ELF/OutputSections.h

//===- OutputSections.h -----------------------------------------*- C++ -*-===//
//
// 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
//
//===----------------------------------------------------------------------===//

#ifndef LLD_ELF_OUTPUT_SECTIONS_H
#define LLD_ELF_OUTPUT_SECTIONS_H

#include "InputSection.h"
#include "LinkerScript.h"
#include "lld/Common/LLVM.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Parallel.h"

#include <array>

namespace lld::elf {

struct PhdrEntry;

struct CompressedData {
  std::unique_ptr<SmallVector<uint8_t, 0>[]> shards;
  uint32_t type = 0;
  uint32_t numShards = 0;
  uint32_t checksum = 0;
  uint64_t uncompressedSize;
};

// This represents a section in an output file.
// It is composed of multiple InputSections.
// The writer creates multiple OutputSections and assign them unique,
// non-overlapping file offsets and VAs.
class OutputSection final : public SectionBase {
public:
  OutputSection(StringRef name, uint32_t type, uint64_t flags);

  static bool classof(const SectionBase *s) {
    return s->kind() == SectionBase::Output;
  }

  uint64_t getLMA() const { return ptLoad ? addr + ptLoad->lmaOffset : addr; }
  template <typename ELFT> void writeHeaderTo(typename ELFT::Shdr *sHdr);

  uint32_t sectionIndex = UINT32_MAX;
  unsigned sortRank;

  uint32_t getPhdrFlags() const;

  // Pointer to the PT_LOAD segment, which this section resides in. This field
  // is used to correctly compute file offset of a section. When two sections
  // share the same load segment, difference between their file offsets should
  // be equal to difference between their virtual addresses. To compute some
  // section offset we use the following formula: Off = Off_first + VA -
  // VA_first, where Off_first and VA_first is file offset and VA of first
  // section in PT_LOAD.
  PhdrEntry *ptLoad = nullptr;

  // Pointer to a relocation section for this section. Usually nullptr because
  // we consume relocations, but if --emit-relocs is specified (which is rare),
  // it may have a non-null value.
  OutputSection *relocationSection = nullptr;

  // Initially this field is the number of InputSections that have been added to
  // the OutputSection so far. Later on, after a call to assignAddresses, it
  // corresponds to the Elf_Shdr member.
  uint64_t size = 0;

  // The following fields correspond to Elf_Shdr members.
  uint64_t offset = 0;
  uint64_t addr = 0;
  uint32_t shName = 0;

  void recordSection(InputSectionBase *isec);
  void commitSection(InputSection *isec);
  void finalizeInputSections();

  // The following members are normally only used in linker scripts.
  MemoryRegion *memRegion = nullptr;
  MemoryRegion *lmaRegion = nullptr;
  Expr addrExpr;
  Expr alignExpr;
  Expr lmaExpr;
  Expr subalignExpr;
  SmallVector<SectionCommand *, 0> commands;
  SmallVector<StringRef, 0> phdrs;
  std::optional<std::array<uint8_t, 4>> filler;
  ConstraintKind constraint = ConstraintKind::NoConstraint;
  std::string location;
  std::string memoryRegionName;
  std::string lmaRegionName;
  bool nonAlloc = false;
  bool typeIsSet = false;
  bool expressionsUseSymbols = false;
  bool usedInExpression = false;
  bool inOverlay = false;

  // Tracks whether the section has ever had an input section added to it, even
  // if the section was later removed (e.g. because it is a synthetic section
  // that wasn't needed). This is needed for orphan placement.
  bool hasInputSections = false;

  // The output section description is specified between DATA_SEGMENT_ALIGN and
  // DATA_RELRO_END.
  bool relro = false;

  void finalize();
  template <class ELFT>
  void writeTo(uint8_t *buf, llvm::parallel::TaskGroup &tg);
  // Check that the addends for dynamic relocations were written correctly.
  void checkDynRelAddends(const uint8_t *bufStart);
  template <class ELFT> void maybeCompress();

  void sort(llvm::function_ref<int(InputSectionBase *s)> order);
  void sortInitFini();
  void sortCtorsDtors();

  // Used for implementation of --compress-debug-sections and
  // --compress-sections.
  CompressedData compressed;

private:
  SmallVector<InputSection *, 0> storage;

  std::array<uint8_t, 4> getFiller();
};

struct OutputDesc final : SectionCommand {
  OutputSection osec;
  OutputDesc(StringRef name, uint32_t type, uint64_t flags)
      : SectionCommand(OutputSectionKind), osec(name, type, flags) {}

  static bool classof(const SectionCommand *c) {
    return c->kind == OutputSectionKind;
  }
};

int getPriority(StringRef s);

InputSection *getFirstInputSection(const OutputSection *os);
llvm::ArrayRef<InputSection *>
getInputSections(const OutputSection &os,
                 SmallVector<InputSection *, 0> &storage);

// All output sections that are handled by the linker specially are
// globally accessible. Writer initializes them, so don't use them
// until Writer is initialized.
struct Out {
  static uint8_t *bufferStart;
  static PhdrEntry *tlsPhdr;
  static OutputSection *elfHeader;
  static OutputSection *programHeaders;
  static OutputSection *preinitArray;
  static OutputSection *initArray;
  static OutputSection *finiArray;
};

uint64_t getHeaderSize();

LLVM_LIBRARY_VISIBILITY extern llvm::SmallVector<OutputSection *, 0>
    outputSections;
} // namespace lld::elf

#endif