//===- LinkerScript.cpp ---------------------------------------------------===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains the parser/evaluator of the linker script.
//
//===----------------------------------------------------------------------===//
#include "LinkerScript.h"
#include "Config.h"
#include "InputSection.h"
#include "OutputSections.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "Writer.h"
#include "lld/Common/Memory.h"
#include "lld/Common/Strings.h"
#include "lld/Common/Threads.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/Path.h"
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <limits>
#include <string>
#include <vector>
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace llvm::support::endian;
using namespace lld;
using namespace lld::elf;
LinkerScript *elf::Script;
static uint64_t getOutputSectionVA(SectionBase *InputSec, StringRef Loc) {
if (OutputSection *OS = InputSec->getOutputSection())
return OS->Addr;
error(Loc + ": unable to evaluate expression: input section " +
InputSec->Name + " has no output section assigned");
return 0;
}
uint64_t ExprValue::getValue() const {
if (Sec)
return alignTo(Sec->getOffset(Val) + getOutputSectionVA(Sec, Loc),
Alignment);
return alignTo(Val, Alignment);
}
uint64_t ExprValue::getSecAddr() const {
if (Sec)
return Sec->getOffset(0) + getOutputSectionVA(Sec, Loc);
return 0;
}
uint64_t ExprValue::getSectionOffset() const {
// If the alignment is trivial, we don't have to compute the full
// value to know the offset. This allows this function to succeed in
// cases where the output section is not yet known.
if (Alignment == 1 && (!Sec || !Sec->getOutputSection()))
return Val;
return getValue() - getSecAddr();
}
OutputSection *LinkerScript::createOutputSection(StringRef Name,
StringRef Location) {
OutputSection *&SecRef = NameToOutputSection[Name];
OutputSection *Sec;
if (SecRef && SecRef->Location.empty()) {
// There was a forward reference.
Sec = SecRef;
} else {
Sec = make<OutputSection>(Name, SHT_NOBITS, 0);
if (!SecRef)
SecRef = Sec;
}
Sec->Location = Location;
return Sec;
}
OutputSection *LinkerScript::getOrCreateOutputSection(StringRef Name) {
OutputSection *&CmdRef = NameToOutputSection[Name];
if (!CmdRef)
CmdRef = make<OutputSection>(Name, SHT_PROGBITS, 0);
return CmdRef;
}
// Expands the memory region by the specified size.
static void expandMemoryRegion(MemoryRegion *MemRegion, uint64_t Size,
StringRef RegionName, StringRef SecName) {
MemRegion->CurPos += Size;
uint64_t NewSize = MemRegion->CurPos - MemRegion->Origin;
if (NewSize > MemRegion->Length)
error("section '" + SecName + "' will not fit in region '" + RegionName +
"': overflowed by " + Twine(NewSize - MemRegion->Length) + " bytes");
}
void LinkerScript::expandMemoryRegions(uint64_t Size) {
if (Ctx->MemRegion)
expandMemoryRegion(Ctx->MemRegion, Size, Ctx->MemRegion->Name,
Ctx->OutSec->Name);
if (Ctx->LMARegion)
expandMemoryRegion(Ctx->LMARegion, Size, Ctx->LMARegion->Name,
Ctx->OutSec->Name);
}
void LinkerScript::expandOutputSection(uint64_t Size) {
Ctx->OutSec->Size += Size;
expandMemoryRegions(Size);
}
void LinkerScript::setDot(Expr E, const Twine &Loc, bool InSec) {
uint64_t Val = E().getValue();
if (Val < Dot && InSec)
error(Loc + ": unable to move location counter backward for: " +
Ctx->OutSec->Name);
// Update to location counter means update to section size.
if (InSec)
expandOutputSection(Val - Dot);
else
expandMemoryRegions(Val - Dot);
Dot = Val;
}
// Used for handling linker symbol assignments, for both finalizing
// their values and doing early declarations. Returns true if symbol
// should be defined from linker script.
static bool shouldDefineSym(SymbolAssignment *Cmd) {
if (Cmd->Name == ".")
return false;
if (!Cmd->Provide)
return true;
// If a symbol was in PROVIDE(), we need to define it only
// when it is a referenced undefined symbol.
Symbol *B = Symtab->find(Cmd->Name);
if (B && !B->isDefined())
return true;
return false;
}
// This function is called from processSectionCommands,
// while we are fixing the output section layout.
void LinkerScript::addSymbol(SymbolAssignment *Cmd) {
if (!shouldDefineSym(Cmd))
return;
// Define a symbol.
Symbol *Sym;
uint8_t Visibility = Cmd->Hidden ? STV_HIDDEN : STV_DEFAULT;
std::tie(Sym, std::ignore) = Symtab->insert(Cmd->Name, /*Type*/ 0, Visibility,
/*CanOmitFromDynSym*/ false,
/*File*/ nullptr);
ExprValue Value = Cmd->Expression();
SectionBase *Sec = Value.isAbsolute() ? nullptr : Value.Sec;
// When this function is called, section addresses have not been
// fixed yet. So, we may or may not know the value of the RHS
// expression.
//
// For example, if an expression is `x = 42`, we know x is always 42.
// However, if an expression is `x = .`, there's no way to know its
// value at the moment.
//
// We want to set symbol values early if we can. This allows us to
// use symbols as variables in linker scripts. Doing so allows us to
// write expressions like this: `alignment = 16; . = ALIGN(., alignment)`.
uint64_t SymValue = Value.Sec ? 0 : Value.getValue();
replaceSymbol<Defined>(Sym, nullptr, Cmd->Name, STB_GLOBAL, Visibility,
STT_NOTYPE, SymValue, 0, Sec);
Cmd->Sym = cast<Defined>(Sym);
}
// This function is called from LinkerScript::declareSymbols.
// It creates a placeholder symbol if needed.
static void declareSymbol(SymbolAssignment *Cmd) {
if (!shouldDefineSym(Cmd))
return;
// We can't calculate final value right now.
Symbol *Sym;
uint8_t Visibility = Cmd->Hidden ? STV_HIDDEN : STV_DEFAULT;
std::tie(Sym, std::ignore) = Symtab->insert(Cmd->Name, /*Type*/ 0, Visibility,
/*CanOmitFromDynSym*/ false,
/*File*/ nullptr);
replaceSymbol<Defined>(Sym, nullptr, Cmd->Name, STB_GLOBAL, Visibility,
STT_NOTYPE, 0, 0, nullptr);
Cmd->Sym = cast<Defined>(Sym);
Cmd->Provide = false;
}
// This method is used to handle INSERT AFTER statement. Here we rebuild
// the list of script commands to mix sections inserted into.
void LinkerScript::processInsertCommands() {
std::vector<BaseCommand *> V;
auto Insert = [&](std::vector<BaseCommand *> &From) {
V.insert(V.end(), From.begin(), From.end());
From.clear();
};
for (BaseCommand *Base : SectionCommands) {
if (auto *OS = dyn_cast<OutputSection>(Base)) {
Insert(InsertBeforeCommands[OS->Name]);
V.push_back(Base);
Insert(InsertAfterCommands[OS->Name]);
continue;
}
V.push_back(Base);
}
for (auto &Cmds : {InsertBeforeCommands, InsertAfterCommands})
for (const std::pair<StringRef, std::vector<BaseCommand *>> &P : Cmds)
if (!P.second.empty())
error("unable to INSERT AFTER/BEFORE " + P.first +
": section not defined");
SectionCommands = std::move(V);
}
// Symbols defined in script should not be inlined by LTO. At the same time
// we don't know their final values until late stages of link. Here we scan
// over symbol assignment commands and create placeholder symbols if needed.
void LinkerScript::declareSymbols() {
assert(!Ctx);
for (BaseCommand *Base : SectionCommands) {
if (auto *Cmd = dyn_cast<SymbolAssignment>(Base)) {
declareSymbol(Cmd);
continue;
}
// If the output section directive has constraints,
// we can't say for sure if it is going to be included or not.
// Skip such sections for now. Improve the checks if we ever
// need symbols from that sections to be declared early.
auto *Sec = cast<OutputSection>(Base);
if (Sec->Constraint != ConstraintKind::NoConstraint)
continue;
for (BaseCommand *Base2 : Sec->SectionCommands)
if (auto *Cmd = dyn_cast<SymbolAssignment>(Base2))
declareSymbol(Cmd);
}
}
// This function is called from assignAddresses, while we are
// fixing the output section addresses. This function is supposed
// to set the final value for a given symbol assignment.
void LinkerScript::assignSymbol(SymbolAssignment *Cmd, bool InSec) {
if (Cmd->Name == ".") {
setDot(Cmd->Expression, Cmd->Location, InSec);
return;
}
if (!Cmd->Sym)
return;
ExprValue V = Cmd->Expression();
if (V.isAbsolute()) {
Cmd->Sym->Section = nullptr;
Cmd->Sym->Value = V.getValue();
} else {
Cmd->Sym->Section = V.Sec;
Cmd->Sym->Value = V.getSectionOffset();
}
}
static std::string getFilename(InputFile *File) {
if (!File)
return "";
if (File->ArchiveName.empty())
return File->getName();
return (File->ArchiveName + "(" + File->getName() + ")").str();
}
bool LinkerScript::shouldKeep(InputSectionBase *S) {
if (KeptSections.empty())
return false;
std::string Filename = getFilename(S->File);
for (InputSectionDescription *ID : KeptSections)
if (ID->FilePat.match(Filename))
for (SectionPattern &P : ID->SectionPatterns)
if (P.SectionPat.match(S->Name))
return true;
return false;
}
// A helper function for the SORT() command.
static std::function<bool(InputSectionBase *, InputSectionBase *)>
getComparator(SortSectionPolicy K) {
switch (K) {
case SortSectionPolicy::Alignment:
return [](InputSectionBase *A, InputSectionBase *B) {
// ">" is not a mistake. Sections with larger alignments are placed
// before sections with smaller alignments in order to reduce the
// amount of padding necessary. This is compatible with GNU.
return A->Alignment > B->Alignment;
};
case SortSectionPolicy::Name:
return [](InputSectionBase *A, InputSectionBase *B) {
return A->Name < B->Name;
};
case SortSectionPolicy::Priority:
return [](InputSectionBase *A, InputSectionBase *B) {
return getPriority(A->Name) < getPriority(B->Name);
};
default:
llvm_unreachable("unknown sort policy");
}
}
// A helper function for the SORT() command.
static bool matchConstraints(ArrayRef<InputSection *> Sections,
ConstraintKind Kind) {
if (Kind == ConstraintKind::NoConstraint)
return true;
bool IsRW = llvm::any_of(
Sections, [](InputSection *Sec) { return Sec->Flags & SHF_WRITE; });
return (IsRW && Kind == ConstraintKind::ReadWrite) ||
(!IsRW && Kind == ConstraintKind::ReadOnly);
}
static void sortSections(MutableArrayRef<InputSection *> Vec,
SortSectionPolicy K) {
if (K != SortSectionPolicy::Default && K != SortSectionPolicy::None)
std::stable_sort(Vec.begin(), Vec.end(), getComparator(K));
}
// Sort sections as instructed by SORT-family commands and --sort-section
// option. Because SORT-family commands can be nested at most two depth
// (e.g. SORT_BY_NAME(SORT_BY_ALIGNMENT(.text.*))) and because the command
// line option is respected even if a SORT command is given, the exact
// behavior we have here is a bit complicated. Here are the rules.
//
// 1. If two SORT commands are given, --sort-section is ignored.
// 2. If one SORT command is given, and if it is not SORT_NONE,
// --sort-section is handled as an inner SORT command.
// 3. If one SORT command is given, and if it is SORT_NONE, don't sort.
// 4. If no SORT command is given, sort according to --sort-section.
static void sortInputSections(MutableArrayRef<InputSection *> Vec,
const SectionPattern &Pat) {
if (Pat.SortOuter == SortSectionPolicy::None)
return;
if (Pat.SortInner == SortSectionPolicy::Default)
sortSections(Vec, Config->SortSection);
else
sortSections(Vec, Pat.SortInner);
sortSections(Vec, Pat.SortOuter);
}
// Compute and remember which sections the InputSectionDescription matches.
std::vector<InputSection *>
LinkerScript::computeInputSections(const InputSectionDescription *Cmd) {
std::vector<InputSection *> Ret;
// Collects all sections that satisfy constraints of Cmd.
for (const SectionPattern &Pat : Cmd->SectionPatterns) {
size_t SizeBefore = Ret.size();
for (InputSectionBase *Sec : InputSections) {
if (!Sec->Live || Sec->Assigned)
continue;
// For -emit-relocs we have to ignore entries like
// .rela.dyn : { *(.rela.data) }
// which are common because they are in the default bfd script.
// We do not ignore SHT_REL[A] linker-synthesized sections here because
// want to support scripts that do custom layout for them.
if (auto *IS = dyn_cast<InputSection>(Sec))
if (IS->getRelocatedSection())
continue;
std::string Filename = getFilename(Sec->File);
if (!Cmd->FilePat.match(Filename) ||
Pat.ExcludedFilePat.match(Filename) ||
!Pat.SectionPat.match(Sec->Name))
continue;
// It is safe to assume that Sec is an InputSection
// because mergeable or EH input sections have already been
// handled and eliminated.
Ret.push_back(cast<InputSection>(Sec));
Sec->Assigned = true;
}
sortInputSections(MutableArrayRef<InputSection *>(Ret).slice(SizeBefore),
Pat);
}
return Ret;
}
void LinkerScript::discard(ArrayRef<InputSection *> V) {
for (InputSection *S : V) {
if (S == InX::ShStrTab || S == InX::Dynamic || S == InX::DynSymTab ||
S == InX::DynStrTab || S == InX::RelaPlt || S == InX::RelaDyn ||
S == InX::RelrDyn)
error("discarding " + S->Name + " section is not allowed");
// You can discard .hash and .gnu.hash sections by linker scripts. Since
// they are synthesized sections, we need to handle them differently than
// other regular sections.
if (S == InX::GnuHashTab)
InX::GnuHashTab = nullptr;
if (S == InX::HashTab)
InX::HashTab = nullptr;
S->Assigned = false;
S->Live = false;
discard(S->DependentSections);
}
}
std::vector<InputSection *>
LinkerScript::createInputSectionList(OutputSection &OutCmd) {
std::vector<InputSection *> Ret;
for (BaseCommand *Base : OutCmd.SectionCommands) {
if (auto *Cmd = dyn_cast<InputSectionDescription>(Base)) {
Cmd->Sections = computeInputSections(Cmd);
Ret.insert(Ret.end(), Cmd->Sections.begin(), Cmd->Sections.end());
}
}
return Ret;
}
void LinkerScript::processSectionCommands() {
// A symbol can be assigned before any section is mentioned in the linker
// script. In an DSO, the symbol values are addresses, so the only important
// section values are:
// * SHN_UNDEF
// * SHN_ABS
// * Any value meaning a regular section.
// To handle that, create a dummy aether section that fills the void before
// the linker scripts switches to another section. It has an index of one
// which will map to whatever the first actual section is.
Aether = make<OutputSection>("", 0, SHF_ALLOC);
Aether->SectionIndex = 1;
// Ctx captures the local AddressState and makes it accessible deliberately.
// This is needed as there are some cases where we cannot just
// thread the current state through to a lambda function created by the
// script parser.
auto Deleter = make_unique<AddressState>();
Ctx = Deleter.get();
Ctx->OutSec = Aether;
size_t I = 0;
// Add input sections to output sections.
for (BaseCommand *Base : SectionCommands) {
// Handle symbol assignments outside of any output section.
if (auto *Cmd = dyn_cast<SymbolAssignment>(Base)) {
addSymbol(Cmd);
continue;
}
if (auto *Sec = dyn_cast<OutputSection>(Base)) {
std::vector<InputSection *> V = createInputSectionList(*Sec);
// The output section name `/DISCARD/' is special.
// Any input section assigned to it is discarded.
if (Sec->Name == "/DISCARD/") {
discard(V);
Sec->SectionCommands.clear();
continue;
}
// This is for ONLY_IF_RO and ONLY_IF_RW. An output section directive
// ".foo : ONLY_IF_R[OW] { ... }" is handled only if all member input
// sections satisfy a given constraint. If not, a directive is handled
// as if it wasn't present from the beginning.
//
// Because we'll iterate over SectionCommands many more times, the easy
// way to "make it as if it wasn't present" is to make it empty.
if (!matchConstraints(V, Sec->Constraint)) {
for (InputSectionBase *S : V)
S->Assigned = false;
Sec->SectionCommands.clear();
continue;
}
// A directive may contain symbol definitions like this:
// ".foo : { ...; bar = .; }". Handle them.
for (BaseCommand *Base : Sec->SectionCommands)
if (auto *OutCmd = dyn_cast<SymbolAssignment>(Base))
addSymbol(OutCmd);
// Handle subalign (e.g. ".foo : SUBALIGN(32) { ... }"). If subalign
// is given, input sections are aligned to that value, whether the
// given value is larger or smaller than the original section alignment.
if (Sec->SubalignExpr) {
uint32_t Subalign = Sec->SubalignExpr().getValue();
for (InputSectionBase *S : V)
S->Alignment = Subalign;
}
// Add input sections to an output section.
for (InputSection *S : V)
Sec->addSection(S);
Sec->SectionIndex = I++;
if (Sec->Noload)
Sec->Type = SHT_NOBITS;
if (Sec->NonAlloc)
Sec->Flags &= ~(uint64_t)SHF_ALLOC;
}
}
Ctx = nullptr;
}
static OutputSection *findByName(ArrayRef<BaseCommand *> Vec,
StringRef Name) {
for (BaseCommand *Base : Vec)
if (auto *Sec = dyn_cast<OutputSection>(Base))
if (Sec->Name == Name)
return Sec;
return nullptr;
}
static OutputSection *createSection(InputSectionBase *IS,
StringRef OutsecName) {
OutputSection *Sec = Script->createOutputSection(OutsecName, "<internal>");
Sec->addSection(cast<InputSection>(IS));
return Sec;
}
static OutputSection *addInputSec(StringMap<OutputSection *> &Map,
InputSectionBase *IS, StringRef OutsecName) {
// 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))
return createSection(IS, OutsecName);
// 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();
if (Out->RelocationSection) {
Out->RelocationSection->addSection(Sec);
return nullptr;
}
Out->RelocationSection = createSection(IS, OutsecName);
return Out->RelocationSection;
}
// When control reaches here, mergeable sections have already been merged into
// synthetic sections. For relocatable case we want to create one output
// section per syntetic section so that they have a valid sh_entsize.
if (Config->Relocatable && (IS->Flags & SHF_MERGE))
return createSection(IS, 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 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.
OutputSection *&Sec = Map[OutsecName];
if (Sec) {
Sec->addSection(cast<InputSection>(IS));
return nullptr;
}
Sec = createSection(IS, OutsecName);
return Sec;
}
// Add sections that didn't match any sections command.
void LinkerScript::addOrphanSections() {
unsigned End = SectionCommands.size();
StringMap<OutputSection *> Map;
std::vector<OutputSection *> V;
auto Add = [&](InputSectionBase *S) {
if (!S->Live || S->Parent)
return;
StringRef Name = getOutputSectionName(S);
if (Config->OrphanHandling == OrphanHandlingPolicy::Error)
error(toString(S) + " is being placed in '" + Name + "'");
else if (Config->OrphanHandling == OrphanHandlingPolicy::Warn)
warn(toString(S) + " is being placed in '" + Name + "'");
if (OutputSection *Sec =
findByName(makeArrayRef(SectionCommands).slice(0, End), Name)) {
Sec->addSection(cast<InputSection>(S));
return;
}
if (OutputSection *OS = addInputSec(Map, S, Name))
V.push_back(OS);
assert(S->getOutputSection()->SectionIndex == UINT32_MAX);
};
// For futher --emit-reloc handling code we need target output section
// to be created before we create relocation output section, so we want
// to create target sections first. We do not want priority handling
// for synthetic sections because them are special.
for (InputSectionBase *IS : InputSections) {
if (auto *Sec = dyn_cast<InputSection>(IS))
if (InputSectionBase *Rel = Sec->getRelocatedSection())
if (auto *RelIS = dyn_cast_or_null<InputSectionBase>(Rel->Parent))
Add(RelIS);
Add(IS);
}
// If no SECTIONS command was given, we should insert sections commands
// before others, so that we can handle scripts which refers them,
// for example: "foo = ABSOLUTE(ADDR(.text)));".
// When SECTIONS command is present we just add all orphans to the end.
if (HasSectionsCommand)
SectionCommands.insert(SectionCommands.end(), V.begin(), V.end());
else
SectionCommands.insert(SectionCommands.begin(), V.begin(), V.end());
}
uint64_t LinkerScript::advance(uint64_t Size, unsigned Alignment) {
bool IsTbss =
(Ctx->OutSec->Flags & SHF_TLS) && Ctx->OutSec->Type == SHT_NOBITS;
uint64_t Start = IsTbss ? Dot + Ctx->ThreadBssOffset : Dot;
Start = alignTo(Start, Alignment);
uint64_t End = Start + Size;
if (IsTbss)
Ctx->ThreadBssOffset = End - Dot;
else
Dot = End;
return End;
}
void LinkerScript::output(InputSection *S) {
uint64_t Before = advance(0, 1);
uint64_t Pos = advance(S->getSize(), S->Alignment);
S->OutSecOff = Pos - S->getSize() - Ctx->OutSec->Addr;
// Update output section size after adding each section. This is so that
// SIZEOF works correctly in the case below:
// .foo { *(.aaa) a = SIZEOF(.foo); *(.bbb) }
expandOutputSection(Pos - Before);
}
void LinkerScript::switchTo(OutputSection *Sec) {
Ctx->OutSec = Sec;
uint64_t Before = advance(0, 1);
Ctx->OutSec->Addr = advance(0, Ctx->OutSec->Alignment);
expandMemoryRegions(Ctx->OutSec->Addr - Before);
}
// This function searches for a memory region to place the given output
// section in. If found, a pointer to the appropriate memory region is
// returned. Otherwise, a nullptr is returned.
MemoryRegion *LinkerScript::findMemoryRegion(OutputSection *Sec) {
// If a memory region name was specified in the output section command,
// then try to find that region first.
if (!Sec->MemoryRegionName.empty()) {
if (MemoryRegion *M = MemoryRegions.lookup(Sec->MemoryRegionName))
return M;
error("memory region '" + Sec->MemoryRegionName + "' not declared");
return nullptr;
}
// If at least one memory region is defined, all sections must
// belong to some memory region. Otherwise, we don't need to do
// anything for memory regions.
if (MemoryRegions.empty())
return nullptr;
// See if a region can be found by matching section flags.
for (auto &Pair : MemoryRegions) {
MemoryRegion *M = Pair.second;
if ((M->Flags & Sec->Flags) && (M->NegFlags & Sec->Flags) == 0)
return M;
}
// Otherwise, no suitable region was found.
if (Sec->Flags & SHF_ALLOC)
error("no memory region specified for section '" + Sec->Name + "'");
return nullptr;
}
// This function assigns offsets to input sections and an output section
// for a single sections command (e.g. ".text { *(.text); }").
void LinkerScript::assignOffsets(OutputSection *Sec) {
if (!(Sec->Flags & SHF_ALLOC))
Dot = 0;
else if (Sec->AddrExpr)
setDot(Sec->AddrExpr, Sec->Location, false);
Ctx->MemRegion = Sec->MemRegion;
Ctx->LMARegion = Sec->LMARegion;
if (Ctx->MemRegion)
Dot = Ctx->MemRegion->CurPos;
switchTo(Sec);
if (Sec->LMAExpr)
Ctx->LMAOffset = Sec->LMAExpr().getValue() - Dot;
if (MemoryRegion *MR = Sec->LMARegion)
Ctx->LMAOffset = MR->CurPos - Dot;
// If neither AT nor AT> is specified for an allocatable section, the linker
// will set the LMA such that the difference between VMA and LMA for the
// section is the same as the preceding output section in the same region
// https://sourceware.org/binutils/docs-2.20/ld/Output-Section-LMA.html
if (PhdrEntry *L = Ctx->OutSec->PtLoad)
L->LMAOffset = Ctx->LMAOffset;
// We can call this method multiple times during the creation of
// thunks and want to start over calculation each time.
Sec->Size = 0;
// We visited SectionsCommands from processSectionCommands to
// layout sections. Now, we visit SectionsCommands again to fix
// section offsets.
for (BaseCommand *Base : Sec->SectionCommands) {
// This handles the assignments to symbol or to the dot.
if (auto *Cmd = dyn_cast<SymbolAssignment>(Base)) {
Cmd->Addr = Dot;
assignSymbol(Cmd, true);
Cmd->Size = Dot - Cmd->Addr;
continue;
}
// Handle BYTE(), SHORT(), LONG(), or QUAD().
if (auto *Cmd = dyn_cast<ByteCommand>(Base)) {
Cmd->Offset = Dot - Ctx->OutSec->Addr;
Dot += Cmd->Size;
expandOutputSection(Cmd->Size);
continue;
}
// Handle a single input section description command.
// It calculates and assigns the offsets for each section and also
// updates the output section size.
auto *Cmd = cast<InputSectionDescription>(Base);
for (InputSection *Sec : Cmd->Sections) {
// We tentatively added all synthetic sections at the beginning and
// removed empty ones afterwards (because there is no way to know
// whether they were going be empty or not other than actually running
// linker scripts.) We need to ignore remains of empty sections.
if (auto *S = dyn_cast<SyntheticSection>(Sec))
if (S->empty())
continue;
if (!Sec->Live)
continue;
assert(Ctx->OutSec == Sec->getParent());
output(Sec);
}
}
}
static bool isDiscardable(OutputSection &Sec) {
// We do not remove empty sections that are explicitly
// assigned to any segment.
if (!Sec.Phdrs.empty())
return false;
// We do not want to remove sections that reference symbols in address and
// other expressions. We add script symbols as undefined, and want to ensure
// all of them are defined in the output, hence have to keep them.
if (Sec.ExpressionsUseSymbols)
return false;
for (BaseCommand *Base : Sec.SectionCommands) {
if (auto Cmd = dyn_cast<SymbolAssignment>(Base))
// Don't create empty output sections just for unreferenced PROVIDE
// symbols.
if (Cmd->Name != "." && !Cmd->Sym)
continue;
if (!isa<InputSectionDescription>(*Base))
return false;
}
return true;
}
void LinkerScript::adjustSectionsBeforeSorting() {
// If the output section contains only symbol assignments, create a
// corresponding output section. The issue is what to do with linker script
// like ".foo : { symbol = 42; }". One option would be to convert it to
// "symbol = 42;". That is, move the symbol out of the empty section
// description. That seems to be what bfd does for this simple case. The
// problem is that this is not completely general. bfd will give up and
// create a dummy section too if there is a ". = . + 1" inside the section
// for example.
// Given that we want to create the section, we have to worry what impact
// it will have on the link. For example, if we just create a section with
// 0 for flags, it would change which PT_LOADs are created.
// We could remember that particular section is dummy and ignore it in
// other parts of the linker, but unfortunately there are quite a few places
// that would need to change:
// * The program header creation.
// * The orphan section placement.
// * The address assignment.
// The other option is to pick flags that minimize the impact the section
// will have on the rest of the linker. That is why we copy the flags from
// the previous sections. Only a few flags are needed to keep the impact low.
uint64_t Flags = SHF_ALLOC;
for (BaseCommand *&Cmd : SectionCommands) {
auto *Sec = dyn_cast<OutputSection>(Cmd);
if (!Sec)
continue;
// Handle align (e.g. ".foo : ALIGN(16) { ... }").
if (Sec->AlignExpr)
Sec->Alignment =
std::max<uint32_t>(Sec->Alignment, Sec->AlignExpr().getValue());
// A live output section means that some input section was added to it. It
// might have been removed (if it was empty synthetic section), but we at
// least know the flags.
if (Sec->Live)
Flags = Sec->Flags;
// We do not want to keep any special flags for output section
// in case it is empty.
bool IsEmpty = getInputSections(Sec).empty();
if (IsEmpty)
Sec->Flags = Flags & (SHF_ALLOC | SHF_WRITE | SHF_EXECINSTR);
if (IsEmpty && isDiscardable(*Sec)) {
Sec->Live = false;
Cmd = nullptr;
}
}
// It is common practice to use very generic linker scripts. So for any
// given run some of the output sections in the script will be empty.
// We could create corresponding empty output sections, but that would
// clutter the output.
// We instead remove trivially empty sections. The bfd linker seems even
// more aggressive at removing them.
llvm::erase_if(SectionCommands, [&](BaseCommand *Base) { return !Base; });
}
void LinkerScript::adjustSectionsAfterSorting() {
// Try and find an appropriate memory region to assign offsets in.
for (BaseCommand *Base : SectionCommands) {
if (auto *Sec = dyn_cast<OutputSection>(Base)) {
if (!Sec->LMARegionName.empty()) {
if (MemoryRegion *M = MemoryRegions.lookup(Sec->LMARegionName))
Sec->LMARegion = M;
else
error("memory region '" + Sec->LMARegionName + "' not declared");
}
Sec->MemRegion = findMemoryRegion(Sec);
}
}
// If output section command doesn't specify any segments,
// and we haven't previously assigned any section to segment,
// then we simply assign section to the very first load segment.
// Below is an example of such linker script:
// PHDRS { seg PT_LOAD; }
// SECTIONS { .aaa : { *(.aaa) } }
std::vector<StringRef> DefPhdrs;
auto FirstPtLoad = llvm::find_if(PhdrsCommands, [](const PhdrsCommand &Cmd) {
return Cmd.Type == PT_LOAD;
});
if (FirstPtLoad != PhdrsCommands.end())
DefPhdrs.push_back(FirstPtLoad->Name);
// Walk the commands and propagate the program headers to commands that don't
// explicitly specify them.
for (BaseCommand *Base : SectionCommands) {
auto *Sec = dyn_cast<OutputSection>(Base);
if (!Sec)
continue;
if (Sec->Phdrs.empty()) {
// To match the bfd linker script behaviour, only propagate program
// headers to sections that are allocated.
if (Sec->Flags & SHF_ALLOC)
Sec->Phdrs = DefPhdrs;
} else {
DefPhdrs = Sec->Phdrs;
}
}
}
static OutputSection *findFirstSection(PhdrEntry *Load) {
for (OutputSection *Sec : OutputSections)
if (Sec->PtLoad == Load)
return Sec;
return nullptr;
}
static uint64_t computeBase(uint64_t Min, bool AllocateHeaders) {
// If there is no SECTIONS or if the linkerscript is explicit about program
// headers, do our best to allocate them.
if (!Script->HasSectionsCommand || AllocateHeaders)
return 0;
// Otherwise only allocate program headers if that would not add a page.
return alignDown(Min, Config->MaxPageSize);
}
// Try to find an address for the file and program headers output sections,
// which were unconditionally added to the first PT_LOAD segment earlier.
//
// When using the default layout, we check if the headers fit below the first
// allocated section. When using a linker script, we also check if the headers
// are covered by the output section. This allows omitting the headers by not
// leaving enough space for them in the linker script; this pattern is common
// in embedded systems.
//
// If there isn't enough space for these sections, we'll remove them from the
// PT_LOAD segment, and we'll also remove the PT_PHDR segment.
void LinkerScript::allocateHeaders(std::vector<PhdrEntry *> &Phdrs) {
uint64_t Min = std::numeric_limits<uint64_t>::max();
for (OutputSection *Sec : OutputSections)
if (Sec->Flags & SHF_ALLOC)
Min = std::min<uint64_t>(Min, Sec->Addr);
auto It = llvm::find_if(
Phdrs, [](const PhdrEntry *E) { return E->p_type == PT_LOAD; });
if (It == Phdrs.end())
return;
PhdrEntry *FirstPTLoad = *It;
bool HasExplicitHeaders =
llvm::any_of(PhdrsCommands, [](const PhdrsCommand &Cmd) {
return Cmd.HasPhdrs || Cmd.HasFilehdr;
});
uint64_t HeaderSize = getHeaderSize();
if (HeaderSize <= Min - computeBase(Min, HasExplicitHeaders)) {
Min = alignDown(Min - HeaderSize, Config->MaxPageSize);
Out::ElfHeader->Addr = Min;
Out::ProgramHeaders->Addr = Min + Out::ElfHeader->Size;
return;
}
// Error if we were explicitly asked to allocate headers.
if (HasExplicitHeaders)
error("could not allocate headers");
Out::ElfHeader->PtLoad = nullptr;
Out::ProgramHeaders->PtLoad = nullptr;
FirstPTLoad->FirstSec = findFirstSection(FirstPTLoad);
llvm::erase_if(Phdrs,
[](const PhdrEntry *E) { return E->p_type == PT_PHDR; });
}
LinkerScript::AddressState::AddressState() {
for (auto &MRI : Script->MemoryRegions) {
MemoryRegion *MR = MRI.second;
MR->CurPos = MR->Origin;
}
}
static uint64_t getInitialDot() {
// By default linker scripts use an initial value of 0 for '.',
// but prefer -image-base if set.
if (Script->HasSectionsCommand)
return Config->ImageBase ? *Config->ImageBase : 0;
uint64_t StartAddr = UINT64_MAX;
// The Sections with -T<section> have been sorted in order of ascending
// address. We must lower StartAddr if the lowest -T<section address> as
// calls to setDot() must be monotonically increasing.
for (auto &KV : Config->SectionStartMap)
StartAddr = std::min(StartAddr, KV.second);
return std::min(StartAddr, Target->getImageBase() + elf::getHeaderSize());
}
// Here we assign addresses as instructed by linker script SECTIONS
// sub-commands. Doing that allows us to use final VA values, so here
// we also handle rest commands like symbol assignments and ASSERTs.
void LinkerScript::assignAddresses() {
Dot = getInitialDot();
auto Deleter = make_unique<AddressState>();
Ctx = Deleter.get();
ErrorOnMissingSection = true;
switchTo(Aether);
for (BaseCommand *Base : SectionCommands) {
if (auto *Cmd = dyn_cast<SymbolAssignment>(Base)) {
Cmd->Addr = Dot;
assignSymbol(Cmd, false);
Cmd->Size = Dot - Cmd->Addr;
continue;
}
assignOffsets(cast<OutputSection>(Base));
}
Ctx = nullptr;
}
// Creates program headers as instructed by PHDRS linker script command.
std::vector<PhdrEntry *> LinkerScript::createPhdrs() {
std::vector<PhdrEntry *> Ret;
// Process PHDRS and FILEHDR keywords because they are not
// real output sections and cannot be added in the following loop.
for (const PhdrsCommand &Cmd : PhdrsCommands) {
PhdrEntry *Phdr = make<PhdrEntry>(Cmd.Type, Cmd.Flags ? *Cmd.Flags : PF_R);
if (Cmd.HasFilehdr)
Phdr->add(Out::ElfHeader);
if (Cmd.HasPhdrs)
Phdr->add(Out::ProgramHeaders);
if (Cmd.LMAExpr) {
Phdr->p_paddr = Cmd.LMAExpr().getValue();
Phdr->HasLMA = true;
}
Ret.push_back(Phdr);
}
// Add output sections to program headers.
for (OutputSection *Sec : OutputSections) {
// Assign headers specified by linker script
for (size_t Id : getPhdrIndices(Sec)) {
Ret[Id]->add(Sec);
if (!PhdrsCommands[Id].Flags.hasValue())
Ret[Id]->p_flags |= Sec->getPhdrFlags();
}
}
return Ret;
}
// Returns true if we should emit an .interp section.
//
// We usually do. But if PHDRS commands are given, and
// no PT_INTERP is there, there's no place to emit an
// .interp, so we don't do that in that case.
bool LinkerScript::needsInterpSection() {
if (PhdrsCommands.empty())
return true;
for (PhdrsCommand &Cmd : PhdrsCommands)
if (Cmd.Type == PT_INTERP)
return true;
return false;
}
ExprValue LinkerScript::getSymbolValue(StringRef Name, const Twine &Loc) {
if (Name == ".") {
if (Ctx)
return {Ctx->OutSec, false, Dot - Ctx->OutSec->Addr, Loc};
error(Loc + ": unable to get location counter value");
return 0;
}
if (Symbol *Sym = Symtab->find(Name)) {
if (auto *DS = dyn_cast<Defined>(Sym))
return {DS->Section, false, DS->Value, Loc};
if (isa<SharedSymbol>(Sym))
if (!ErrorOnMissingSection)
return {nullptr, false, 0, Loc};
}
error(Loc + ": symbol not found: " + Name);
return 0;
}
// Returns the index of the segment named Name.
static Optional<size_t> getPhdrIndex(ArrayRef<PhdrsCommand> Vec,
StringRef Name) {
for (size_t I = 0; I < Vec.size(); ++I)
if (Vec[I].Name == Name)
return I;
return None;
}
// Returns indices of ELF headers containing specific section. Each index is a
// zero based number of ELF header listed within PHDRS {} script block.
std::vector<size_t> LinkerScript::getPhdrIndices(OutputSection *Cmd) {
std::vector<size_t> Ret;
for (StringRef S : Cmd->Phdrs) {
if (Optional<size_t> Idx = getPhdrIndex(PhdrsCommands, S))
Ret.push_back(*Idx);
else if (S != "NONE")
error(Cmd->Location + ": section header '" + S +
"' is not listed in PHDRS");
}
return Ret;
}