llvm-project/lld/ELF/OutputSections.cpp

605 lines
21 KiB
C++

//===- OutputSections.cpp -------------------------------------------------===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
#include "OutputSections.h"
#include "Config.h"
#include "LinkerScript.h"
#include "Memory.h"
#include "Strings.h"
#include "SymbolTable.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "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"
using namespace llvm;
using namespace llvm::dwarf;
using namespace llvm::object;
using namespace llvm::support::endian;
using namespace llvm::ELF;
using namespace lld;
using namespace lld::elf;
uint8_t Out::First;
OutputSection *Out::Opd;
uint8_t *Out::OpdBuf;
PhdrEntry *Out::TlsPhdr;
OutputSection *Out::DebugInfo;
OutputSection *Out::ElfHeader;
OutputSection *Out::ProgramHeaders;
OutputSection *Out::PreinitArray;
OutputSection *Out::InitArray;
OutputSection *Out::FiniArray;
std::vector<OutputSection *> elf::OutputSections;
uint32_t OutputSection::getPhdrFlags() const {
uint32_t 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),
SectionIndex(INT_MAX) {
Live = false;
}
static uint64_t updateOffset(uint64_t Off, InputSection *S) {
Off = alignTo(Off, S->Alignment);
S->OutSecOff = Off;
return Off + S->getSize();
}
void OutputSection::addSection(InputSection *S) {
assert(S->Live);
Live = true;
S->Parent = this;
this->updateAlignment(S->Alignment);
// The actual offsets will be computed by assignAddresses. For now, use
// crude approximation so that it is at least easy for other code to know the
// section order. It is also used to calculate the output section size early
// for compressed debug sections.
this->Size = updateOffset(Size, S);
// If this section contains a table of fixed-size entries, sh_entsize
// holds the element size. Consequently, if this contains two or more
// input sections, all of them must have the same sh_entsize. However,
// you can put different types of input sections into one output
// sectin by using linker scripts. I don't know what to do here.
// Probably we sholuld handle that as an error. But for now we just
// pick the largest sh_entsize.
this->Entsize = std::max(this->Entsize, S->Entsize);
if (!S->Assigned) {
S->Assigned = true;
if (Commands.empty() || !isa<InputSectionDescription>(Commands.back()))
Commands.push_back(make<InputSectionDescription>(""));
auto *ISD = cast<InputSectionDescription>(Commands.back());
ISD->Sections.push_back(S);
}
}
static SectionKey createKey(InputSectionBase *C, StringRef OutsecName) {
// The ELF spec just says
// ----------------------------------------------------------------
// In the first phase, input sections that match in name, type and
// attribute flags should be concatenated into single sections.
// ----------------------------------------------------------------
//
// However, it is clear that at least some flags have to be ignored for
// section merging. At the very least SHF_GROUP and SHF_COMPRESSED have to be
// ignored. We should not have two output .text sections just because one was
// in a group and another was not for example.
//
// It also seems that that wording was a late addition and didn't get the
// necessary scrutiny.
//
// Merging sections with different flags is expected by some users. One
// reason is that if one file has
//
// int *const bar __attribute__((section(".foo"))) = (int *)0;
//
// gcc with -fPIC will produce a read only .foo section. But if another
// file has
//
// int zed;
// int *const bar __attribute__((section(".foo"))) = (int *)&zed;
//
// gcc with -fPIC will produce a read write section.
//
// Last but not least, when using linker script the merge rules are forced by
// the script. Unfortunately, linker scripts are name based. This means that
// expressions like *(.foo*) can refer to multiple input sections with
// different flags. We cannot put them in different output sections or we
// would produce wrong results for
//
// start = .; *(.foo.*) end = .; *(.bar)
//
// and a mapping of .foo1 and .bar1 to one section and .foo2 and .bar2 to
// another. The problem is that there is no way to layout those output
// sections such that the .foo sections are the only thing between the start
// and end symbols.
//
// Given the above issues, we instead merge sections by name and error on
// incompatible types and flags.
uint32_t Alignment = 0;
uint64_t Flags = 0;
if (Config->Relocatable && (C->Flags & SHF_MERGE)) {
Alignment = std::max<uint64_t>(C->Alignment, C->Entsize);
Flags = C->Flags & (SHF_MERGE | SHF_STRINGS);
}
return SectionKey{OutsecName, Flags, Alignment};
}
OutputSectionFactory::OutputSectionFactory() {}
static uint64_t getIncompatibleFlags(uint64_t Flags) {
return Flags & (SHF_ALLOC | SHF_TLS);
}
// 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;
}
void elf::sortByOrder(MutableArrayRef<InputSection *> In,
std::function<int(InputSectionBase *S)> Order) {
typedef std::pair<int, InputSection *> Pair;
auto Comp = [](const Pair &A, const Pair &B) { return A.first < B.first; };
std::vector<Pair> V;
for (InputSection *S : In)
V.push_back({Order(S), S});
std::stable_sort(V.begin(), V.end(), Comp);
for (size_t I = 0; I < V.size(); ++I)
In[I] = V[I].second;
}
void elf::reportDiscarded(InputSectionBase *IS) {
if (!Config->PrintGcSections)
return;
message("removing unused section from '" + IS->Name + "' in file '" +
IS->File->getName() + "'");
}
static OutputSection *addSection(InputSectionBase *IS, StringRef OutsecName,
OutputSection *Sec) {
if (Sec && Sec->Live) {
if (getIncompatibleFlags(Sec->Flags) != getIncompatibleFlags(IS->Flags))
error("incompatible section flags for " + Sec->Name + "\n>>> " +
toString(IS) + ": 0x" + utohexstr(IS->Flags) +
"\n>>> output section " + Sec->Name + ": 0x" +
utohexstr(Sec->Flags));
if (Sec->Type != IS->Type) {
if (canMergeToProgbits(Sec->Type) && canMergeToProgbits(IS->Type))
Sec->Type = SHT_PROGBITS;
else
error("section type mismatch for " + IS->Name + "\n>>> " +
toString(IS) + ": " +
getELFSectionTypeName(Config->EMachine, IS->Type) +
"\n>>> output section " + Sec->Name + ": " +
getELFSectionTypeName(Config->EMachine, Sec->Type));
}
Sec->Flags |= IS->Flags;
} else {
if (!Sec) {
Sec = Script->createOutputSection(OutsecName, "<internal>");
Script->Opt.Commands.push_back(Sec);
}
Sec->Type = IS->Type;
Sec->Flags = IS->Flags;
}
Sec->addSection(cast<InputSection>(IS));
return Sec;
}
void OutputSectionFactory::addInputSec(InputSectionBase *IS,
StringRef OutsecName,
OutputSection *OS) {
if (!IS->Live) {
reportDiscarded(IS);
return;
}
// If we have destination output section - use it directly.
if (OS) {
addSection(IS, OutsecName, OS);
return;
}
// Sections with SHT_GROUP or SHF_GROUP attributes reach here only when the -r
// option is given. A section with SHT_GROUP defines a "section group", and
// its members have SHF_GROUP attribute. Usually these flags have already been
// stripped by InputFiles.cpp as section groups are processed and uniquified.
// However, for the -r option, we want to pass through all section groups
// as-is because adding/removing members or merging them with other groups
// change their semantics.
if (IS->Type == SHT_GROUP || (IS->Flags & SHF_GROUP)) {
addSection(IS, OutsecName, nullptr);
return;
}
// Imagine .zed : { *(.foo) *(.bar) } script. Both foo and bar may have
// relocation sections .rela.foo and .rela.bar for example. Most tools do
// not allow multiple REL[A] sections for output section. Hence we
// should combine these relocation sections into single output.
// We skip synthetic sections because it can be .rela.dyn/.rela.plt or any
// other REL[A] sections created by linker itself.
if (!isa<SyntheticSection>(IS) &&
(IS->Type == SHT_REL || IS->Type == SHT_RELA)) {
auto *Sec = cast<InputSection>(IS);
OutputSection *Out = Sec->getRelocatedSection()->getOutputSection();
Out->RelocationSection = addSection(IS, OutsecName, Out->RelocationSection);
return;
}
SectionKey Key = createKey(IS, OutsecName);
OutputSection *&Sec = Map[Key];
Sec = addSection(IS, OutsecName, Sec);
}
OutputSectionFactory::~OutputSectionFactory() {}
SectionKey DenseMapInfo<SectionKey>::getEmptyKey() {
return SectionKey{DenseMapInfo<StringRef>::getEmptyKey(), 0, 0};
}
SectionKey DenseMapInfo<SectionKey>::getTombstoneKey() {
return SectionKey{DenseMapInfo<StringRef>::getTombstoneKey(), 0, 0};
}
unsigned DenseMapInfo<SectionKey>::getHashValue(const SectionKey &Val) {
return hash_combine(Val.Name, Val.Flags, Val.Alignment);
}
bool DenseMapInfo<SectionKey>::isEqual(const SectionKey &LHS,
const SectionKey &RHS) {
return DenseMapInfo<StringRef>::isEqual(LHS.Name, RHS.Name) &&
LHS.Flags == RHS.Flags && LHS.Alignment == RHS.Alignment;
}
uint64_t elf::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(std::function<int(InputSectionBase *S)> Order) {
assert(Commands.size() == 1);
sortByOrder(cast<InputSectionDescription>(Commands[0])->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, uint32_t Filler) {
size_t I = 0;
for (; I + 4 < Size; I += 4)
memcpy(Buf + I, &Filler, 4);
memcpy(Buf + I, &Filler, Size - I);
}
// Compress section contents if this section contains debug info.
template <class ELFT> void OutputSection::maybeCompress() {
typedef typename ELFT::Chdr Elf_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());
if (Error E = zlib::compress(toStringRef(Buf), CompressedData))
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, Config->Endianness);
else if (Size == 4)
write32(Buf, Data, Config->Endianness);
else if (Size == 8)
write64(Buf, Data, Config->Endianness);
else
llvm_unreachable("unsupported Size argument");
}
template <class ELFT> void OutputSection::writeTo(uint8_t *Buf) {
if (Type == SHT_NOBITS)
return;
Loc = Buf;
// If -compress-debug-section is specified and if this is a debug seciton,
// 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;
for (BaseCommand *Cmd : Commands)
if (auto *ISD = dyn_cast<InputSectionDescription>(Cmd))
for (InputSection *IS : ISD->Sections)
if (IS->Live)
Sections.push_back(IS);
uint32_t Filler = getFiller();
if (Filler)
fill(Buf, Sections.empty() ? Size : Sections[0]->OutSecOff, Filler);
parallelForEachN(0, Sections.size(), [=](size_t I) {
InputSection *IS = Sections[I];
IS->writeTo<ELFT>(Buf);
// Fill gaps between sections.
if (Filler) {
uint8_t *Start = Buf + IS->OutSecOff + IS->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 : Commands)
if (auto *Data = dyn_cast<BytesDataCommand>(Base))
writeInt(Buf + Data->Offset, Data->Expression().getValue(), Data->Size);
}
static bool compareByFilePosition(InputSection *A, InputSection *B) {
// Synthetic doesn't have link order dependecy, stable_sort will keep it last
if (A->kind() == InputSectionBase::Synthetic ||
B->kind() == InputSectionBase::Synthetic)
return false;
InputSection *LA = A->getLinkOrderDep();
InputSection *LB = B->getLinkOrderDep();
OutputSection *AOut = LA->getParent();
OutputSection *BOut = LB->getParent();
if (AOut != BOut)
return AOut->SectionIndex < BOut->SectionIndex;
return LA->OutSecOff < LB->OutSecOff;
}
template <class ELFT>
static void finalizeShtGroup(OutputSection *OS,
ArrayRef<InputSection *> Sections) {
assert(Config->Relocatable && Sections.size() == 1);
// sh_link field for SHT_GROUP sections should contain the section index of
// the symbol table.
OS->Link = InX::SymTab->getParent()->SectionIndex;
// sh_info then contain index of an entry in symbol table section which
// provides signature of the section group.
ObjFile<ELFT> *Obj = Sections[0]->getFile<ELFT>();
ArrayRef<SymbolBody *> Symbols = Obj->getSymbols();
OS->Info = InX::SymTab->getSymbolIndex(Symbols[Sections[0]->Info]);
}
template <class ELFT> void OutputSection::finalize() {
// Link order may be distributed across several InputSectionDescriptions
// but sort must consider them all at once.
std::vector<InputSection **> ScriptSections;
std::vector<InputSection *> Sections;
for (BaseCommand *Base : Commands)
if (auto *ISD = dyn_cast<InputSectionDescription>(Base))
for (InputSection *&IS : ISD->Sections) {
ScriptSections.push_back(&IS);
Sections.push_back(IS);
}
if (Flags & SHF_LINK_ORDER) {
std::stable_sort(Sections.begin(), Sections.end(), compareByFilePosition);
for (int I = 0, N = Sections.size(); I < N; ++I)
*ScriptSections[I] = Sections[I];
// 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 *D = Sections.front()->getLinkOrderDep())
Link = D->getParent()->SectionIndex;
}
if (Type == SHT_GROUP) {
finalizeShtGroup<ELFT>(this, Sections);
return;
}
if (!Config->CopyRelocs || (Type != SHT_RELA && Type != SHT_REL))
return;
InputSection *First = Sections[0];
if (isa<SyntheticSection>(First))
return;
Link = InX::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 matches /Filename.?\.o$/.
static bool isCrtBeginEnd(StringRef S, StringRef Filename) {
if (!S.endswith(".o"))
return false;
S = S.drop_back(2);
if (S.endswith(Filename))
return true;
return !S.empty() && S.drop_back().endswith(Filename);
}
static bool isCrtbegin(StringRef S) { return isCrtBeginEnd(S, "crtbegin"); }
static bool isCrtend(StringRef S) { return isCrtBeginEnd(S, "crtend"); }
// .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);
if (X.empty() && Y.empty())
return false;
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(Commands.size() == 1);
auto *ISD = cast<InputSectionDescription>(Commands[0]);
std::stable_sort(ISD->Sections.begin(), ISD->Sections.end(), 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 elf::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;
}
// 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); });
}
uint32_t OutputSection::getFiller() {
if (Filler)
return *Filler;
if (Flags & SHF_EXECINSTR)
return Target->TrapInstr;
return 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>();
template void OutputSection::finalize<ELF32LE>();
template void OutputSection::finalize<ELF32BE>();
template void OutputSection::finalize<ELF64LE>();
template void OutputSection::finalize<ELF64BE>();