//===- OutputSections.cpp -------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "OutputSections.h"
#include "Config.h"
#include "LinkerScript.h"
#include "SymbolTable.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "lld/Common/Memory.h"
#include "lld/Common/Strings.h"
#include "lld/Common/Threads.h"
#include "llvm/BinaryFormat/Dwarf.h"
#include "llvm/Support/Compression.h"
#include "llvm/Support/MD5.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/SHA1.h"
#include <regex>
using namespace llvm;
using namespace llvm::dwarf;
using namespace llvm::object;
using namespace llvm::support::endian;
using namespace llvm::ELF;
namespace lld {
namespace elf {
uint8_t *Out::bufferStart;
uint8_t Out::first;
PhdrEntry *Out::tlsPhdr;
OutputSection *Out::elfHeader;
OutputSection *Out::programHeaders;
OutputSection *Out::preinitArray;
OutputSection *Out::initArray;
OutputSection *Out::finiArray;
std::vector<OutputSection *> outputSections;
uint32_t OutputSection::getPhdrFlags() const {
uint32_t ret = 0;
if (config->emachine != EM_ARM || !(flags & SHF_ARM_PURECODE))
ret |= PF_R;
if (flags & SHF_WRITE)
ret |= PF_W;
if (flags & SHF_EXECINSTR)
ret |= PF_X;
return ret;
}
template <class ELFT>
void OutputSection::writeHeaderTo(typename ELFT::Shdr *shdr) {
shdr->sh_entsize = entsize;
shdr->sh_addralign = alignment;
shdr->sh_type = type;
shdr->sh_offset = offset;
shdr->sh_flags = flags;
shdr->sh_info = info;
shdr->sh_link = link;
shdr->sh_addr = addr;
shdr->sh_size = size;
shdr->sh_name = shName;
}
OutputSection::OutputSection(StringRef name, uint32_t type, uint64_t flags)
: BaseCommand(OutputSectionKind),
SectionBase(Output, name, flags, /*Entsize*/ 0, /*Alignment*/ 1, type,
/*Info*/ 0, /*Link*/ 0) {}
// We allow sections of types listed below to merged into a
// single progbits section. This is typically done by linker
// scripts. Merging nobits and progbits will force disk space
// to be allocated for nobits sections. Other ones don't require
// any special treatment on top of progbits, so there doesn't
// seem to be a harm in merging them.
static bool canMergeToProgbits(unsigned type) {
return type == SHT_NOBITS || type == SHT_PROGBITS || type == SHT_INIT_ARRAY ||
type == SHT_PREINIT_ARRAY || type == SHT_FINI_ARRAY ||
type == SHT_NOTE;
}
// Record that isec will be placed in the OutputSection. isec does not become
// permanent until finalizeInputSections() is called. The function should not be
// used after finalizeInputSections() is called. If you need to add an
// InputSection post finalizeInputSections(), then you must do the following:
//
// 1. Find or create an InputSectionDescription to hold InputSection.
// 2. Add the InputSection to the InputSectionDescription::sections.
// 3. Call commitSection(isec).
void OutputSection::recordSection(InputSectionBase *isec) {
partition = isec->partition;
isec->parent = this;
if (sectionCommands.empty() ||
!isa<InputSectionDescription>(sectionCommands.back()))
sectionCommands.push_back(make<InputSectionDescription>(""));
auto *isd = cast<InputSectionDescription>(sectionCommands.back());
isd->sectionBases.push_back(isec);
}
// Update fields (type, flags, alignment, etc) according to the InputSection
// isec. Also check whether the InputSection flags and type are consistent with
// other InputSections.
void OutputSection::commitSection(InputSection *isec) {
if (!hasInputSections) {
// If IS is the first section to be added to this section,
// initialize type, entsize and flags from isec.
hasInputSections = true;
type = isec->type;
entsize = isec->entsize;
flags = isec->flags;
} else {
// Otherwise, check if new type or flags are compatible with existing ones.
if ((flags ^ isec->flags) & SHF_TLS)
error("incompatible section flags for " + name + "\n>>> " + toString(isec) +
": 0x" + utohexstr(isec->flags) + "\n>>> output section " + name +
": 0x" + utohexstr(flags));
if (type != isec->type) {
if (!canMergeToProgbits(type) || !canMergeToProgbits(isec->type))
error("section type mismatch for " + isec->name + "\n>>> " +
toString(isec) + ": " +
getELFSectionTypeName(config->emachine, isec->type) +
"\n>>> output section " + name + ": " +
getELFSectionTypeName(config->emachine, type));
type = SHT_PROGBITS;
}
}
if (noload)
type = SHT_NOBITS;
isec->parent = this;
uint64_t andMask =
config->emachine == EM_ARM ? (uint64_t)SHF_ARM_PURECODE : 0;
uint64_t orMask = ~andMask;
uint64_t andFlags = (flags & isec->flags) & andMask;
uint64_t orFlags = (flags | isec->flags) & orMask;
flags = andFlags | orFlags;
if (nonAlloc)
flags &= ~(uint64_t)SHF_ALLOC;
alignment = std::max(alignment, isec->alignment);
// If this section contains a table of fixed-size entries, sh_entsize
// holds the element size. If it contains elements of different size we
// set sh_entsize to 0.
if (entsize != isec->entsize)
entsize = 0;
}
// This function scans over the InputSectionBase list sectionBases to create
// InputSectionDescription::sections.
//
// It removes MergeInputSections from the input section array and adds
// new synthetic sections at the location of the first input section
// that it replaces. It then finalizes each synthetic section in order
// to compute an output offset for each piece of each input section.
void OutputSection::finalizeInputSections() {
std::vector<MergeSyntheticSection *> mergeSections;
for (BaseCommand *base : sectionCommands) {
auto *cmd = dyn_cast<InputSectionDescription>(base);
if (!cmd)
continue;
cmd->sections.reserve(cmd->sectionBases.size());
for (InputSectionBase *s : cmd->sectionBases) {
MergeInputSection *ms = dyn_cast<MergeInputSection>(s);
if (!ms) {
cmd->sections.push_back(cast<InputSection>(s));
continue;
}
// We do not want to handle sections that are not alive, so just remove
// them instead of trying to merge.
if (!ms->isLive())
continue;
auto i = llvm::find_if(mergeSections, [=](MergeSyntheticSection *sec) {
// While we could create a single synthetic section for two different
// values of Entsize, it is better to take Entsize into consideration.
//
// With a single synthetic section no two pieces with different Entsize
// could be equal, so we may as well have two sections.
//
// Using Entsize in here also allows us to propagate it to the synthetic
// section.
//
// SHF_STRINGS section with different alignments should not be merged.
return sec->flags == ms->flags && sec->entsize == ms->entsize &&
(sec->alignment == ms->alignment || !(sec->flags & SHF_STRINGS));
});
if (i == mergeSections.end()) {
MergeSyntheticSection *syn =
createMergeSynthetic(name, ms->type, ms->flags, ms->alignment);
mergeSections.push_back(syn);
i = std::prev(mergeSections.end());
syn->entsize = ms->entsize;
cmd->sections.push_back(syn);
}
(*i)->addSection(ms);
}
// sectionBases should not be used from this point onwards. Clear it to
// catch misuses.
cmd->sectionBases.clear();
// Some input sections may be removed from the list after ICF.
for (InputSection *s : cmd->sections)
commitSection(s);
}
for (auto *ms : mergeSections)
ms->finalizeContents();
}
static void sortByOrder(MutableArrayRef<InputSection *> in,
llvm::function_ref<int(InputSectionBase *s)> order) {
std::vector<std::pair<int, InputSection *>> v;
for (InputSection *s : in)
v.push_back({order(s), s});
llvm::stable_sort(v, less_first());
for (size_t i = 0; i < v.size(); ++i)
in[i] = v[i].second;
}
uint64_t getHeaderSize() {
if (config->oFormatBinary)
return 0;
return Out::elfHeader->size + Out::programHeaders->size;
}
bool OutputSection::classof(const BaseCommand *c) {
return c->kind == OutputSectionKind;
}
void OutputSection::sort(llvm::function_ref<int(InputSectionBase *s)> order) {
assert(isLive());
for (BaseCommand *b : sectionCommands)
if (auto *isd = dyn_cast<InputSectionDescription>(b))
sortByOrder(isd->sections, order);
}
// Fill [Buf, Buf + Size) with Filler.
// This is used for linker script "=fillexp" command.
static void fill(uint8_t *buf, size_t size,
const std::array<uint8_t, 4> &filler) {
size_t i = 0;
for (; i + 4 < size; i += 4)
memcpy(buf + i, filler.data(), 4);
memcpy(buf + i, filler.data(), size - i);
}
// Compress section contents if this section contains debug info.
template <class ELFT> void OutputSection::maybeCompress() {
using Elf_Chdr = typename ELFT::Chdr;
// Compress only DWARF debug sections.
if (!config->compressDebugSections || (flags & SHF_ALLOC) ||
!name.startswith(".debug_"))
return;
// Create a section header.
zDebugHeader.resize(sizeof(Elf_Chdr));
auto *hdr = reinterpret_cast<Elf_Chdr *>(zDebugHeader.data());
hdr->ch_type = ELFCOMPRESS_ZLIB;
hdr->ch_size = size;
hdr->ch_addralign = alignment;
// Write section contents to a temporary buffer and compress it.
std::vector<uint8_t> buf(size);
writeTo<ELFT>(buf.data());
// We chose 1 as the default compression level because it is the fastest. If
// -O2 is given, we use level 6 to compress debug info more by ~15%. We found
// that level 7 to 9 doesn't make much difference (~1% more compression) while
// they take significant amount of time (~2x), so level 6 seems enough.
if (Error e = zlib::compress(toStringRef(buf), compressedData,
config->optimize >= 2 ? 6 : 1))
fatal("compress failed: " + llvm::toString(std::move(e)));
// Update section headers.
size = sizeof(Elf_Chdr) + compressedData.size();
flags |= SHF_COMPRESSED;
}
static void writeInt(uint8_t *buf, uint64_t data, uint64_t size) {
if (size == 1)
*buf = data;
else if (size == 2)
write16(buf, data);
else if (size == 4)
write32(buf, data);
else if (size == 8)
write64(buf, data);
else
llvm_unreachable("unsupported Size argument");
}
template <class ELFT> void OutputSection::writeTo(uint8_t *buf) {
if (type == SHT_NOBITS)
return;
// If -compress-debug-section is specified and if this is a debug section,
// we've already compressed section contents. If that's the case,
// just write it down.
if (!compressedData.empty()) {
memcpy(buf, zDebugHeader.data(), zDebugHeader.size());
memcpy(buf + zDebugHeader.size(), compressedData.data(),
compressedData.size());
return;
}
// Write leading padding.
std::vector<InputSection *> sections = getInputSections(this);
std::array<uint8_t, 4> filler = getFiller();
bool nonZeroFiller = read32(filler.data()) != 0;
if (nonZeroFiller)
fill(buf, sections.empty() ? size : sections[0]->outSecOff, filler);
parallelForEachN(0, sections.size(), [&](size_t i) {
InputSection *isec = sections[i];
isec->writeTo<ELFT>(buf);
// Fill gaps between sections.
if (nonZeroFiller) {
uint8_t *start = buf + isec->outSecOff + isec->getSize();
uint8_t *end;
if (i + 1 == sections.size())
end = buf + size;
else
end = buf + sections[i + 1]->outSecOff;
fill(start, end - start, filler);
}
});
// Linker scripts may have BYTE()-family commands with which you
// can write arbitrary bytes to the output. Process them if any.
for (BaseCommand *base : sectionCommands)
if (auto *data = dyn_cast<ByteCommand>(base))
writeInt(buf + data->offset, data->expression().getValue(), data->size);
}
static void finalizeShtGroup(OutputSection *os,
InputSection *section) {
assert(config->relocatable);
// sh_link field for SHT_GROUP sections should contain the section index of
// the symbol table.
os->link = in.symTab->getParent()->sectionIndex;
// sh_info then contain index of an entry in symbol table section which
// provides signature of the section group.
ArrayRef<Symbol *> symbols = section->file->getSymbols();
os->info = in.symTab->getSymbolIndex(symbols[section->info]);
}
void OutputSection::finalize() {
std::vector<InputSection *> v = getInputSections(this);
InputSection *first = v.empty() ? nullptr : v[0];
if (flags & SHF_LINK_ORDER) {
// We must preserve the link order dependency of sections with the
// SHF_LINK_ORDER flag. The dependency is indicated by the sh_link field. We
// need to translate the InputSection sh_link to the OutputSection sh_link,
// all InputSections in the OutputSection have the same dependency.
if (auto *ex = dyn_cast<ARMExidxSyntheticSection>(first))
link = ex->getLinkOrderDep()->getParent()->sectionIndex;
else if (first->flags & SHF_LINK_ORDER)
if (auto *d = first->getLinkOrderDep())
link = d->getParent()->sectionIndex;
}
if (type == SHT_GROUP) {
finalizeShtGroup(this, first);
return;
}
if (!config->copyRelocs || (type != SHT_RELA && type != SHT_REL))
return;
if (isa<SyntheticSection>(first))
return;
link = in.symTab->getParent()->sectionIndex;
// sh_info for SHT_REL[A] sections should contain the section header index of
// the section to which the relocation applies.
InputSectionBase *s = first->getRelocatedSection();
info = s->getOutputSection()->sectionIndex;
flags |= SHF_INFO_LINK;
}
// Returns true if S is in one of the many forms the compiler driver may pass
// crtbegin files.
//
// Gcc uses any of crtbegin[<empty>|S|T].o.
// Clang uses Gcc's plus clang_rt.crtbegin[<empty>|S|T][-<arch>|<empty>].o.
static bool isCrtbegin(StringRef s) {
static std::regex re(R"((clang_rt\.)?crtbegin[ST]?(-.*)?\.o)");
s = sys::path::filename(s);
return std::regex_match(s.begin(), s.end(), re);
}
static bool isCrtend(StringRef s) {
static std::regex re(R"((clang_rt\.)?crtend[ST]?(-.*)?\.o)");
s = sys::path::filename(s);
return std::regex_match(s.begin(), s.end(), re);
}
// .ctors and .dtors are sorted by this priority from highest to lowest.
//
// 1. The section was contained in crtbegin (crtbegin contains
// some sentinel value in its .ctors and .dtors so that the runtime
// can find the beginning of the sections.)
//
// 2. The section has an optional priority value in the form of ".ctors.N"
// or ".dtors.N" where N is a number. Unlike .{init,fini}_array,
// they are compared as string rather than number.
//
// 3. The section is just ".ctors" or ".dtors".
//
// 4. The section was contained in crtend, which contains an end marker.
//
// In an ideal world, we don't need this function because .init_array and
// .ctors are duplicate features (and .init_array is newer.) However, there
// are too many real-world use cases of .ctors, so we had no choice to
// support that with this rather ad-hoc semantics.
static bool compCtors(const InputSection *a, const InputSection *b) {
bool beginA = isCrtbegin(a->file->getName());
bool beginB = isCrtbegin(b->file->getName());
if (beginA != beginB)
return beginA;
bool endA = isCrtend(a->file->getName());
bool endB = isCrtend(b->file->getName());
if (endA != endB)
return endB;
StringRef x = a->name;
StringRef y = b->name;
assert(x.startswith(".ctors") || x.startswith(".dtors"));
assert(y.startswith(".ctors") || y.startswith(".dtors"));
x = x.substr(6);
y = y.substr(6);
return x < y;
}
// Sorts input sections by the special rules for .ctors and .dtors.
// Unfortunately, the rules are different from the one for .{init,fini}_array.
// Read the comment above.
void OutputSection::sortCtorsDtors() {
assert(sectionCommands.size() == 1);
auto *isd = cast<InputSectionDescription>(sectionCommands[0]);
llvm::stable_sort(isd->sections, compCtors);
}
// If an input string is in the form of "foo.N" where N is a number,
// return N. Otherwise, returns 65536, which is one greater than the
// lowest priority.
int getPriority(StringRef s) {
size_t pos = s.rfind('.');
if (pos == StringRef::npos)
return 65536;
int v;
if (!to_integer(s.substr(pos + 1), v, 10))
return 65536;
return v;
}
std::vector<InputSection *> getInputSections(OutputSection *os) {
std::vector<InputSection *> ret;
for (BaseCommand *base : os->sectionCommands)
if (auto *isd = dyn_cast<InputSectionDescription>(base))
ret.insert(ret.end(), isd->sections.begin(), isd->sections.end());
return ret;
}
// Sorts input sections by section name suffixes, so that .foo.N comes
// before .foo.M if N < M. Used to sort .{init,fini}_array.N sections.
// We want to keep the original order if the priorities are the same
// because the compiler keeps the original initialization order in a
// translation unit and we need to respect that.
// For more detail, read the section of the GCC's manual about init_priority.
void OutputSection::sortInitFini() {
// Sort sections by priority.
sort([](InputSectionBase *s) { return getPriority(s->name); });
}
std::array<uint8_t, 4> OutputSection::getFiller() {
if (filler)
return *filler;
if (flags & SHF_EXECINSTR)
return target->trapInstr;
return {0, 0, 0, 0};
}
template void OutputSection::writeHeaderTo<ELF32LE>(ELF32LE::Shdr *Shdr);
template void OutputSection::writeHeaderTo<ELF32BE>(ELF32BE::Shdr *Shdr);
template void OutputSection::writeHeaderTo<ELF64LE>(ELF64LE::Shdr *Shdr);
template void OutputSection::writeHeaderTo<ELF64BE>(ELF64BE::Shdr *Shdr);
template void OutputSection::writeTo<ELF32LE>(uint8_t *Buf);
template void OutputSection::writeTo<ELF32BE>(uint8_t *Buf);
template void OutputSection::writeTo<ELF64LE>(uint8_t *Buf);
template void OutputSection::writeTo<ELF64BE>(uint8_t *Buf);
template void OutputSection::maybeCompress<ELF32LE>();
template void OutputSection::maybeCompress<ELF32BE>();
template void OutputSection::maybeCompress<ELF64LE>();
template void OutputSection::maybeCompress<ELF64BE>();
} // namespace elf
} // namespace lld
