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llvm-project/lld/ELF/Symbols.cpp

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//===- Symbols.cpp --------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "Symbols.h"
#include "InputFiles.h"
#include "InputSection.h"
#include "OutputSections.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "Writer.h"
#include "lld/Common/ErrorHandler.h"
#include "lld/Common/Strings.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/Path.h"
#include <cstring>
using namespace llvm;
using namespace llvm::object;
using namespace llvm::ELF;
namespace lld {
// Returns a symbol for an error message.
static std::string demangle(StringRef symName) {
if (elf::config->demangle)
return demangleItanium(symName);
return symName;
}
std::string toString(const elf::Symbol &b) { return demangle(b.getName()); }
std::string toELFString(const Archive::Symbol &b) {
return demangle(b.getName());
}
namespace elf {
Defined *ElfSym::bss;
Defined *ElfSym::etext1;
Defined *ElfSym::etext2;
Defined *ElfSym::edata1;
Defined *ElfSym::edata2;
Defined *ElfSym::end1;
Defined *ElfSym::end2;
Defined *ElfSym::globalOffsetTable;
Defined *ElfSym::mipsGp;
Defined *ElfSym::mipsGpDisp;
Defined *ElfSym::mipsLocalGp;
Defined *ElfSym::relaIpltStart;
Defined *ElfSym::relaIpltEnd;
Defined *ElfSym::riscvGlobalPointer;
Defined *ElfSym::tlsModuleBase;
static uint64_t getSymVA(const Symbol &sym, int64_t &addend) {
switch (sym.kind()) {
case Symbol::DefinedKind: {
auto &d = cast<Defined>(sym);
SectionBase *isec = d.section;
// This is an absolute symbol.
if (!isec)
return d.value;
assert(isec != &InputSection::discarded);
isec = isec->repl;
uint64_t offset = d.value;
// An object in an SHF_MERGE section might be referenced via a
// section symbol (as a hack for reducing the number of local
// symbols).
// Depending on the addend, the reference via a section symbol
// refers to a different object in the merge section.
// Since the objects in the merge section are not necessarily
// contiguous in the output, the addend can thus affect the final
// VA in a non-linear way.
// To make this work, we incorporate the addend into the section
// offset (and zero out the addend for later processing) so that
// we find the right object in the section.
if (d.isSection()) {
offset += addend;
addend = 0;
}
// In the typical case, this is actually very simple and boils
// down to adding together 3 numbers:
// 1. The address of the output section.
// 2. The offset of the input section within the output section.
// 3. The offset within the input section (this addition happens
// inside InputSection::getOffset).
//
// If you understand the data structures involved with this next
// line (and how they get built), then you have a pretty good
// understanding of the linker.
uint64_t va = isec->getVA(offset);
// MIPS relocatable files can mix regular and microMIPS code.
// Linker needs to distinguish such code. To do so microMIPS
// symbols has the `STO_MIPS_MICROMIPS` flag in the `st_other`
// field. Unfortunately, the `MIPS::relocateOne()` method has
// a symbol value only. To pass type of the symbol (regular/microMIPS)
// to that routine as well as other places where we write
// a symbol value as-is (.dynamic section, `Elf_Ehdr::e_entry`
// field etc) do the same trick as compiler uses to mark microMIPS
// for CPU - set the less-significant bit.
if (config->emachine == EM_MIPS && isMicroMips() &&
((sym.stOther & STO_MIPS_MICROMIPS) || sym.needsPltAddr))
va |= 1;
if (d.isTls() && !config->relocatable) {
// Use the address of the TLS segment's first section rather than the
// segment's address, because segment addresses aren't initialized until
// after sections are finalized. (e.g. Measuring the size of .rela.dyn
// for Android relocation packing requires knowing TLS symbol addresses
// during section finalization.)
if (!Out::tlsPhdr || !Out::tlsPhdr->firstSec)
fatal(toString(d.file) +
" has an STT_TLS symbol but doesn't have an SHF_TLS section");
return va - Out::tlsPhdr->firstSec->addr;
}
return va;
}
case Symbol::SharedKind:
case Symbol::UndefinedKind:
return 0;
case Symbol::LazyArchiveKind:
case Symbol::LazyObjectKind:
assert(sym.isUsedInRegularObj && "lazy symbol reached writer");
return 0;
case Symbol::CommonKind:
llvm_unreachable("common symbol reached writer");
case Symbol::PlaceholderKind:
llvm_unreachable("placeholder symbol reached writer");
}
llvm_unreachable("invalid symbol kind");
}
uint64_t Symbol::getVA(int64_t addend) const {
uint64_t outVA = getSymVA(*this, addend);
return outVA + addend;
}
uint64_t Symbol::getGotVA() const {
if (gotInIgot)
return in.igotPlt->getVA() + getGotPltOffset();
return in.got->getVA() + getGotOffset();
}
uint64_t Symbol::getGotOffset() const { return gotIndex * config->wordsize; }
uint64_t Symbol::getGotPltVA() const {
if (isInIplt)
return in.igotPlt->getVA() + getGotPltOffset();
return in.gotPlt->getVA() + getGotPltOffset();
}
uint64_t Symbol::getGotPltOffset() const {
if (isInIplt)
return pltIndex * config->wordsize;
return (pltIndex + target->gotPltHeaderEntriesNum) * config->wordsize;
}
uint64_t Symbol::getPltVA() const {
uint64_t outVA = isInIplt
? in.iplt->getVA() + pltIndex * target->ipltEntrySize
: in.plt->getVA() + in.plt->headerSize +
pltIndex * target->pltEntrySize;
// While linking microMIPS code PLT code are always microMIPS
// code. Set the less-significant bit to track that fact.
// See detailed comment in the `getSymVA` function.
if (config->emachine == EM_MIPS && isMicroMips())
outVA |= 1;
return outVA;
}
uint64_t Symbol::getSize() const {
if (const auto *dr = dyn_cast<Defined>(this))
return dr->size;
return cast<SharedSymbol>(this)->size;
}
OutputSection *Symbol::getOutputSection() const {
if (auto *s = dyn_cast<Defined>(this)) {
if (auto *sec = s->section)
return sec->repl->getOutputSection();
return nullptr;
}
return nullptr;
}
// If a symbol name contains '@', the characters after that is
// a symbol version name. This function parses that.
void Symbol::parseSymbolVersion() {
StringRef s = getName();
size_t pos = s.find('@');
if (pos == 0 || pos == StringRef::npos)
return;
StringRef verstr = s.substr(pos + 1);
if (verstr.empty())
return;
// Truncate the symbol name so that it doesn't include the version string.
nameSize = pos;
// If this is not in this DSO, it is not a definition.
if (!isDefined())
return;
// '@@' in a symbol name means the default version.
// It is usually the most recent one.
bool isDefault = (verstr[0] == '@');
if (isDefault)
verstr = verstr.substr(1);
for (const VersionDefinition &ver : namedVersionDefs()) {
if (ver.name != verstr)
continue;
if (isDefault)
versionId = ver.id;
else
versionId = ver.id | VERSYM_HIDDEN;
return;
}
// It is an error if the specified version is not defined.
// Usually version script is not provided when linking executable,
// but we may still want to override a versioned symbol from DSO,
// so we do not report error in this case. We also do not error
// if the symbol has a local version as it won't be in the dynamic
// symbol table.
if (config->shared && versionId != VER_NDX_LOCAL)
error(toString(file) + ": symbol " + s + " has undefined version " +
verstr);
}
void Symbol::fetch() const {
if (auto *sym = dyn_cast<LazyArchive>(this)) {
cast<ArchiveFile>(sym->file)->fetch(sym->sym);
return;
}
if (auto *sym = dyn_cast<LazyObject>(this)) {
dyn_cast<LazyObjFile>(sym->file)->fetch();
return;
}
llvm_unreachable("Symbol::fetch() is called on a non-lazy symbol");
}
MemoryBufferRef LazyArchive::getMemberBuffer() {
Archive::Child c =
CHECK(sym.getMember(),
"could not get the member for symbol " + toELFString(sym));
return CHECK(c.getMemoryBufferRef(),
"could not get the buffer for the member defining symbol " +
toELFString(sym));
}
uint8_t Symbol::computeBinding() const {
if (config->relocatable)
return binding;
if ((visibility != STV_DEFAULT && visibility != STV_PROTECTED) ||
versionId == VER_NDX_LOCAL)
return STB_LOCAL;
if (!config->gnuUnique && binding == STB_GNU_UNIQUE)
return STB_GLOBAL;
return binding;
}
bool Symbol::includeInDynsym() const {
if (!config->hasDynSymTab)
return false;
if (computeBinding() == STB_LOCAL)
return false;
// If a PIE binary was not linked against any shared libraries, then we can
// safely drop weak undef symbols from .dynsym.
if (isUndefWeak() && config->pie && sharedFiles.empty())
return false;
return isUndefined() || isShared() || exportDynamic || inDynamicList;
}
// Print out a log message for --trace-symbol.
void printTraceSymbol(const Symbol *sym) {
std::string s;
if (sym->isUndefined())
s = ": reference to ";
else if (sym->isLazy())
s = ": lazy definition of ";
else if (sym->isShared())
s = ": shared definition of ";
else if (sym->isCommon())
s = ": common definition of ";
else
s = ": definition of ";
message(toString(sym->file) + s + sym->getName());
}
void maybeWarnUnorderableSymbol(const Symbol *sym) {
if (!config->warnSymbolOrdering)
return;
// If UnresolvedPolicy::Ignore is used, no "undefined symbol" error/warning
// is emitted. It makes sense to not warn on undefined symbols.
//
// Note, ld.bfd --symbol-ordering-file= does not warn on undefined symbols,
// but we don't have to be compatible here.
if (sym->isUndefined() &&
config->unresolvedSymbols == UnresolvedPolicy::Ignore)
return;
const InputFile *file = sym->file;
auto *d = dyn_cast<Defined>(sym);
auto report = [&](StringRef s) { warn(toString(file) + s + sym->getName()); };
if (sym->isUndefined())
report(": unable to order undefined symbol: ");
else if (sym->isShared())
report(": unable to order shared symbol: ");
else if (d && !d->section)
report(": unable to order absolute symbol: ");
else if (d && isa<OutputSection>(d->section))
report(": unable to order synthetic symbol: ");
else if (d && !d->section->repl->isLive())
report(": unable to order discarded symbol: ");
}
// Returns true if a symbol can be replaced at load-time by a symbol
// with the same name defined in other ELF executable or DSO.
bool computeIsPreemptible(const Symbol &sym) {
assert(!sym.isLocal());
// Only symbols with default visibility that appear in dynsym can be
// preempted. Symbols with protected visibility cannot be preempted.
if (!sym.includeInDynsym() || sym.visibility != STV_DEFAULT)
return false;
// At this point copy relocations have not been created yet, so any
// symbol that is not defined locally is preemptible.
if (!sym.isDefined())
return true;
if (!config->shared)
return false;
// If the dynamic list is present, it specifies preemptable symbols in a DSO.
if (config->hasDynamicList)
return sym.inDynamicList;
// -Bsymbolic means that definitions are not preempted.
if (config->bsymbolic || (config->bsymbolicFunctions && sym.isFunc()))
return false;
return true;
}
static uint8_t getMinVisibility(uint8_t va, uint8_t vb) {
if (va == STV_DEFAULT)
return vb;
if (vb == STV_DEFAULT)
return va;
return std::min(va, vb);
}
// Merge symbol properties.
//
// When we have many symbols of the same name, we choose one of them,
// and that's the result of symbol resolution. However, symbols that
// were not chosen still affect some symbol properties.
void Symbol::mergeProperties(const Symbol &other) {
if (other.exportDynamic)
exportDynamic = true;
if (other.isUsedInRegularObj)
isUsedInRegularObj = true;
// DSO symbols do not affect visibility in the output.
if (!other.isShared())
visibility = getMinVisibility(visibility, other.visibility);
}
void Symbol::resolve(const Symbol &other) {
mergeProperties(other);
if (isPlaceholder()) {
replace(other);
return;
}
switch (other.kind()) {
case Symbol::UndefinedKind:
resolveUndefined(cast<Undefined>(other));
break;
case Symbol::CommonKind:
resolveCommon(cast<CommonSymbol>(other));
break;
case Symbol::DefinedKind:
resolveDefined(cast<Defined>(other));
break;
case Symbol::LazyArchiveKind:
resolveLazy(cast<LazyArchive>(other));
break;
case Symbol::LazyObjectKind:
resolveLazy(cast<LazyObject>(other));
break;
case Symbol::SharedKind:
resolveShared(cast<SharedSymbol>(other));
break;
case Symbol::PlaceholderKind:
llvm_unreachable("bad symbol kind");
}
}
void Symbol::resolveUndefined(const Undefined &other) {
// An undefined symbol with non default visibility must be satisfied
// in the same DSO.
//
// If this is a non-weak defined symbol in a discarded section, override the
// existing undefined symbol for better error message later.
if ((isShared() && other.visibility != STV_DEFAULT) ||
(isUndefined() && other.binding != STB_WEAK && other.discardedSecIdx)) {
replace(other);
return;
}
if (traced)
printTraceSymbol(&other);
if (isLazy()) {
// An undefined weak will not fetch archive members. See comment on Lazy in
// Symbols.h for the details.
if (other.binding == STB_WEAK) {
binding = STB_WEAK;
type = other.type;
return;
}
// Do extra check for --warn-backrefs.
//
// --warn-backrefs is an option to prevent an undefined reference from
// fetching an archive member written earlier in the command line. It can be
// used to keep compatibility with GNU linkers to some degree.
// I'll explain the feature and why you may find it useful in this comment.
//
// lld's symbol resolution semantics is more relaxed than traditional Unix
// linkers. For example,
//
// ld.lld foo.a bar.o
//
// succeeds even if bar.o contains an undefined symbol that has to be
// resolved by some object file in foo.a. Traditional Unix linkers don't
// allow this kind of backward reference, as they visit each file only once
// from left to right in the command line while resolving all undefined
// symbols at the moment of visiting.
//
// In the above case, since there's no undefined symbol when a linker visits
// foo.a, no files are pulled out from foo.a, and because the linker forgets
// about foo.a after visiting, it can't resolve undefined symbols in bar.o
// that could have been resolved otherwise.
//
// That lld accepts more relaxed form means that (besides it'd make more
// sense) you can accidentally write a command line or a build file that
// works only with lld, even if you have a plan to distribute it to wider
// users who may be using GNU linkers. With --warn-backrefs, you can detect
// a library order that doesn't work with other Unix linkers.
//
// The option is also useful to detect cyclic dependencies between static
// archives. Again, lld accepts
//
// ld.lld foo.a bar.a
//
// even if foo.a and bar.a depend on each other. With --warn-backrefs, it is
// handled as an error.
//
// Here is how the option works. We assign a group ID to each file. A file
// with a smaller group ID can pull out object files from an archive file
// with an equal or greater group ID. Otherwise, it is a reverse dependency
// and an error.
//
// A file outside --{start,end}-group gets a fresh ID when instantiated. All
// files within the same --{start,end}-group get the same group ID. E.g.
//
// ld.lld A B --start-group C D --end-group E
//
// A forms group 0. B form group 1. C and D (including their member object
// files) form group 2. E forms group 3. I think that you can see how this
// group assignment rule simulates the traditional linker's semantics.
bool backref = config->warnBackrefs && other.file &&
file->groupId < other.file->groupId;
fetch();
// We don't report backward references to weak symbols as they can be
// overridden later.
if (backref && !isWeak())
warn("backward reference detected: " + other.getName() + " in " +
toString(other.file) + " refers to " + toString(file));
return;
}
// Undefined symbols in a SharedFile do not change the binding.
if (dyn_cast_or_null<SharedFile>(other.file))
return;
if (isUndefined() || isShared()) {
// The binding will be weak if there is at least one reference and all are
// weak. The binding has one opportunity to change to weak: if the first
// reference is weak.
if (other.binding != STB_WEAK || !referenced)
binding = other.binding;
referenced = true;
}
}
// Using .symver foo,foo@@VER unfortunately creates two symbols: foo and
// foo@@VER. We want to effectively ignore foo, so give precedence to
// foo@@VER.
// FIXME: If users can transition to using
// .symver foo,foo@@@VER
// we can delete this hack.
static int compareVersion(StringRef a, StringRef b) {
bool x = a.contains("@@");
bool y = b.contains("@@");
if (!x && y)
return 1;
if (x && !y)
return -1;
return 0;
}
// Compare two symbols. Return 1 if the new symbol should win, -1 if
// the new symbol should lose, or 0 if there is a conflict.
int Symbol::compare(const Symbol *other) const {
assert(other->isDefined() || other->isCommon());
if (!isDefined() && !isCommon())
return 1;
if (int cmp = compareVersion(getName(), other->getName()))
return cmp;
if (other->isWeak())
return -1;
if (isWeak())
return 1;
if (isCommon() && other->isCommon()) {
if (config->warnCommon)
warn("multiple common of " + getName());
return 0;
}
if (isCommon()) {
if (config->warnCommon)
warn("common " + getName() + " is overridden");
return 1;
}
if (other->isCommon()) {
if (config->warnCommon)
warn("common " + getName() + " is overridden");
return -1;
}
auto *oldSym = cast<Defined>(this);
auto *newSym = cast<Defined>(other);
if (dyn_cast_or_null<BitcodeFile>(other->file))
return 0;
if (!oldSym->section && !newSym->section && oldSym->value == newSym->value &&
newSym->binding == STB_GLOBAL)
return -1;
return 0;
}
static void reportDuplicate(Symbol *sym, InputFile *newFile,
InputSectionBase *errSec, uint64_t errOffset) {
if (config->allowMultipleDefinition)
return;
Defined *d = cast<Defined>(sym);
if (!d->section || !errSec) {
error("duplicate symbol: " + toString(*sym) + "\n>>> defined in " +
toString(sym->file) + "\n>>> defined in " + toString(newFile));
return;
}
// Construct and print an error message in the form of:
//
// ld.lld: error: duplicate symbol: foo
// >>> defined at bar.c:30
// >>> bar.o (/home/alice/src/bar.o)
// >>> defined at baz.c:563
// >>> baz.o in archive libbaz.a
auto *sec1 = cast<InputSectionBase>(d->section);
std::string src1 = sec1->getSrcMsg(*sym, d->value);
std::string obj1 = sec1->getObjMsg(d->value);
std::string src2 = errSec->getSrcMsg(*sym, errOffset);
std::string obj2 = errSec->getObjMsg(errOffset);
std::string msg = "duplicate symbol: " + toString(*sym) + "\n>>> defined at ";
if (!src1.empty())
msg += src1 + "\n>>> ";
msg += obj1 + "\n>>> defined at ";
if (!src2.empty())
msg += src2 + "\n>>> ";
msg += obj2;
error(msg);
}
void Symbol::resolveCommon(const CommonSymbol &other) {
int cmp = compare(&other);
if (cmp < 0)
return;
if (cmp > 0) {
if (auto *s = dyn_cast<SharedSymbol>(this)) {
// Increase st_size if the shared symbol has a larger st_size. The shared
// symbol may be created from common symbols. The fact that some object
// files were linked into a shared object first should not change the
// regular rule that picks the largest st_size.
uint64_t size = s->size;
replace(other);
if (size > cast<CommonSymbol>(this)->size)
cast<CommonSymbol>(this)->size = size;
} else {
replace(other);
}
return;
}
CommonSymbol *oldSym = cast<CommonSymbol>(this);
oldSym->alignment = std::max(oldSym->alignment, other.alignment);
if (oldSym->size < other.size) {
oldSym->file = other.file;
oldSym->size = other.size;
}
}
void Symbol::resolveDefined(const Defined &other) {
int cmp = compare(&other);
if (cmp > 0)
replace(other);
else if (cmp == 0)
reportDuplicate(this, other.file,
dyn_cast_or_null<InputSectionBase>(other.section),
other.value);
}
template <class LazyT> void Symbol::resolveLazy(const LazyT &other) {
if (!isUndefined())
return;
// An undefined weak will not fetch archive members. See comment on Lazy in
// Symbols.h for the details.
if (isWeak()) {
uint8_t ty = type;
replace(other);
type = ty;
binding = STB_WEAK;
return;
}
other.fetch();
}
void Symbol::resolveShared(const SharedSymbol &other) {
if (isCommon()) {
// See the comment in resolveCommon() above.
if (other.size > cast<CommonSymbol>(this)->size)
cast<CommonSymbol>(this)->size = other.size;
return;
}
if (visibility == STV_DEFAULT && (isUndefined() || isLazy())) {
// An undefined symbol with non default visibility must be satisfied
// in the same DSO.
uint8_t bind = binding;
replace(other);
binding = bind;
referenced = true;
}
}
} // namespace elf
} // namespace lld
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