBssLabel;
} else if (strcmp(portion.symbol_name,
kIsolateSnapshotInstructionsAsmSymbol) == 0) {
size = BSS::kIsolateGroupEntryCount * compiler::target::kWordSize;
symbol_name = kIsolateSnapshotBssAsmSymbol;
label = kIsolateBssLabel;
} else {
// Not VM or isolate text.
UNREACHABLE();
continue;
}
uint8_t* bytes = nullptr;
if (type_ == Type::Snapshot) {
// Ideally the BSS segment would take no space in the object, but
// Android's "strip" utility truncates the memory-size of our segments to
// their file-size.
//
// Therefore we must insert zero-filled data for the BSS.
bytes = zone_->Alloc<uint8_t>(size);
memset(bytes, 0, size);
}
// For the BSS section, we add the section symbols as local symbols in the
// static symbol table, as these addresses are only used for relocation.
// (This matches the behavior in the assembly output.)
auto* symbols = new (zone_) ZoneGrowableArray<Elf::SymbolData>();
symbols->Add({symbol_name, elf::STT_SECTION, 0, size, label});
bss_container->AddPortion(bytes, size, /*relocations=*/nullptr, symbols);
}
section_table_->Add(bss_container, kBssName);
}
void Elf::AddROData(const char* name,
intptr_t label,
const uint8_t* bytes,
intptr_t size,
const ZoneGrowableArray<Relocation>* relocations,
const ZoneGrowableArray<SymbolData>* symbols) {
auto* const container = new (zone_) DataSection(type_);
container->AddPortion(bytes, size, relocations, symbols, name, label);
section_table_->Add(container, kDataName);
}
#if defined(DART_PRECOMPILER)
class DwarfElfStream : public DwarfWriteStream {
public:
DwarfElfStream(Zone* zone, NonStreamingWriteStream* stream)
: zone_(ASSERT_NOTNULL(zone)),
stream_(ASSERT_NOTNULL(stream)),
relocations_(new(zone) ZoneGrowableArray<Elf::Relocation>()) {}
const uint8_t* buffer() const { return stream_->buffer(); }
intptr_t bytes_written() const { return stream_->bytes_written(); }
intptr_t Position() const { return stream_->Position(); }
void sleb128(intptr_t value) { stream_->WriteSLEB128(value); }
void uleb128(uintptr_t value) { stream_->WriteLEB128(value); }
void u1(uint8_t value) { stream_->WriteByte(value); }
void u2(uint16_t value) { stream_->WriteFixed(value); }
void u4(uint32_t value) { stream_->WriteFixed(value); }
void u8(uint64_t value) { stream_->WriteFixed(value); }
void string(const char* cstr) { // NOLINT
// Unlike stream_->WriteString(), we want the null terminator written.
stream_->WriteBytes(cstr, strlen(cstr) + 1);
}
// The prefix is ignored for DwarfElfStreams.
void WritePrefixedLength(const char* unused, std::function<void()> body) {
const intptr_t fixup = stream_->Position();
// We assume DWARF v2 currently, so all sizes are 32-bit.
u4(0);
// All sizes for DWARF sections measure the size of the section data _after_
// the size value.
const intptr_t start = stream_->Position();
body();
const intptr_t end = stream_->Position();
stream_->SetPosition(fixup);
u4(end - start);
stream_->SetPosition(end);
}
// Shorthand for when working directly with DwarfElfStreams.
void WritePrefixedLength(std::function<void()> body) {
WritePrefixedLength(nullptr, body);
}
void OffsetFromSymbol(intptr_t label, intptr_t offset) {
relocations_->Add({kAddressSize, stream_->Position(),
Elf::Relocation::kSnapshotRelative, 0, label, offset});
addr(0); // Resolved later.
}
template <typename T>
void RelativeSymbolOffset(intptr_t label) {
relocations_->Add({sizeof(T), stream_->Position(),
Elf::Relocation::kSelfRelative, 0, label, 0});
stream_->WriteFixed<T>(0); // Resolved later.
}
void InitializeAbstractOrigins(intptr_t size) {
abstract_origins_size_ = size;
abstract_origins_ = zone_->Alloc<uint32_t>(abstract_origins_size_);
}
void RegisterAbstractOrigin(intptr_t index) {
ASSERT(abstract_origins_ != nullptr);
ASSERT(index < abstract_origins_size_);
abstract_origins_[index] = stream_->Position();
}
void AbstractOrigin(intptr_t index) { u4(abstract_origins_[index]); }
const ZoneGrowableArray<Elf::Relocation>* relocations() const {
return relocations_;
}
protected:
#if defined(TARGET_ARCH_IS_32_BIT)
static constexpr intptr_t kAddressSize = kInt32Size;
#else
static constexpr intptr_t kAddressSize = kInt64Size;
#endif
void addr(uword value) {
#if defined(TARGET_ARCH_IS_32_BIT)
u4(value);
#else
u8(value);
#endif
}
Zone* const zone_;
NonStreamingWriteStream* const stream_;
ZoneGrowableArray<Elf::Relocation>* relocations_ = nullptr;
uint32_t* abstract_origins_ = nullptr;
intptr_t abstract_origins_size_ = -1;
private:
DISALLOW_COPY_AND_ASSIGN(DwarfElfStream);
};
static constexpr intptr_t kInitialDwarfBufferSize = 64 * KB;
#endif
void SymbolTable::Initialize(const GrowableArray<Section*>& sections) {
for (auto* const section : sections) {
// The values of all added symbols are memory addresses.
if (!section->IsAllocated()) continue;
if (auto* const bits = section->AsBitsContainer()) {
for (const auto& portion : section->AsBitsContainer()->portions()) {
if (portion.symbol_name != nullptr) {
// Global dynamic symbols for the content of a given section, which is
// always a single structured element (and thus we use STT_OBJECT).
const intptr_t binding = elf::STB_GLOBAL;
const intptr_t type = elf::STT_OBJECT;
// Some tools assume the static symbol table is a superset of the
// dynamic symbol table when it exists and only use it, so put all
// dynamic symbols there also. (see dartbug.com/41783).
AddSymbol(portion.symbol_name, binding, type, portion.size,
section->index, portion.offset, portion.label);
}
if (!dynamic_ && portion.symbols != nullptr) {
for (const auto& symbol_data : *portion.symbols) {
// Local static-only symbols, e.g., code payloads or RO objects.
AddSymbol(symbol_data.name, elf::STB_LOCAL, symbol_data.type,
symbol_data.size, section->index,
portion.offset + symbol_data.offset, symbol_data.label);
}
}
}
}
}
}
void Elf::InitializeSymbolTables() {
// Not idempotent.
ASSERT(symtab_ == nullptr);
// Create static and dynamic symbol tables.
auto* const dynstrtab = new (zone_) StringTable(zone_, /*allocate=*/true);
section_table_->Add(dynstrtab, ".dynstr");
auto* const dynsym =
new (zone_) SymbolTable(zone_, dynstrtab, /*dynamic=*/true);
section_table_->Add(dynsym, ".dynsym");
dynsym->Initialize(section_table_->sections());
// Now the dynamic symbol table is populated, set up the hash table and
// dynamic table.
auto* const hash = new (zone_) SymbolHashTable(zone_, dynsym);
section_table_->Add(hash, ".hash");
auto* const dynamic = new (zone_) DynamicTable(zone_, dynsym, hash);
section_table_->Add(dynamic, kDynamicTableName);
// We only add the static string and symbol tables to the section table if
// this is an unstripped output, but we always create them as they are used
// to resolve relocations.
auto* const strtab = new (zone_) StringTable(zone_, /*allocate=*/false);
if (!IsStripped()) {
section_table_->Add(strtab, ".strtab");
}
symtab_ = new (zone_) SymbolTable(zone_, strtab, /*dynamic=*/false);
if (!IsStripped()) {
section_table_->Add(symtab_, ".symtab");
}
symtab_->Initialize(section_table_->sections());
}
void Elf::FinalizeEhFrame() {
#if !defined(TARGET_ARCH_IA32)
#if defined(TARGET_ARCH_X64)
// The x86_64 psABI defines the DWARF register numbers, which differ from
// the registers' usual encoding within instructions.
const intptr_t DWARF_RA = 16; // No corresponding register.
const intptr_t DWARF_FP = 6; // RBP
#else
const intptr_t DWARF_RA = ConcreteRegister(LINK_REGISTER);
const intptr_t DWARF_FP = FP;
#endif
// No text section added means no .eh_frame.
TextSection* text_section = nullptr;
if (auto* const section = section_table_->Find(kTextName)) {
text_section = section->AsTextSection();
ASSERT(text_section != nullptr);
}
// No text section added means no .eh_frame.
if (text_section == nullptr) return;
#if defined(DART_TARGET_OS_WINDOWS) && \
(defined(TARGET_ARCH_X64) || defined(TARGET_ARCH_ARM64))
// Append Windows unwinding instructions to the end of .text section.
{ // NOLINT
auto* const unwinding_instructions_frame = new (zone_) TextSection(type_);
ZoneWriteStream stream(
zone(),
/*initial_size=*/UnwindingRecordsPlatform::SizeInBytes());
uint8_t* unwinding_instructions =
zone()->Alloc<uint8_t>(UnwindingRecordsPlatform::SizeInBytes());
intptr_t start_offset =
Utils::RoundUp(text_section->FileSize(), text_section->alignment);
stream.WriteBytes(UnwindingRecords::GenerateRecordsInto(
start_offset, unwinding_instructions),
UnwindingRecordsPlatform::SizeInBytes());
unwinding_instructions_frame->AddPortion(stream.buffer(),
stream.bytes_written());
section_table_->Add(unwinding_instructions_frame, kTextName);
}
#endif
// Multiplier which will be used to scale operands of DW_CFA_offset and
// DW_CFA_val_offset.
const intptr_t kDataAlignment = -compiler::target::kWordSize;
static constexpr uint8_t DW_EH_PE_pcrel = 0x10;
static constexpr uint8_t DW_EH_PE_sdata4 = 0x0b;
ZoneWriteStream stream(zone(), kInitialDwarfBufferSize);
DwarfElfStream dwarf_stream(zone_, &stream);
// Emit CIE.
// Used to calculate offset to CIE in FDEs.
const intptr_t cie_start = dwarf_stream.Position();
dwarf_stream.WritePrefixedLength([&] {
dwarf_stream.u4(0); // CIE
dwarf_stream.u1(1); // Version (must be 1 or 3)
// Augmentation String
dwarf_stream.string("zR"); // NOLINT
dwarf_stream.uleb128(1); // Code alignment (must be 1).
dwarf_stream.sleb128(kDataAlignment); // Data alignment
dwarf_stream.u1(DWARF_RA); // Return address register
dwarf_stream.uleb128(1); // Augmentation size
dwarf_stream.u1(DW_EH_PE_pcrel | DW_EH_PE_sdata4); // FDE encoding.
// CFA is caller's SP (FP+kCallerSpSlotFromFp*kWordSize)
dwarf_stream.u1(Dwarf::DW_CFA_def_cfa);
dwarf_stream.uleb128(DWARF_FP);
dwarf_stream.uleb128(kCallerSpSlotFromFp * compiler::target::kWordSize);
});
// Emit rule defining that |reg| value is stored at CFA+offset.
const auto cfa_offset = [&](intptr_t reg, intptr_t offset) {
const intptr_t scaled_offset = offset / kDataAlignment;
RELEASE_ASSERT(scaled_offset >= 0);
dwarf_stream.u1(Dwarf::DW_CFA_offset | reg);
dwarf_stream.uleb128(scaled_offset);
};
// Emit an FDE covering each .text section.
for (const auto& portion : text_section->portions()) {
#if defined(DART_TARGET_OS_WINDOWS) && \
(defined(TARGET_ARCH_X64) || defined(TARGET_ARCH_ARM64))
if (portion.label == 0) {
// Unwinding instructions sections doesn't have label, doesn't dwarf
continue;
}
#endif
ASSERT(portion.label != 0); // Needed for relocations.
dwarf_stream.WritePrefixedLength([&]() {
// Offset to CIE. Note that unlike pcrel this offset is encoded
// backwards: it will be subtracted from the current position.
dwarf_stream.u4(stream.Position() - cie_start);
// Start address as a PC relative reference.
dwarf_stream.RelativeSymbolOffset<int32_t>(portion.label);
dwarf_stream.u4(portion.size); // Size.
dwarf_stream.u1(0); // Augmentation Data length.
// Caller FP at FP+kSavedCallerPcSlotFromFp*kWordSize,
// where FP is CFA - kCallerSpSlotFromFp*kWordSize.
COMPILE_ASSERT((kSavedCallerFpSlotFromFp - kCallerSpSlotFromFp) <= 0);
cfa_offset(DWARF_FP, (kSavedCallerFpSlotFromFp - kCallerSpSlotFromFp) *
compiler::target::kWordSize);
// Caller LR at FP+kSavedCallerPcSlotFromFp*kWordSize,
// where FP is CFA - kCallerSpSlotFromFp*kWordSize
COMPILE_ASSERT((kSavedCallerPcSlotFromFp - kCallerSpSlotFromFp) <= 0);
cfa_offset(DWARF_RA, (kSavedCallerPcSlotFromFp - kCallerSpSlotFromFp) *
compiler::target::kWordSize);
});
}
dwarf_stream.u4(0); // end of section (FDE with zero length)
auto* const eh_frame = new (zone_)
BitsContainer(type_, /*writable=*/false, /*executable=*/false);
eh_frame->AddPortion(dwarf_stream.buffer(), dwarf_stream.bytes_written(),
dwarf_stream.relocations());
section_table_->Add(eh_frame, ".eh_frame");
#endif // !defined(TARGET_ARCH_IA32)
}
void Elf::FinalizeDwarfSections() {
if (dwarf_ == nullptr) return;
// Currently we only output DWARF information involving code.
ASSERT(section_table_->HasSectionNamed(kTextName));
auto add_debug = [&](const char* name, const DwarfElfStream& stream) {
auto const container =
new (zone_) BitsContainer(elf::SectionHeaderType::SHT_PROGBITS);
container->AddPortion(stream.buffer(), stream.bytes_written(),
stream.relocations());
section_table_->Add(container, name);
};
{
ZoneWriteStream stream(zone(), kInitialDwarfBufferSize);
DwarfElfStream dwarf_stream(zone_, &stream);
dwarf_->WriteAbbreviations(&dwarf_stream);
add_debug(".debug_abbrev", dwarf_stream);
}
{
ZoneWriteStream stream(zone(), kInitialDwarfBufferSize);
DwarfElfStream dwarf_stream(zone_, &stream);
dwarf_->WriteDebugInfo(&dwarf_stream);
add_debug(".debug_info", dwarf_stream);
}
{
ZoneWriteStream stream(zone(), kInitialDwarfBufferSize);
DwarfElfStream dwarf_stream(zone_, &stream);
dwarf_->WriteLineNumberProgram(&dwarf_stream);
add_debug(".debug_line", dwarf_stream);
}
}
ProgramTable* SectionTable::CreateProgramTable(SymbolTable* symtab) {
const intptr_t num_sections = sections_.length();
// Should have at least the reserved entry in sections_.
ASSERT(!sections_.is_empty());
ASSERT_EQUAL(sections_[0]->alignment, 0);
// The new program table that collects the segments for allocated sections
// and a few special segments.
auto* const program_table = new (zone_) ProgramTable(zone_);
GrowableArray<Section*> reordered_sections(zone_, num_sections);
// Maps the old indices of sections to the new ones.
GrowableArray<intptr_t> index_map(zone_, num_sections);
index_map.FillWith(0, 0, num_sections);
Segment* current_segment = nullptr;
// Only called for sections in the section table (i.e., not special sections
// appearing in segments only or the section table itself).
auto add_to_reordered_sections = [&](Section* section) {
intptr_t new_index = reordered_sections.length();
index_map[section->index] = new_index;
section->index = new_index;
reordered_sections.Add(section);
if (section->IsAllocated()) {
ASSERT(current_segment != nullptr);
if (!current_segment->Add(section)) {
// The current segment is incompatible for the current sectioni, so
// create a new one.
current_segment = new (zone_)
Segment(zone_, section, elf::ProgramHeaderType::PT_LOAD);
program_table->Add(current_segment);
}
}
};
// The first section in the section header table is always a reserved
// entry containing only 0 values, so copy it over from sections_.
add_to_reordered_sections(sections_[0]);
// There are few important invariants originating from Android idiosyncrasies
// we are trying to maintain when ordering sections:
//
// - Android requires the program header table be in the first load segment,
// so create PseudoSections representing the ELF header and program header
// table to initialize that segment.
//
// - The Android dynamic linker in Jelly Bean incorrectly assumes that all
// non-writable segments are contiguous. Thus we write them all together.
// The bug is here: https://github.com/aosp-mirror/platform_bionic/blob/94963af28e445384e19775a838a29e6a71708179/linker/linker.c#L1991-L2001
//
// - On Android native libraries can be mapped directly from an APK
// they are stored uncompressed in it. In such situations the name
// of the mapping no longer provides enough information for libunwindstack
// to find the original ELF file and instead it has to rely on heuristics
// to locate program header table. These heuristics currently assume that
// program header table will be located in the RO mapping which precedes
// RX mapping.
//
// These invariants imply the following order of segments: RO (program
// header, .note.gnu.build-id, .dynstr, .dynsym, .hash, .rodata
// and .eh_frame), RX (.text), RW (.dynamic and .bss).
//
auto* const elf_header = new (zone_) ElfHeader(*program_table, *this);
// Self-reference to program header table. Required by Android but not by
// Linux. Must appear before any PT_LOAD entries.
program_table->Add(new (zone_) Segment(zone_, program_table,
elf::ProgramHeaderType::PT_PHDR));
// Create the initial load segment which contains the ELF header and program
// table.
current_segment =
new (zone_) Segment(zone_, elf_header, elf::ProgramHeaderType::PT_LOAD);
program_table->Add(current_segment);
current_segment->Add(program_table);
// We now do several passes over the collected sections to reorder them in
// a way that minimizes segments (and thus padding) in the resulting snapshot.
auto add_sections_matching =
[&](const std::function<bool(Section*)>& should_add) {
// We order the sections in a segment so all non-NOBITS sections come
// before NOBITS sections, since the former sections correspond to the
// file contents for the segment.
for (auto* const section : sections_) {
if (!section->HasBits()) continue;
if (should_add(section)) {
add_to_reordered_sections(section);
}
}
for (auto* const section : sections_) {
if (section->HasBits()) continue;
if (should_add(section)) {
add_to_reordered_sections(section);
}
}
};
// If a build ID was created, we put it right after the program table so it
// can be read with a minimum number of bytes from the ELF file.
auto* const build_id = Find(Elf::kBuildIdNoteName);
if (build_id != nullptr) {
ASSERT(build_id->type == elf::SectionHeaderType::SHT_NOTE);
add_to_reordered_sections(build_id);
}
// Now add the other non-writable, non-executable allocated sections.
add_sections_matching([&](Section* section) -> bool {
if (section == build_id) return false; // Already added.
return section->IsAllocated() && !section->IsWritable() &&
!section->IsExecutable();
});
// Now add the executable sections in a new segment.
add_sections_matching([](Section* section) -> bool {
return section->IsExecutable(); // Implies IsAllocated() && !IsWritable()
});
// Now add all the writable sections.
add_sections_matching([](Section* section) -> bool {
return section->IsWritable(); // Implies IsAllocated() && !IsExecutable()
});
// We put all non-reserved unallocated sections last. Otherwise, they would
// affect the file offset but not the memory offset of any following allocated
// sections. Doing it in this order makes it easier to keep file and memory
// offsets page-aligned with respect to each other, which is required for
// some loaders.
add_sections_matching([](Section* section) -> bool {
// Don't re-add the initial reserved section.
return !section->IsReservedSection() && !section->IsAllocated();
});
// All sections should have been accounted for in the loops above.
ASSERT_EQUAL(sections_.length(), reordered_sections.length());
// Replace the content of sections_ with the reordered sections.
sections_.Clear();
sections_.AddArray(reordered_sections);
// This must be true for uses of the map to be correct.
ASSERT_EQUAL(index_map[elf::SHN_UNDEF], elf::SHN_UNDEF);
// Since the section indices have been updated, change links to match
// and update the indexes of symbols in any symbol tables.
for (auto* const section : sections_) {
// SHN_UNDEF maps to SHN_UNDEF, so no need to check for it.
section->link = index_map[section->link];
if (auto* const table = section->AsSymbolTable()) {
table->UpdateSectionIndices(index_map);
}
}
if (symtab->index == elf::SHN_UNDEF) {
// The output is stripped, so this wasn't finalized during the loop above.
symtab->UpdateSectionIndices(index_map);
}
// Add any special non-load segments.
if (build_id != nullptr) {
// Add a PT_NOTE segment for the build ID.
program_table->Add(
new (zone_) Segment(zone_, build_id, elf::ProgramHeaderType::PT_NOTE));
}
// Add a PT_DYNAMIC segment for the dynamic symbol table.
ASSERT(HasSectionNamed(Elf::kDynamicTableName));
auto* const dynamic = Find(Elf::kDynamicTableName)->AsDynamicTable();
program_table->Add(
new (zone_) Segment(zone_, dynamic, elf::ProgramHeaderType::PT_DYNAMIC));
// Add a PT_GNU_STACK segment to prevent the loading of our snapshot from
// switch the stack to be executable.
auto* const gnu_stack = new (zone_) GnuStackSection();
program_table->Add(new (zone_) Segment(zone_, gnu_stack,
elf::ProgramHeaderType::PT_GNU_STACK));
return program_table;
}
void Elf::Finalize() {
// Generate the build ID now that we have all user-provided sections.
GenerateBuildId();
// We add a BSS section in all cases, even to the separate debugging
// information, to ensure that relocated addresses are consistent between ELF
// snapshots and the corresponding separate debugging information.
CreateBSS();
FinalizeEhFrame();
FinalizeDwarfSections();
// Create and initialize the dynamic and static symbol tables and any
// other associated sections now that all other sections have been added.
InitializeSymbolTables();
// Creates an appropriate program table containing load segments for allocated
// sections and any other segments needed. May reorder sections to minimize
// the number of load segments, so also takes the static symbol table so
// symbol section indices can be adjusted if needed.
program_table_ = section_table_->CreateProgramTable(symtab_);
// Calculate file and memory offsets, and finalizes symbol values in any
// symbol tables.
ComputeOffsets();
#if defined(DEBUG)
if (type_ == Type::Snapshot) {
// For files that will be dynamically loaded, ensure the file offsets
// of allocated sections are page aligned to the memory offsets.
for (auto* const segment : program_table_->segments()) {
for (auto* const section : segment->sections()) {
ASSERT_EQUAL(section->file_offset() % Elf::kPageSize,
section->memory_offset() % Elf::kPageSize);
}
}
}
#endif
// Finally, write the ELF file contents.
ElfWriteStream wrapped(unwrapped_stream_, *this);
auto write_section = [&](const Section* section) {
wrapped.Align(section->alignment);
ASSERT_EQUAL(wrapped.Position(), section->file_offset());
section->Write(&wrapped);
ASSERT_EQUAL(wrapped.Position(),
section->file_offset() + section->FileSize());
};
// To match ComputeOffsets, first we write allocated sections and then
// unallocated sections. We access the allocated sections via the load
// segments so we can properly align the stream for each entered segment.
intptr_t section_index = 1; // We don't visit the reserved section.
for (auto* const segment : program_table_->segments()) {
if (segment->type != elf::ProgramHeaderType::PT_LOAD) continue;
wrapped.Align(segment->Alignment());
for (auto* const section : segment->sections()) {
ASSERT(section->IsAllocated());
write_section(section);
if (!section->IsPseudoSection()) {
ASSERT_EQUAL(section->index, section_index);
section_index++;
}
}
}
const auto& sections = section_table_->sections();
for (; section_index < sections.length(); section_index++) {
auto* const section = sections[section_index];
ASSERT(!section->IsAllocated());
write_section(section);
}
// Finally, write the section table.
write_section(section_table_);
}
// For the build ID, we generate a 128-bit hash, where each 32 bits is a hash of
// the contents of the following segments in order:
//
// .text(VM) | .text(Isolate) | .rodata(VM) | .rodata(Isolate)
static constexpr const char* kBuildIdSegmentNames[]{
kVmSnapshotInstructionsAsmSymbol,
kIsolateSnapshotInstructionsAsmSymbol,
kVmSnapshotDataAsmSymbol,
kIsolateSnapshotDataAsmSymbol,
};
static constexpr intptr_t kBuildIdSegmentNamesLength =
ARRAY_SIZE(kBuildIdSegmentNames);
// Includes the note name, but not the description.
static constexpr intptr_t kBuildIdHeaderSize =
sizeof(elf::Note) + sizeof(elf::ELF_NOTE_GNU);
void Elf::GenerateBuildId() {
// Not idempotent.
ASSERT(section_table_->Find(kBuildIdNoteName) == nullptr);
uint32_t hashes[kBuildIdSegmentNamesLength];
// Currently, we construct the build ID out of data from two different
// sections: the .text section and the .rodata section.
//
// TODO(dartbug.com/43274): Generate build IDs for separate debugging
// information for assembly snapshots.
auto* const text_section = section_table_->Find(kTextName);
if (text_section == nullptr) return;
ASSERT(text_section->IsTextSection());
auto* const text_bits = text_section->AsBitsContainer();
auto* const data_section = section_table_->Find(kDataName);
ASSERT(data_section == nullptr || data_section->IsDataSection());
// Hash each component by first hashing the associated text section and, if
// there's not one, hashing the associated data section (if any).
//
// Any component of the build ID which does not have an associated section
// in the result is kept as 0.
bool has_any_text = false;
for (intptr_t i = 0; i < kBuildIdSegmentNamesLength; i++) {
auto* const name = kBuildIdSegmentNames[i];
hashes[i] = text_bits->Hash(name);
if (hashes[i] != 0) {
has_any_text = true;
} else if (data_section != nullptr) {
hashes[i] = data_section->AsBitsContainer()->Hash(name);
}
}
// If none of the sections in the hash were text sections, then we don't need
// a build ID, as it is only used to symbolicize non-symbolic stack traces.
if (!has_any_text) return;
auto const description_bytes = reinterpret_cast<uint8_t*>(hashes);
const size_t description_length = sizeof(hashes);
// Now that we have the description field contents, create the section.
ZoneWriteStream stream(zone(), kBuildIdHeaderSize + description_length);
stream.WriteFixed<decltype(elf::Note::name_size)>(sizeof(elf::ELF_NOTE_GNU));
stream.WriteFixed<decltype(elf::Note::description_size)>(description_length);
stream.WriteFixed<decltype(elf::Note::type)>(elf::NoteType::NT_GNU_BUILD_ID);
ASSERT_EQUAL(stream.Position(), sizeof(elf::Note));
stream.WriteBytes(elf::ELF_NOTE_GNU, sizeof(elf::ELF_NOTE_GNU));
ASSERT_EQUAL(stream.bytes_written(), kBuildIdHeaderSize);
stream.WriteBytes(description_bytes, description_length);
auto* const container = new (zone_) NoteSection();
container->AddPortion(stream.buffer(), stream.bytes_written(),
/*relocations=*/nullptr, /*symbols=*/nullptr,
kSnapshotBuildIdAsmSymbol, kBuildIdLabel);
section_table_->Add(container, kBuildIdNoteName);
}
void Elf::ComputeOffsets() {
intptr_t file_offset = 0;
intptr_t memory_offset = 0;
// Maps indices of allocated sections in the section table to memory offsets.
const intptr_t num_sections = section_table_->SectionCount();
GrowableArray<intptr_t> address_map(zone_, num_sections);
address_map.Add(0); // Don't adjust offsets for symbols with index SHN_UNDEF.
auto calculate_section_offsets = [&](Section* section) {
file_offset = Utils::RoundUp(file_offset, section->alignment);
section->set_file_offset(file_offset);
file_offset += section->FileSize();
if (section->IsAllocated()) {
memory_offset = Utils::RoundUp(memory_offset, section->alignment);
section->set_memory_offset(memory_offset);
memory_offset += section->MemorySize();
}
};
intptr_t section_index = 1; // We don't visit the reserved section.
for (auto* const segment : program_table_->segments()) {
if (segment->type != elf::ProgramHeaderType::PT_LOAD) continue;
// Adjust file and memory offsets for segment alignment on entry.
file_offset = Utils::RoundUp(file_offset, segment->Alignment());
memory_offset = Utils::RoundUp(memory_offset, segment->Alignment());
for (auto* const section : segment->sections()) {
ASSERT(section->IsAllocated());
calculate_section_offsets(section);
if (!section->IsPseudoSection()) {
// Note: this assumes that the sections in the section header has all
// allocated sections before all (non-reserved) unallocated sections and
// in the same order as the load segments in in the program table.
address_map.Add(section->memory_offset());
ASSERT_EQUAL(section->index, section_index);
section_index++;
}
}
}
const auto& sections = section_table_->sections();
for (; section_index < sections.length(); section_index++) {
auto* const section = sections[section_index];
ASSERT(!section->IsAllocated());
calculate_section_offsets(section);
}
ASSERT_EQUAL(section_index, sections.length());
// Now that all sections have been handled, set the file offset for the
// section table, as it will be written after the last section.
calculate_section_offsets(section_table_);
#if defined(DEBUG)
// Double check that segment starts are aligned as expected.
for (auto* const segment : program_table_->segments()) {
ASSERT(Utils::IsAligned(segment->MemoryOffset(), segment->Alignment()));
}
#endif
// This must be true for uses of the map to be correct.
ASSERT_EQUAL(address_map[elf::SHN_UNDEF], 0);
// Adjust addresses in symbol tables as we now have section memory offsets.
// Also finalize the entries of the dynamic table, as some are memory offsets.
for (auto* const section : sections) {
if (auto* const table = section->AsSymbolTable()) {
table->Finalize(address_map);
} else if (auto* const dynamic = section->AsDynamicTable()) {
dynamic->Finalize();
}
}
// Also adjust addresses in symtab for stripped snapshots.
if (IsStripped()) {
ASSERT_EQUAL(symtab_->index, elf::SHN_UNDEF);
symtab_->Finalize(address_map);
}
}
void ElfHeader::Write(ElfWriteStream* stream) const {
ASSERT_EQUAL(file_offset(), 0);
ASSERT_EQUAL(memory_offset(), 0);
#if defined(TARGET_ARCH_IS_32_BIT)
uint8_t size = elf::ELFCLASS32;
#else
uint8_t size = elf::ELFCLASS64;
#endif
uint8_t e_ident[16] = {0x7f,
'E',
'L',
'F',
size,
elf::ELFDATA2LSB,
elf::EV_CURRENT,
elf::ELFOSABI_SYSV,
0,
0,
0,
0,
0,
0,
0,
0};
stream->WriteBytes(e_ident, 16);
stream->WriteHalf(elf::ET_DYN); // Shared library.
#if defined(TARGET_ARCH_IA32)
stream->WriteHalf(elf::EM_386);
#elif defined(TARGET_ARCH_X64)
stream->WriteHalf(elf::EM_X86_64);
#elif defined(TARGET_ARCH_ARM)
stream->WriteHalf(elf::EM_ARM);
#elif defined(TARGET_ARCH_ARM64)
stream->WriteHalf(elf::EM_AARCH64);
#elif defined(TARGET_ARCH_RISCV32) || defined(TARGET_ARCH_RISCV64)
stream->WriteHalf(elf::EM_RISCV);
#else
FATAL("Unknown ELF architecture");
#endif
stream->WriteWord(elf::EV_CURRENT); // Version
stream->WriteAddr(0); // "Entry point"
stream->WriteOff(program_table_.file_offset());
stream->WriteOff(section_table_.file_offset());
#if defined(TARGET_ARCH_ARM)
uword flags = elf::EF_ARM_ABI | (TargetCPUFeatures::hardfp_supported()
? elf::EF_ARM_ABI_FLOAT_HARD
: elf::EF_ARM_ABI_FLOAT_SOFT);
#else
uword flags = 0;
#endif
stream->WriteWord(flags);
stream->WriteHalf(sizeof(elf::ElfHeader));
stream->WriteHalf(program_table_.entry_size);
stream->WriteHalf(program_table_.SegmentCount());
stream->WriteHalf(section_table_.entry_size);
stream->WriteHalf(section_table_.SectionCount());
stream->WriteHalf(stream->elf().section_table().StringTableIndex());
}
void ProgramTable::Write(ElfWriteStream* stream) const {
ASSERT(segments_.length() > 0);
// Make sure all relevant segments were created by checking the type of the
// first.
ASSERT(segments_[0]->type == elf::ProgramHeaderType::PT_PHDR);
const intptr_t start = stream->Position();
// Should be immediately following the ELF header.
ASSERT_EQUAL(start, sizeof(elf::ElfHeader));
#if defined(DEBUG)
// Here, we count the number of times that a PT_LOAD writable segment is
// followed by a non-writable segment. We initialize last_writable to true
// so that we catch the case where the first segment is non-writable.
bool last_writable = true;
int non_writable_groups = 0;
#endif
for (intptr_t i = 0; i < segments_.length(); i++) {
const Segment* const segment = segments_[i];
ASSERT(segment->type != elf::ProgramHeaderType::PT_NULL);
ASSERT_EQUAL(i == 0, segment->type == elf::ProgramHeaderType::PT_PHDR);
#if defined(DEBUG)
if (segment->type == elf::ProgramHeaderType::PT_LOAD) {
if (last_writable && !segment->IsWritable()) {
non_writable_groups++;
}
last_writable = segment->IsWritable();
}
#endif
const intptr_t start = stream->Position();
segment->WriteProgramHeader(stream);
const intptr_t end = stream->Position();
ASSERT_EQUAL(end - start, entry_size);
}
#if defined(DEBUG)
// All PT_LOAD non-writable segments must be contiguous. If not, some older
// Android dynamic linkers fail to handle writable segments between
// non-writable ones. See https://github.com/flutter/flutter/issues/43259.
ASSERT(non_writable_groups <= 1);
#endif
}
void SectionTable::Write(ElfWriteStream* stream) const {
for (intptr_t i = 0; i < sections_.length(); i++) {
const Section* const section = sections_[i];
ASSERT_EQUAL(i == 0, section->IsReservedSection());
ASSERT_EQUAL(section->index, i);
ASSERT(section->link < sections_.length());
const intptr_t start = stream->Position();
section->WriteSectionHeader(stream);
const intptr_t end = stream->Position();
ASSERT_EQUAL(end - start, entry_size);
}
}
#endif // DART_PRECOMPILER
} // namespace dart