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llvm-project/lld/ELF/LinkerScript.cpp

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//===- LinkerScript.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
//
//===----------------------------------------------------------------------===//
//
// 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 "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/Parallel.h"
#include "llvm/Support/Path.h"
#include "llvm/Support/TimeProfiler.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 *sec) {
OutputSection *os = sec->getOutputSection();
assert(os && "input section has no output section assigned");
return os ? os->addr : 0;
}
uint64_t ExprValue::getValue() const {
if (sec)
return alignTo(sec->getOffset(val) + getOutputSectionVA(sec),
alignment);
return alignTo(val, alignment);
}
uint64_t ExprValue::getSecAddr() const {
if (sec)
return sec->getOffset(0) + getOutputSectionVA(sec);
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)
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_PROGBITS, 0);
if (!secRef)
secRef = sec;
}
sec->location = std::string(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)().getValue();
uint64_t length = (memRegion->length)().getValue();
if (newSize > length)
error("section '" + secName + "' will not fit in region '" + regionName +
"': overflowed by " + Twine(newSize - length) + " bytes");
}
void LinkerScript::expandMemoryRegions(uint64_t size) {
if (ctx->memRegion)
expandMemoryRegion(ctx->memRegion, size, ctx->memRegion->name,
ctx->outSec->name);
// Only expand the LMARegion if it is different from memRegion.
if (ctx->lmaRegion && ctx->memRegion != 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);
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;
}
// Called by processSymbolAssignments() to assign definitions to
// linker-script-defined symbols.
void LinkerScript::addSymbol(SymbolAssignment *cmd) {
if (!shouldDefineSym(cmd))
return;
// Define a symbol.
ExprValue value = cmd->expression();
SectionBase *sec = value.isAbsolute() ? nullptr : value.sec;
uint8_t visibility = cmd->hidden ? STV_HIDDEN : STV_DEFAULT;
// 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();
Defined newSym(nullptr, cmd->name, STB_GLOBAL, visibility, value.type,
symValue, 0, sec);
Symbol *sym = symtab->insert(cmd->name);
sym->mergeProperties(newSym);
sym->replace(newSym);
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;
uint8_t visibility = cmd->hidden ? STV_HIDDEN : STV_DEFAULT;
Defined newSym(nullptr, cmd->name, STB_GLOBAL, visibility, STT_NOTYPE, 0, 0,
nullptr);
// We can't calculate final value right now.
Symbol *sym = symtab->insert(cmd->name);
sym->mergeProperties(newSym);
sym->replace(newSym);
cmd->sym = cast<Defined>(sym);
cmd->provide = false;
sym->scriptDefined = true;
}
using SymbolAssignmentMap =
DenseMap<const Defined *, std::pair<SectionBase *, uint64_t>>;
// Collect section/value pairs of linker-script-defined symbols. This is used to
// check whether symbol values converge.
static SymbolAssignmentMap
getSymbolAssignmentValues(const std::vector<BaseCommand *> &sectionCommands) {
SymbolAssignmentMap ret;
for (BaseCommand *base : sectionCommands) {
if (auto *cmd = dyn_cast<SymbolAssignment>(base)) {
if (cmd->sym) // sym is nullptr for dot.
ret.try_emplace(cmd->sym,
std::make_pair(cmd->sym->section, cmd->sym->value));
continue;
}
for (BaseCommand *sub_base : cast<OutputSection>(base)->sectionCommands)
if (auto *cmd = dyn_cast<SymbolAssignment>(sub_base))
if (cmd->sym)
ret.try_emplace(cmd->sym,
std::make_pair(cmd->sym->section, cmd->sym->value));
}
return ret;
}
// Returns the lexicographical smallest (for determinism) Defined whose
// section/value has changed.
static const Defined *
getChangedSymbolAssignment(const SymbolAssignmentMap &oldValues) {
const Defined *changed = nullptr;
for (auto &it : oldValues) {
const Defined *sym = it.first;
if (std::make_pair(sym->section, sym->value) != it.second &&
(!changed || sym->getName() < changed->getName()))
changed = sym;
}
return changed;
}
// Process INSERT [AFTER|BEFORE] commands. For each command, we move the
// specified output section to the designated place.
void LinkerScript::processInsertCommands() {
std::vector<OutputSection *> moves;
for (const InsertCommand &cmd : insertCommands) {
for (StringRef name : cmd.names) {
// If base is empty, it may have been discarded by
// adjustSectionsBeforeSorting(). We do not handle such output sections.
auto from = llvm::find_if(sectionCommands, [&](BaseCommand *base) {
return isa<OutputSection>(base) &&
cast<OutputSection>(base)->name == name;
});
if (from == sectionCommands.end())
continue;
moves.push_back(cast<OutputSection>(*from));
sectionCommands.erase(from);
}
auto insertPos = llvm::find_if(sectionCommands, [&cmd](BaseCommand *base) {
auto *to = dyn_cast<OutputSection>(base);
return to != nullptr && to->name == cmd.where;
});
if (insertPos == sectionCommands.end()) {
error("unable to insert " + cmd.names[0] +
(cmd.isAfter ? " after " : " before ") + cmd.where);
} else {
if (cmd.isAfter)
++insertPos;
sectionCommands.insert(insertPos, moves.begin(), moves.end());
}
moves.clear();
}
}
// 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();
}
cmd->sym->type = v.type;
}
static inline StringRef getFilename(const InputFile *file) {
return file ? file->getNameForScript() : StringRef();
}
bool InputSectionDescription::matchesFile(const InputFile *file) const {
if (filePat.isTrivialMatchAll())
return true;
if (!matchesFileCache || matchesFileCache->first != file)
matchesFileCache.emplace(file, filePat.match(getFilename(file)));
return matchesFileCache->second;
}
bool SectionPattern::excludesFile(const InputFile *file) const {
if (excludedFilePat.empty())
return false;
if (!excludesFileCache || excludesFileCache->first != file)
excludesFileCache.emplace(file, excludedFilePat.match(getFilename(file)));
return excludesFileCache->second;
}
bool LinkerScript::shouldKeep(InputSectionBase *s) {
for (InputSectionDescription *id : keptSections)
if (id->matchesFile(s->file))
for (SectionPattern &p : id->sectionPatterns)
if (p.sectionPat.match(s->name) &&
(s->flags & id->withFlags) == id->withFlags &&
(s->flags & id->withoutFlags) == 0)
return true;
return false;
}
// A helper function for the SORT() command.
static bool matchConstraints(ArrayRef<InputSectionBase *> sections,
ConstraintKind kind) {
if (kind == ConstraintKind::NoConstraint)
return true;
bool isRW = llvm::any_of(
sections, [](InputSectionBase *sec) { return sec->flags & SHF_WRITE; });
return (isRW && kind == ConstraintKind::ReadWrite) ||
(!isRW && kind == ConstraintKind::ReadOnly);
}
static void sortSections(MutableArrayRef<InputSectionBase *> vec,
SortSectionPolicy k) {
auto alignmentComparator = [](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;
};
auto nameComparator = [](InputSectionBase *a, InputSectionBase *b) {
return a->name < b->name;
};
auto priorityComparator = [](InputSectionBase *a, InputSectionBase *b) {
return getPriority(a->name) < getPriority(b->name);
};
switch (k) {
case SortSectionPolicy::Default:
case SortSectionPolicy::None:
return;
case SortSectionPolicy::Alignment:
return llvm::stable_sort(vec, alignmentComparator);
case SortSectionPolicy::Name:
return llvm::stable_sort(vec, nameComparator);
case SortSectionPolicy::Priority:
return llvm::stable_sort(vec, priorityComparator);
}
}
// 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<InputSectionBase *> vec,
SortSectionPolicy outer,
SortSectionPolicy inner) {
if (outer == SortSectionPolicy::None)
return;
if (inner == SortSectionPolicy::Default)
sortSections(vec, config->sortSection);
else
sortSections(vec, inner);
sortSections(vec, outer);
}
// Compute and remember which sections the InputSectionDescription matches.
std::vector<InputSectionBase *>
LinkerScript::computeInputSections(const InputSectionDescription *cmd,
ArrayRef<InputSectionBase *> sections) {
std::vector<InputSectionBase *> ret;
std::vector<size_t> indexes;
DenseSet<size_t> seen;
auto sortByPositionThenCommandLine = [&](size_t begin, size_t end) {
llvm::sort(MutableArrayRef<size_t>(indexes).slice(begin, end - begin));
for (size_t i = begin; i != end; ++i)
ret[i] = sections[indexes[i]];
sortInputSections(
MutableArrayRef<InputSectionBase *>(ret).slice(begin, end - begin),
config->sortSection, SortSectionPolicy::None);
};
// Collects all sections that satisfy constraints of Cmd.
size_t sizeAfterPrevSort = 0;
for (const SectionPattern &pat : cmd->sectionPatterns) {
size_t sizeBeforeCurrPat = ret.size();
for (size_t i = 0, e = sections.size(); i != e; ++i) {
// Skip if the section is dead or has been matched by a previous input
// section description or a previous pattern.
InputSectionBase *sec = sections[i];
if (!sec->isLive() || sec->parent || seen.contains(i))
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 (isa<InputSection>(sec) &&
cast<InputSection>(sec)->getRelocatedSection())
continue;
// Check the name early to improve performance in the common case.
if (!pat.sectionPat.match(sec->name))
continue;
if (!cmd->matchesFile(sec->file) || pat.excludesFile(sec->file) ||
(sec->flags & cmd->withFlags) != cmd->withFlags ||
(sec->flags & cmd->withoutFlags) != 0)
continue;
ret.push_back(sec);
indexes.push_back(i);
seen.insert(i);
}
if (pat.sortOuter == SortSectionPolicy::Default)
continue;
// Matched sections are ordered by radix sort with the keys being (SORT*,
// --sort-section, input order), where SORT* (if present) is most
// significant.
//
// Matched sections between the previous SORT* and this SORT* are sorted by
// (--sort-alignment, input order).
sortByPositionThenCommandLine(sizeAfterPrevSort, sizeBeforeCurrPat);
// Matched sections by this SORT* pattern are sorted using all 3 keys.
// ret[sizeBeforeCurrPat,ret.size()) are already in the input order, so we
// just sort by sortOuter and sortInner.
sortInputSections(
MutableArrayRef<InputSectionBase *>(ret).slice(sizeBeforeCurrPat),
pat.sortOuter, pat.sortInner);
sizeAfterPrevSort = ret.size();
}
// Matched sections after the last SORT* are sorted by (--sort-alignment,
// input order).
sortByPositionThenCommandLine(sizeAfterPrevSort, ret.size());
return ret;
}
void LinkerScript::discard(InputSectionBase *s) {
if (s == in.shStrTab || s == mainPart->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 == mainPart->gnuHashTab)
mainPart->gnuHashTab = nullptr;
if (s == mainPart->hashTab)
mainPart->hashTab = nullptr;
s->markDead();
s->parent = nullptr;
for (InputSection *ds : s->dependentSections)
discard(ds);
}
void LinkerScript::discardSynthetic(OutputSection &outCmd) {
for (Partition &part : partitions) {
if (!part.armExidx || !part.armExidx->isLive())
continue;
std::vector<InputSectionBase *> secs(part.armExidx->exidxSections.begin(),
part.armExidx->exidxSections.end());
for (BaseCommand *base : outCmd.sectionCommands)
if (auto *cmd = dyn_cast<InputSectionDescription>(base)) {
std::vector<InputSectionBase *> matches =
computeInputSections(cmd, secs);
for (InputSectionBase *s : matches)
discard(s);
}
}
}
std::vector<InputSectionBase *>
LinkerScript::createInputSectionList(OutputSection &outCmd) {
std::vector<InputSectionBase *> ret;
for (BaseCommand *base : outCmd.sectionCommands) {
if (auto *cmd = dyn_cast<InputSectionDescription>(base)) {
cmd->sectionBases = computeInputSections(cmd, inputSections);
for (InputSectionBase *s : cmd->sectionBases)
s->parent = &outCmd;
ret.insert(ret.end(), cmd->sectionBases.begin(), cmd->sectionBases.end());
}
}
return ret;
}
// Create output sections described by SECTIONS commands.
void LinkerScript::processSectionCommands() {
auto process = [this](OutputSection *osec) {
std::vector<InputSectionBase *> v = createInputSectionList(*osec);
// The output section name `/DISCARD/' is special.
// Any input section assigned to it is discarded.
if (osec->name == "/DISCARD/") {
for (InputSectionBase *s : v)
discard(s);
discardSynthetic(*osec);
osec->sectionCommands.clear();
return false;
}
// 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, osec->constraint)) {
for (InputSectionBase *s : v)
s->parent = nullptr;
osec->sectionCommands.clear();
return false;
}
// 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 (osec->subalignExpr) {
uint32_t subalign = osec->subalignExpr().getValue();
for (InputSectionBase *s : v)
s->alignment = subalign;
}
// Set the partition field the same way OutputSection::recordSection()
// does. Partitions cannot be used with the SECTIONS command, so this is
// always 1.
osec->partition = 1;
return true;
};
// Process OVERWRITE_SECTIONS first so that it can overwrite the main script
// or orphans.
DenseMap<StringRef, OutputSection *> map;
size_t i = 0;
for (OutputSection *osec : overwriteSections)
if (process(osec) && !map.try_emplace(osec->name, osec).second)
warn("OVERWRITE_SECTIONS specifies duplicate " + osec->name);
for (BaseCommand *&base : sectionCommands)
if (auto *osec = dyn_cast<OutputSection>(base)) {
if (OutputSection *overwrite = map.lookup(osec->name)) {
log(overwrite->location + " overwrites " + osec->name);
overwrite->sectionIndex = i++;
base = overwrite;
} else if (process(osec)) {
osec->sectionIndex = i++;
}
}
// If an OVERWRITE_SECTIONS specified output section is not in
// sectionCommands, append it to the end. The section will be inserted by
// orphan placement.
for (OutputSection *osec : overwriteSections)
if (osec->partition == 1 && osec->sectionIndex == UINT32_MAX)
sectionCommands.push_back(osec);
}
void LinkerScript::processSymbolAssignments() {
// Dot outside an output section still represents a relative address, whose
// sh_shndx should not be SHN_UNDEF or SHN_ABS. Create a dummy aether section
// that fills the void outside a section. It has an index of one, which is
// indistinguishable from any other regular section index.
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.
AddressState state;
ctx = &state;
ctx->outSec = aether;
for (BaseCommand *base : sectionCommands) {
if (auto *cmd = dyn_cast<SymbolAssignment>(base))
addSymbol(cmd);
else
for (BaseCommand *sub_base : cast<OutputSection>(base)->sectionCommands)
if (auto *cmd = dyn_cast<SymbolAssignment>(sub_base))
addSymbol(cmd);
}
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 *isec,
StringRef outsecName) {
OutputSection *sec = script->createOutputSection(outsecName, "<internal>");
sec->recordSection(isec);
return sec;
}
static OutputSection *
addInputSec(StringMap<TinyPtrVector<OutputSection *>> &map,
InputSectionBase *isec, 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 (isec->type == SHT_GROUP || (isec->flags & SHF_GROUP))
return createSection(isec, 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>(isec) &&
(isec->type == SHT_REL || isec->type == SHT_RELA)) {
auto *sec = cast<InputSection>(isec);
OutputSection *out = sec->getRelocatedSection()->getOutputSection();
if (out->relocationSection) {
out->relocationSection->recordSection(sec);
return nullptr;
}
out->relocationSection = createSection(isec, outsecName);
return out->relocationSection;
}
// 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.
TinyPtrVector<OutputSection *> &v = map[outsecName];
for (OutputSection *sec : v) {
if (sec->partition != isec->partition)
continue;
if (config->relocatable && (isec->flags & SHF_LINK_ORDER)) {
// Merging two SHF_LINK_ORDER sections with different sh_link fields will
// change their semantics, so we only merge them in -r links if they will
// end up being linked to the same output section. The casts are fine
// because everything in the map was created by the orphan placement code.
auto *firstIsec = cast<InputSectionBase>(
cast<InputSectionDescription>(sec->sectionCommands[0])
->sectionBases[0]);
OutputSection *firstIsecOut =
firstIsec->flags & SHF_LINK_ORDER
? firstIsec->getLinkOrderDep()->getOutputSection()
: nullptr;
if (firstIsecOut != isec->getLinkOrderDep()->getOutputSection())
continue;
}
sec->recordSection(isec);
return nullptr;
}
OutputSection *sec = createSection(isec, outsecName);
v.push_back(sec);
return sec;
}
// Add sections that didn't match any sections command.
void LinkerScript::addOrphanSections() {
StringMap<TinyPtrVector<OutputSection *>> map;
std::vector<OutputSection *> v;
std::function<void(InputSectionBase *)> add;
add = [&](InputSectionBase *s) {
if (s->isLive() && !s->parent) {
orphanSections.push_back(s);
StringRef name = getOutputSectionName(s);
if (config->unique) {
v.push_back(createSection(s, name));
} else if (OutputSection *sec = findByName(sectionCommands, name)) {
sec->recordSection(s);
} else {
if (OutputSection *os = addInputSec(map, s, name))
v.push_back(os);
assert(isa<MergeInputSection>(s) ||
s->getOutputSection()->sectionIndex == UINT32_MAX);
}
}
if (config->relocatable)
for (InputSectionBase *depSec : s->dependentSections)
if (depSec->flags & SHF_LINK_ORDER)
add(depSec);
};
// For further --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 *isec : inputSections) {
// In -r links, SHF_LINK_ORDER sections are added while adding their parent
// sections because we need to know the parent's output section before we
// can select an output section for the SHF_LINK_ORDER section.
if (config->relocatable && (isec->flags & SHF_LINK_ORDER))
continue;
if (auto *sec = dyn_cast<InputSection>(isec))
if (InputSectionBase *rel = sec->getRelocatedSection())
if (auto *relIS = dyn_cast_or_null<InputSectionBase>(rel->parent))
add(relIS);
add(isec);
}
// 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());
}
void LinkerScript::diagnoseOrphanHandling() const {
llvm::TimeTraceScope timeScope("Diagnose orphan sections");
if (config->orphanHandling == OrphanHandlingPolicy::Place)
return;
for (const InputSectionBase *sec : orphanSections) {
// Input SHT_REL[A] retained by --emit-relocs are ignored by
// computeInputSections(). Don't warn/error.
if (isa<InputSection>(sec) &&
cast<InputSection>(sec)->getRelocatedSection())
continue;
StringRef name = getOutputSectionName(sec);
if (config->orphanHandling == OrphanHandlingPolicy::Error)
error(toString(sec) + " is being placed in '" + name + "'");
else
warn(toString(sec) + " is being placed in '" + name + "'");
}
}
uint64_t LinkerScript::advance(uint64_t size, unsigned alignment) {
dot = alignTo(dot, alignment) + size;
return dot;
}
void LinkerScript::output(InputSection *s) {
assert(ctx->outSec == s->getParent());
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 pos = advance(0, 1);
if (sec->addrExpr && script->hasSectionsCommand) {
// The alignment is ignored.
ctx->outSec->addr = pos;
} else {
// ctx->outSec->alignment is the max of ALIGN and the maximum of input
// section alignments.
ctx->outSec->addr = advance(0, ctx->outSec->alignment);
expandMemoryRegions(ctx->outSec->addr - pos);
}
}
// 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;
}
static OutputSection *findFirstSection(PhdrEntry *load) {
for (OutputSection *sec : outputSections)
if (sec->ptLoad == load)
return sec;
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) {
const bool isTbss = (sec->flags & SHF_TLS) && sec->type == SHT_NOBITS;
const bool sameMemRegion = ctx->memRegion == sec->memRegion;
const bool prevLMARegionIsDefault = ctx->lmaRegion == nullptr;
const uint64_t savedDot = dot;
ctx->memRegion = sec->memRegion;
ctx->lmaRegion = sec->lmaRegion;
if (!(sec->flags & SHF_ALLOC)) {
// Non-SHF_ALLOC sections have zero addresses.
dot = 0;
} else if (isTbss) {
// Allow consecutive SHF_TLS SHT_NOBITS output sections. The address range
// starts from the end address of the previous tbss section.
if (ctx->tbssAddr == 0)
ctx->tbssAddr = dot;
else
dot = ctx->tbssAddr;
} else {
if (ctx->memRegion)
dot = ctx->memRegion->curPos;
if (sec->addrExpr)
setDot(sec->addrExpr, sec->location, false);
// If the address of the section has been moved forward by an explicit
// expression so that it now starts past the current curPos of the enclosing
// region, we need to expand the current region to account for the space
// between the previous section, if any, and the start of this section.
if (ctx->memRegion && ctx->memRegion->curPos < dot)
expandMemoryRegion(ctx->memRegion, dot - ctx->memRegion->curPos,
ctx->memRegion->name, sec->name);
}
switchTo(sec);
// ctx->lmaOffset is LMA minus VMA. If LMA is explicitly specified via AT() or
// AT>, recompute ctx->lmaOffset; otherwise, if both previous/current LMA
// region is the default, and the two sections are in the same memory region,
// reuse previous lmaOffset; otherwise, reset lmaOffset to 0. This emulates
// heuristics described in
// https://sourceware.org/binutils/docs/ld/Output-Section-LMA.html
if (sec->lmaExpr)
ctx->lmaOffset = sec->lmaExpr().getValue() - dot;
else if (MemoryRegion *mr = sec->lmaRegion)
ctx->lmaOffset = alignTo(mr->curPos, sec->alignment) - dot;
else if (!sameMemRegion || !prevLMARegionIsDefault)
ctx->lmaOffset = 0;
// Propagate ctx->lmaOffset to the first "non-header" section.
if (PhdrEntry *l = ctx->outSec->ptLoad)
if (sec == findFirstSection(l))
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.
for (InputSection *sec : cast<InputSectionDescription>(base)->sections)
output(sec);
}
// Non-SHF_ALLOC sections do not affect the addresses of other OutputSections
// as they are not part of the process image.
if (!(sec->flags & SHF_ALLOC)) {
dot = savedDot;
} else if (isTbss) {
// NOBITS TLS sections are similar. Additionally save the end address.
ctx->tbssAddr = dot;
dot = savedDot;
}
}
static bool isDiscardable(OutputSection &sec) {
if (sec.name == "/DISCARD/")
return true;
// We do not want to remove OutputSections with expressions that reference
// symbols even if the OutputSection is empty. We want to ensure that the
// expressions can be evaluated and report an error if they cannot.
if (sec.expressionsUseSymbols)
return false;
// OutputSections may be referenced by name in ADDR and LOADADDR expressions,
// as an empty Section can has a valid VMA and LMA we keep the OutputSection
// to maintain the integrity of the other Expression.
if (sec.usedInExpression)
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;
}
static void maybePropagatePhdrs(OutputSection &sec,
std::vector<StringRef> &phdrs) {
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 = phdrs;
} else {
phdrs = sec.phdrs;
}
}
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;
std::vector<StringRef> defPhdrs;
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());
// The input section might have been removed (if it was an empty synthetic
// section), but we at least know the flags.
if (sec->hasInputSections)
flags = sec->flags;
// We do not want to keep any special flags for output section
// in case it is empty.
bool isEmpty = (getFirstInputSection(sec) == nullptr);
if (isEmpty)
sec->flags = flags & ((sec->nonAlloc ? 0 : (uint64_t)SHF_ALLOC) |
SHF_WRITE | SHF_EXECINSTR);
// The code below may remove empty output sections. We should save the
// specified program headers (if exist) and propagate them to subsequent
// sections which do not specify program headers.
// An example of such a linker script is:
// SECTIONS { .empty : { *(.empty) } :rw
// .foo : { *(.foo) } }
// Note: at this point the order of output sections has not been finalized,
// because orphans have not been inserted into their expected positions. We
// will handle them in adjustSectionsAfterSorting().
if (sec->sectionIndex != UINT32_MAX)
maybePropagatePhdrs(*sec, defPhdrs);
if (isEmpty && isDiscardable(*sec)) {
sec->markDead();
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)
if (auto *sec = dyn_cast<OutputSection>(base))
maybePropagatePhdrs(*sec, defPhdrs);
}
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);
}
// When the SECTIONS command is used, try to find an address for the file and
// program headers output sections, which can be added to the first PT_LOAD
// segment when program headers are created.
//
// We check if the headers fit below the first allocated section. 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;
});
bool paged = !config->omagic && !config->nmagic;
uint64_t headerSize = getHeaderSize();
if ((paged || hasExplicitHeaders) &&
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)().getValue();
}
}
// 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.
// Returns a symbol that has changed its section or value, or nullptr if no
// symbol has changed.
const Defined *LinkerScript::assignAddresses() {
if (script->hasSectionsCommand) {
// With a linker script, assignment of addresses to headers is covered by
// allocateHeaders().
dot = config->imageBase.getValueOr(0);
} else {
// Assign addresses to headers right now.
dot = target->getImageBase();
Out::elfHeader->addr = dot;
Out::programHeaders->addr = dot + Out::elfHeader->size;
dot += getHeaderSize();
}
auto deleter = std::make_unique<AddressState>();
ctx = deleter.get();
errorOnMissingSection = true;
switchTo(aether);
SymbolAssignmentMap oldValues = getSymbolAssignmentValues(sectionCommands);
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;
return getChangedSymbolAssignment(oldValues);
}
// 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)) {
ExprValue v{ds->section, false, ds->value, loc};
// Retain the original st_type, so that the alias will get the same
// behavior in relocation processing. Any operation will reset st_type to
// STT_NOTYPE.
v.type = ds->type;
return v;
}
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 + ": program header '" + s +
"' is not listed in PHDRS");
}
return ret;
}
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