llvm/lld/ELF/SyntheticSections.h

//===- SyntheticSection.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
//
//===----------------------------------------------------------------------===//
//
// Synthetic sections represent chunks of linker-created data. If you
// need to create a chunk of data that to be included in some section
// in the result, you probably want to create that as a synthetic section.
//
// Synthetic sections are designed as input sections as opposed to
// output sections because we want to allow them to be manipulated
// using linker scripts just like other input sections from regular
// files.
//
//===----------------------------------------------------------------------===//

#ifndef LLD_ELF_SYNTHETIC_SECTIONS_H
#define LLD_ELF_SYNTHETIC_SECTIONS_H

#include "Config.h"
#include "DWARF.h"
#include "InputSection.h"
#include "Symbols.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/FoldingSet.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/STLFunctionalExtras.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/DebugInfo/DWARF/DWARFAcceleratorTable.h"
#include "llvm/MC/StringTableBuilder.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/Parallel.h"
#include "llvm/Support/Threading.h"

namespace lld::elf {
class Defined;
struct PhdrEntry;
class SymbolTableBaseSection;

struct CieRecord { … };

// Section for .eh_frame.
class EhFrameSection final : public SyntheticSection { … };

class GotSection final : public SyntheticSection { … };

// .note.GNU-stack section.
class GnuStackSection : public SyntheticSection { … };

class GnuPropertySection final : public SyntheticSection { … };

// .note.gnu.build-id section.
class BuildIdSection : public SyntheticSection { … };

// BssSection is used to reserve space for copy relocations and common symbols.
// We create three instances of this class for .bss, .bss.rel.ro and "COMMON",
// that are used for writable symbols, read-only symbols and common symbols,
// respectively.
class BssSection final : public SyntheticSection { … };

class MipsGotSection final : public SyntheticSection { … };

class GotPltSection final : public SyntheticSection { … };

// The IgotPltSection is a Got associated with the PltSection for GNU Ifunc
// Symbols that will be relocated by Target->IRelativeRel.
// On most Targets the IgotPltSection will immediately follow the GotPltSection
// on ARM the IgotPltSection will immediately follow the GotSection.
class IgotPltSection final : public SyntheticSection { … };

class StringTableSection final : public SyntheticSection { … };

class DynamicReloc { … };

template <class ELFT> class DynamicSection final : public SyntheticSection { … };

class RelocationBaseSection : public SyntheticSection { … };

template <>
inline void RelocationBaseSection::addReloc<true>(const DynamicReloc &reloc) { … }

template <class ELFT>
class RelocationSection final : public RelocationBaseSection { … };

template <class ELFT>
class AndroidPackedRelocationSection final : public RelocationBaseSection { … };

struct RelativeReloc { … };

class RelrBaseSection : public SyntheticSection { … };

// RelrSection is used to encode offsets for relative relocations.
// Proposal for adding SHT_RELR sections to generic-abi is here:
//   https://groups.google.com/forum/#!topic/generic-abi/bX460iggiKg
// For more details, see the comment in RelrSection::updateAllocSize(Ctx &ctx).
template <class ELFT> class RelrSection final : public RelrBaseSection { … };

struct SymbolTableEntry { … };

class SymbolTableBaseSection : public SyntheticSection { … };

template <class ELFT>
class SymbolTableSection final : public SymbolTableBaseSection { … };

class SymtabShndxSection final : public SyntheticSection { … };

// Outputs GNU Hash section. For detailed explanation see:
// https://blogs.oracle.com/ali/entry/gnu_hash_elf_sections
class GnuHashTableSection final : public SyntheticSection { … };

class HashTableSection final : public SyntheticSection { … };

// Used for PLT entries. It usually has a PLT header for lazy binding. Each PLT
// entry is associated with a JUMP_SLOT relocation, which may be resolved lazily
// at runtime.
//
// On PowerPC, this section contains lazy symbol resolvers. A branch instruction
// jumps to a PLT call stub, which will then jump to the target (BIND_NOW) or a
// lazy symbol resolver.
//
// On x86 when IBT is enabled, this section (.plt.sec) contains PLT call stubs.
// A call instruction jumps to a .plt.sec entry, which will then jump to the
// target (BIND_NOW) or a .plt entry.
class PltSection : public SyntheticSection { … };

// Used for non-preemptible ifuncs. It does not have a header. Each entry is
// associated with an IRELATIVE relocation, which will be resolved eagerly at
// runtime. PltSection can only contain entries associated with JUMP_SLOT
// relocations, so IPLT entries are in a separate section.
class IpltSection final : public SyntheticSection { … };

class PPC32GlinkSection : public PltSection { … };

// This is x86-only.
class IBTPltSection : public SyntheticSection { … };

// Used to align the end of the PT_GNU_RELRO segment and the associated PT_LOAD
// segment to a common-page-size boundary. This padding section ensures that all
// pages in the PT_LOAD segment is covered by at least one section.
class RelroPaddingSection final : public SyntheticSection { … };

// Used by the merged DWARF32 .debug_names (a per-module index). If we
// move to DWARF64, most of this data will need to be re-sized.
class DebugNamesBaseSection : public SyntheticSection { … };

// Complement DebugNamesBaseSection for ELFT-aware code: reading offsets,
// relocating string offsets, and writeTo.
template <class ELFT>
class DebugNamesSection final : public DebugNamesBaseSection { … };

class GdbIndexSection final : public SyntheticSection { … };

// --eh-frame-hdr option tells linker to construct a header for all the
// .eh_frame sections. This header is placed to a section named .eh_frame_hdr
// and also to a PT_GNU_EH_FRAME segment.
// At runtime the unwinder then can find all the PT_GNU_EH_FRAME segments by
// calling dl_iterate_phdr.
// This section contains a lookup table for quick binary search of FDEs.
// Detailed info about internals can be found in Ian Lance Taylor's blog:
// http://www.airs.com/blog/archives/460 (".eh_frame")
// http://www.airs.com/blog/archives/462 (".eh_frame_hdr")
class EhFrameHeader final : public SyntheticSection { … };

// For more information about .gnu.version and .gnu.version_r see:
// https://www.akkadia.org/drepper/symbol-versioning

// The .gnu.version_d section which has a section type of SHT_GNU_verdef shall
// contain symbol version definitions. The number of entries in this section
// shall be contained in the DT_VERDEFNUM entry of the .dynamic section.
// The section shall contain an array of Elf_Verdef structures, optionally
// followed by an array of Elf_Verdaux structures.
class VersionDefinitionSection final : public SyntheticSection { … };

// The .gnu.version section specifies the required version of each symbol in the
// dynamic symbol table. It contains one Elf_Versym for each dynamic symbol
// table entry. An Elf_Versym is just a 16-bit integer that refers to a version
// identifier defined in the either .gnu.version_r or .gnu.version_d section.
// The values 0 and 1 are reserved. All other values are used for versions in
// the own object or in any of the dependencies.
class VersionTableSection final : public SyntheticSection { … };

// The .gnu.version_r section defines the version identifiers used by
// .gnu.version. It contains a linked list of Elf_Verneed data structures. Each
// Elf_Verneed specifies the version requirements for a single DSO, and contains
// a reference to a linked list of Elf_Vernaux data structures which define the
// mapping from version identifiers to version names.
template <class ELFT>
class VersionNeedSection final : public SyntheticSection { … };

// MergeSyntheticSection is a class that allows us to put mergeable sections
// with different attributes in a single output sections. To do that
// we put them into MergeSyntheticSection synthetic input sections which are
// attached to regular output sections.
class MergeSyntheticSection : public SyntheticSection { … };

class MergeTailSection final : public MergeSyntheticSection { … };

class MergeNoTailSection final : public MergeSyntheticSection { … };

// .MIPS.abiflags section.
template <class ELFT>
class MipsAbiFlagsSection final : public SyntheticSection { … };

// .MIPS.options section.
template <class ELFT> class MipsOptionsSection final : public SyntheticSection { … };

// MIPS .reginfo section.
template <class ELFT> class MipsReginfoSection final : public SyntheticSection { … };

// This is a MIPS specific section to hold a space within the data segment
// of executable file which is pointed to by the DT_MIPS_RLD_MAP entry.
// See "Dynamic section" in Chapter 5 in the following document:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
class MipsRldMapSection final : public SyntheticSection { … };

// Representation of the combined .ARM.Exidx input sections. We process these
// as a SyntheticSection like .eh_frame as we need to merge duplicate entries
// and add terminating sentinel entries.
//
// The .ARM.exidx input sections after SHF_LINK_ORDER processing is done form
// a table that the unwinder can derive (Addresses are encoded as offsets from
// table):
// | Address of function | Unwind instructions for function |
// where the unwind instructions are either a small number of unwind or the
// special EXIDX_CANTUNWIND entry representing no unwinding information.
// When an exception is thrown from an address A, the unwinder searches the
// table for the closest table entry with Address of function <= A. This means
// that for two consecutive table entries:
// | A1 | U1 |
// | A2 | U2 |
// The range of addresses described by U1 is [A1, A2)
//
// There are two cases where we need a linker generated table entry to fixup
// the address ranges in the table
// Case 1:
// - A sentinel entry added with an address higher than all
// executable sections. This was needed to work around libunwind bug pr31091.
// - After address assignment we need to find the highest addressed executable
// section and use the limit of that section so that the unwinder never
// matches it.
// Case 2:
// - InputSections without a .ARM.exidx section (usually from Assembly)
// need a table entry so that they terminate the range of the previously
// function. This is pr40277.
//
// Instead of storing pointers to the .ARM.exidx InputSections from
// InputObjects, we store pointers to the executable sections that need
// .ARM.exidx sections. We can then use the dependentSections of these to
// either find the .ARM.exidx section or know that we need to generate one.
class ARMExidxSyntheticSection : public SyntheticSection { … };

// A container for one or more linker generated thunks. Instances of these
// thunks including ARM interworking and Mips LA25 PI to non-PI thunks.
class ThunkSection final : public SyntheticSection { … };

// Cortex-M Security Extensions. Prefix for functions that should be exported
// for the non-secure world.
const char ACLESESYM_PREFIX[] = …;
const int ACLESESYM_SIZE = …;

class ArmCmseSGVeneer;

class ArmCmseSGSection final : public SyntheticSection { … };

// Used to compute outSecOff of .got2 in each object file. This is needed to
// synthesize PLT entries for PPC32 Secure PLT ABI.
class PPC32Got2Section final : public SyntheticSection { … };

// This section is used to store the addresses of functions that are called
// in range-extending thunks on PowerPC64. When producing position dependent
// code the addresses are link-time constants and the table is written out to
// the binary. When producing position-dependent code the table is allocated and
// filled in by the dynamic linker.
class PPC64LongBranchTargetSection final : public SyntheticSection { … };

template <typename ELFT>
class PartitionElfHeaderSection final : public SyntheticSection { … };

template <typename ELFT>
class PartitionProgramHeadersSection final : public SyntheticSection { … };

class PartitionIndexSection final : public SyntheticSection { … };

// See the following link for the Android-specific loader code that operates on
// this section:
// https://cs.android.com/android/platform/superproject/+/master:bionic/libc/bionic/libc_init_static.cpp;drc=9425b16978f9c5aa8f2c50c873db470819480d1d;l=192
class MemtagAndroidNote final : public SyntheticSection { … };

class PackageMetadataNote final : public SyntheticSection { … };

class MemtagGlobalDescriptors final : public SyntheticSection { … };

template <class ELFT> void createSyntheticSections(Ctx &);
InputSection *createInterpSection(Ctx &);
MergeInputSection *createCommentSection();
template <class ELFT> void splitSections(Ctx &);
void combineEhSections(Ctx &);

bool hasMemtag(Ctx &);
bool canHaveMemtagGlobals(Ctx &);

template <typename ELFT> void writeEhdr(uint8_t *buf, Partition &part);
template <typename ELFT> void writePhdrs(uint8_t *buf, Partition &part);

Defined *addSyntheticLocal(StringRef name, uint8_t type, uint64_t value,
                           uint64_t size, InputSectionBase &section);

void addVerneed(Symbol *ss);

// Linker generated per-partition sections.
struct Partition { … };

inline Partition &SectionBase::getPartition() const { … }

} // namespace lld::elf

#endif