lld/ELF/MarkLive.cpp

//===- MarkLive.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 implements --gc-sections, which is a feature to remove unused
// sections from output. Unused sections are sections that are not reachable
// from known GC-root symbols or sections. Naturally the feature is
// implemented as a mark-sweep garbage collector.
//
// Here's how it works. Each InputSectionBase has a "Live" bit. The bit is off
// by default. Starting with GC-root symbols or sections, markLive function
// defined in this file visits all reachable sections to set their Live
// bits. Writer will then ignore sections whose Live bits are off, so that
// such sections are not included into output.
//
//===----------------------------------------------------------------------===//

#include "MarkLive.h"
#include "InputSection.h"
#include "LinkerScript.h"
#include "OutputSections.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "lld/Common/Memory.h"
#include "lld/Common/Strings.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Object/ELF.h"
#include <functional>
#include <vector>

using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;

namespace endian = llvm::support::endian;

namespace lld {
namespace elf {
namespace {
template <class ELFT> class MarkLive {
public:
  MarkLive(unsigned partition) : partition(partition) {}

  void run();
  void moveToMain();

private:
  void enqueue(InputSectionBase *sec, uint64_t offset);
  void markSymbol(Symbol *sym);
  void mark();

  template <class RelTy>
  void resolveReloc(InputSectionBase &sec, RelTy &rel, bool isLSDA);

  template <class RelTy>
  void scanEhFrameSection(EhInputSection &eh, ArrayRef<RelTy> rels);

  // The index of the partition that we are currently processing.
  unsigned partition;

  // A list of sections to visit.
  SmallVector<InputSection *, 256> queue;

  // There are normally few input sections whose names are valid C
  // identifiers, so we just store a std::vector instead of a multimap.
  DenseMap<StringRef, std::vector<InputSectionBase *>> cNamedSections;
};
} // namespace

template <class ELFT>
static uint64_t getAddend(InputSectionBase &sec,
                          const typename ELFT::Rel &rel) {
  return target->getImplicitAddend(sec.data().begin() + rel.r_offset,
                                   rel.getType(config->isMips64EL));
}

template <class ELFT>
static uint64_t getAddend(InputSectionBase &sec,
                          const typename ELFT::Rela &rel) {
  return rel.r_addend;
}

template <class ELFT>
template <class RelTy>
void MarkLive<ELFT>::resolveReloc(InputSectionBase &sec, RelTy &rel,
                                  bool isLSDA) {
  Symbol &sym = sec.getFile<ELFT>()->getRelocTargetSym(rel);

  // If a symbol is referenced in a live section, it is used.
  sym.used = true;

  if (auto *d = dyn_cast<Defined>(&sym)) {
    auto *relSec = dyn_cast_or_null<InputSectionBase>(d->section);
    if (!relSec)
      return;

    uint64_t offset = d->value;
    if (d->isSection())
      offset += getAddend<ELFT>(sec, rel);

    if (!isLSDA || !(relSec->flags & SHF_EXECINSTR))
      enqueue(relSec, offset);
    return;
  }

  if (auto *ss = dyn_cast<SharedSymbol>(&sym))
    if (!ss->isWeak())
      ss->getFile().isNeeded = true;

  for (InputSectionBase *sec : cNamedSections.lookup(sym.getName()))
    enqueue(sec, 0);
}

// The .eh_frame section is an unfortunate special case.
// The section is divided in CIEs and FDEs and the relocations it can have are
// * CIEs can refer to a personality function.
// * FDEs can refer to a LSDA
// * FDEs refer to the function they contain information about
// The last kind of relocation cannot keep the referred section alive, or they
// would keep everything alive in a common object file. In fact, each FDE is
// alive if the section it refers to is alive.
// To keep things simple, in here we just ignore the last relocation kind. The
// other two keep the referred section alive.
//
// A possible improvement would be to fully process .eh_frame in the middle of
// the gc pass. With that we would be able to also gc some sections holding
// LSDAs and personality functions if we found that they were unused.
template <class ELFT>
template <class RelTy>
void MarkLive<ELFT>::scanEhFrameSection(EhInputSection &eh,
                                        ArrayRef<RelTy> rels) {
  for (size_t i = 0, end = eh.pieces.size(); i < end; ++i) {
    EhSectionPiece &piece = eh.pieces[i];
    size_t firstRelI = piece.firstRelocation;
    if (firstRelI == (unsigned)-1)
      continue;

    if (endian::read32<ELFT::TargetEndianness>(piece.data().data() + 4) == 0) {
      // This is a CIE, we only need to worry about the first relocation. It is
      // known to point to the personality function.
      resolveReloc(eh, rels[firstRelI], false);
      continue;
    }

    // This is a FDE. The relocations point to the described function or to
    // a LSDA. We only need to keep the LSDA alive, so ignore anything that
    // points to executable sections.
    uint64_t pieceEnd = piece.inputOff + piece.size;
    for (size_t j = firstRelI, end2 = rels.size(); j < end2; ++j)
      if (rels[j].r_offset < pieceEnd)
        resolveReloc(eh, rels[j], true);
  }
}

// Some sections are used directly by the loader, so they should never be
// garbage-collected. This function returns true if a given section is such
// section.
static bool isReserved(InputSectionBase *sec) {
  switch (sec->type) {
  case SHT_FINI_ARRAY:
  case SHT_INIT_ARRAY:
  case SHT_PREINIT_ARRAY:
    return true;
  case SHT_NOTE:
    // SHT_NOTE sections in a group are subject to garbage collection.
    return !sec->nextInSectionGroup;
  default:
    StringRef s = sec->name;
    return s.startswith(".ctors") || s.startswith(".dtors") ||
           s.startswith(".init") || s.startswith(".fini") ||
           s.startswith(".jcr");
  }
}

template <class ELFT>
void MarkLive<ELFT>::enqueue(InputSectionBase *sec, uint64_t offset) {
  // Skip over discarded sections. This in theory shouldn't happen, because
  // the ELF spec doesn't allow a relocation to point to a deduplicated
  // COMDAT section directly. Unfortunately this happens in practice (e.g.
  // .eh_frame) so we need to add a check.
  if (sec == &InputSection::discarded)
    return;

  // Usually, a whole section is marked as live or dead, but in mergeable
  // (splittable) sections, each piece of data has independent liveness bit.
  // So we explicitly tell it which offset is in use.
  if (auto *ms = dyn_cast<MergeInputSection>(sec))
    ms->getSectionPiece(offset)->live = true;

  // Set Sec->Partition to the meet (i.e. the "minimum") of Partition and
  // Sec->Partition in the following lattice: 1 < other < 0. If Sec->Partition
  // doesn't change, we don't need to do anything.
  if (sec->partition == 1 || sec->partition == partition)
    return;
  sec->partition = sec->partition ? 1 : partition;

  // Add input section to the queue.
  if (InputSection *s = dyn_cast<InputSection>(sec))
    queue.push_back(s);
}

template <class ELFT> void MarkLive<ELFT>::markSymbol(Symbol *sym) {
  if (auto *d = dyn_cast_or_null<Defined>(sym))
    if (auto *isec = dyn_cast_or_null<InputSectionBase>(d->section))
      enqueue(isec, d->value);
}

// This is the main function of the garbage collector.
// Starting from GC-root sections, this function visits all reachable
// sections to set their "Live" bits.
template <class ELFT> void MarkLive<ELFT>::run() {
  // Add GC root symbols.

  // Preserve externally-visible symbols if the symbols defined by this
  // file can interrupt other ELF file's symbols at runtime.
  for (Symbol *sym : symtab->symbols())
    if (sym->includeInDynsym() && sym->partition == partition)
      markSymbol(sym);

  // If this isn't the main partition, that's all that we need to preserve.
  if (partition != 1) {
    mark();
    return;
  }

  markSymbol(symtab->find(config->entry));
  markSymbol(symtab->find(config->init));
  markSymbol(symtab->find(config->fini));
  for (StringRef s : config->undefined)
    markSymbol(symtab->find(s));
  for (StringRef s : script->referencedSymbols)
    markSymbol(symtab->find(s));

  // Preserve special sections and those which are specified in linker
  // script KEEP command.
  for (InputSectionBase *sec : inputSections) {
    // Mark .eh_frame sections as live because there are usually no relocations
    // that point to .eh_frames. Otherwise, the garbage collector would drop
    // all of them. We also want to preserve personality routines and LSDA
    // referenced by .eh_frame sections, so we scan them for that here.
    if (auto *eh = dyn_cast<EhInputSection>(sec)) {
      eh->markLive();
      if (!eh->numRelocations)
        continue;

      if (eh->areRelocsRela)
        scanEhFrameSection(*eh, eh->template relas<ELFT>());
      else
        scanEhFrameSection(*eh, eh->template rels<ELFT>());
    }

    if (sec->flags & SHF_LINK_ORDER)
      continue;

    if (isReserved(sec) || script->shouldKeep(sec)) {
      enqueue(sec, 0);
    } else if (isValidCIdentifier(sec->name)) {
      cNamedSections[saver.save("__start_" + sec->name)].push_back(sec);
      cNamedSections[saver.save("__stop_" + sec->name)].push_back(sec);
    }
  }

  mark();
}

template <class ELFT> void MarkLive<ELFT>::mark() {
  // Mark all reachable sections.
  while (!queue.empty()) {
    InputSectionBase &sec = *queue.pop_back_val();

    if (sec.areRelocsRela) {
      for (const typename ELFT::Rela &rel : sec.template relas<ELFT>())
        resolveReloc(sec, rel, false);
    } else {
      for (const typename ELFT::Rel &rel : sec.template rels<ELFT>())
        resolveReloc(sec, rel, false);
    }

    for (InputSectionBase *isec : sec.dependentSections)
      enqueue(isec, 0);

    // Mark the next group member.
    if (sec.nextInSectionGroup)
      enqueue(sec.nextInSectionGroup, 0);
  }
}

// Move the sections for some symbols to the main partition, specifically ifuncs
// (because they can result in an IRELATIVE being added to the main partition's
// GOT, which means that the ifunc must be available when the main partition is
// loaded) and TLS symbols (because we only know how to correctly process TLS
// relocations for the main partition).
//
// We also need to move sections whose names are C identifiers that are referred
// to from __start_/__stop_ symbols because there will only be one set of
// symbols for the whole program.
template <class ELFT> void MarkLive<ELFT>::moveToMain() {
  for (InputFile *file : objectFiles)
    for (Symbol *s : file->getSymbols())
      if (auto *d = dyn_cast<Defined>(s))
        if ((d->type == STT_GNU_IFUNC || d->type == STT_TLS) && d->section &&
            d->section->isLive())
          markSymbol(s);

  for (InputSectionBase *sec : inputSections) {
    if (!sec->isLive() || !isValidCIdentifier(sec->name))
      continue;
    if (symtab->find(("__start_" + sec->name).str()) ||
        symtab->find(("__stop_" + sec->name).str()))
      enqueue(sec, 0);
  }

  mark();
}

// Before calling this function, Live bits are off for all
// input sections. This function make some or all of them on
// so that they are emitted to the output file.
template <class ELFT> void markLive() {
  // If -gc-sections is not given, no sections are removed.
  if (!config->gcSections) {
    for (InputSectionBase *sec : inputSections)
      sec->markLive();

    // If a DSO defines a symbol referenced in a regular object, it is needed.
    for (Symbol *sym : symtab->symbols())
      if (auto *s = dyn_cast<SharedSymbol>(sym))
        if (s->isUsedInRegularObj && !s->isWeak())
          s->getFile().isNeeded = true;
    return;
  }

  // Otherwise, do mark-sweep GC.
  //
  // The -gc-sections option works only for SHF_ALLOC sections
  // (sections that are memory-mapped at runtime). So we can
  // unconditionally make non-SHF_ALLOC sections alive except
  // SHF_LINK_ORDER and SHT_REL/SHT_RELA sections.
  //
  // Usually, non-SHF_ALLOC sections are not removed even if they are
  // unreachable through relocations because reachability is not
  // a good signal whether they are garbage or not (e.g. there is
  // usually no section referring to a .comment section, but we
  // want to keep it.).
  //
  // Note on SHF_LINK_ORDER: Such sections contain metadata and they
  // have a reverse dependency on the InputSection they are linked with.
  // We are able to garbage collect them.
  //
  // Note on SHF_REL{,A}: Such sections reach here only when -r
  // or -emit-reloc were given. And they are subject of garbage
  // collection because, if we remove a text section, we also
  // remove its relocation section.
  //
  // Note on nextInSectionGroup: The ELF spec says that group sections are
  // included or omitted as a unit. We take the interpretation that:
  //
  // - Group members (nextInSectionGroup != nullptr) are subject to garbage
  //   collection.
  // - Groups members are retained or discarded as a unit.
  for (InputSectionBase *sec : inputSections) {
    bool isAlloc = (sec->flags & SHF_ALLOC);
    bool isLinkOrder = (sec->flags & SHF_LINK_ORDER);
    bool isRel = (sec->type == SHT_REL || sec->type == SHT_RELA);

    if (!isAlloc && !isLinkOrder && !isRel && !sec->nextInSectionGroup)
      sec->markLive();
  }

  // Follow the graph to mark all live sections.
  for (unsigned curPart = 1; curPart <= partitions.size(); ++curPart)
    MarkLive<ELFT>(curPart).run();

  // If we have multiple partitions, some sections need to live in the main
  // partition even if they were allocated to a loadable partition. Move them
  // there now.
  if (partitions.size() != 1)
    MarkLive<ELFT>(1).moveToMain();

  // Report garbage-collected sections.
  if (config->printGcSections)
    for (InputSectionBase *sec : inputSections)
      if (!sec->isLive())
        message("removing unused section " + toString(sec));
}

template void markLive<ELF32LE>();
template void markLive<ELF32BE>();
template void markLive<ELF64LE>();
template void markLive<ELF64BE>();

} // namespace elf
} // namespace lld