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llvm-project/lld/COFF/Chunks.cpp

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//===- Chunks.cpp ---------------------------------------------------------===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
#include "Chunks.h"
#include "Error.h"
#include "InputFiles.h"
#include "Symbols.h"
#include "Writer.h"
#include "llvm/ADT/Twine.h"
#include "llvm/BinaryFormat/COFF.h"
#include "llvm/Object/COFF.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
using namespace llvm;
using namespace llvm::object;
using namespace llvm::support::endian;
using namespace llvm::COFF;
using llvm::support::ulittle32_t;
namespace lld {
namespace coff {
SectionChunk::SectionChunk(ObjFile *F, const coff_section *H)
: Chunk(SectionKind), Repl(this), Header(H), File(F),
Relocs(File->getCOFFObj()->getRelocations(Header)),
NumRelocs(std::distance(Relocs.begin(), Relocs.end())) {
// Initialize SectionName.
File->getCOFFObj()->getSectionName(Header, SectionName);
Alignment = Header->getAlignment();
// Chunks may be discarded during comdat merging.
Discarded = false;
// If linker GC is disabled, every chunk starts out alive. If linker GC is
// enabled, treat non-comdat sections as roots. Generally optimized object
// files will be built with -ffunction-sections or /Gy, so most things worth
// stripping will be in a comdat.
Live = !Config->DoGC || !isCOMDAT();
}
static void add16(uint8_t *P, int16_t V) { write16le(P, read16le(P) + V); }
static void add32(uint8_t *P, int32_t V) { write32le(P, read32le(P) + V); }
static void add64(uint8_t *P, int64_t V) { write64le(P, read64le(P) + V); }
static void or16(uint8_t *P, uint16_t V) { write16le(P, read16le(P) | V); }
static void or32(uint8_t *P, uint32_t V) { write32le(P, read32le(P) | V); }
static void applySecRel(const SectionChunk *Sec, uint8_t *Off,
OutputSection *OS, uint64_t S) {
if (!OS) {
if (Sec->isCodeView())
return;
fatal("SECREL relocation cannot be applied to absolute symbols");
}
uint64_t SecRel = S - OS->getRVA();
assert(SecRel < INT32_MAX && "overflow in SECREL relocation");
add32(Off, SecRel);
}
static void applySecIdx(uint8_t *Off, OutputSection *OS) {
// If we have no output section, this must be an absolute symbol. Use the
// sentinel absolute symbol section index.
uint16_t SecIdx = OS ? OS->SectionIndex : DefinedAbsolute::OutputSectionIndex;
add16(Off, SecIdx);
}
void SectionChunk::applyRelX64(uint8_t *Off, uint16_t Type, OutputSection *OS,
uint64_t S, uint64_t P) const {
switch (Type) {
case IMAGE_REL_AMD64_ADDR32: add32(Off, S + Config->ImageBase); break;
case IMAGE_REL_AMD64_ADDR64: add64(Off, S + Config->ImageBase); break;
case IMAGE_REL_AMD64_ADDR32NB: add32(Off, S); break;
case IMAGE_REL_AMD64_REL32: add32(Off, S - P - 4); break;
case IMAGE_REL_AMD64_REL32_1: add32(Off, S - P - 5); break;
case IMAGE_REL_AMD64_REL32_2: add32(Off, S - P - 6); break;
case IMAGE_REL_AMD64_REL32_3: add32(Off, S - P - 7); break;
case IMAGE_REL_AMD64_REL32_4: add32(Off, S - P - 8); break;
case IMAGE_REL_AMD64_REL32_5: add32(Off, S - P - 9); break;
case IMAGE_REL_AMD64_SECTION: applySecIdx(Off, OS); break;
case IMAGE_REL_AMD64_SECREL: applySecRel(this, Off, OS, S); break;
default:
fatal("unsupported relocation type 0x" + Twine::utohexstr(Type));
}
}
void SectionChunk::applyRelX86(uint8_t *Off, uint16_t Type, OutputSection *OS,
uint64_t S, uint64_t P) const {
switch (Type) {
case IMAGE_REL_I386_ABSOLUTE: break;
case IMAGE_REL_I386_DIR32: add32(Off, S + Config->ImageBase); break;
case IMAGE_REL_I386_DIR32NB: add32(Off, S); break;
case IMAGE_REL_I386_REL32: add32(Off, S - P - 4); break;
case IMAGE_REL_I386_SECTION: applySecIdx(Off, OS); break;
case IMAGE_REL_I386_SECREL: applySecRel(this, Off, OS, S); break;
default:
fatal("unsupported relocation type 0x" + Twine::utohexstr(Type));
}
}
static void applyMOV(uint8_t *Off, uint16_t V) {
write16le(Off, (read16le(Off) & 0xfbf0) | ((V & 0x800) >> 1) | ((V >> 12) & 0xf));
write16le(Off + 2, (read16le(Off + 2) & 0x8f00) | ((V & 0x700) << 4) | (V & 0xff));
}
static uint16_t readMOV(uint8_t *Off) {
uint16_t Opcode1 = read16le(Off);
uint16_t Opcode2 = read16le(Off + 2);
uint16_t Imm = (Opcode2 & 0x00ff) | ((Opcode2 >> 4) & 0x0700);
Imm |= ((Opcode1 << 1) & 0x0800) | ((Opcode1 & 0x000f) << 12);
return Imm;
}
void applyMOV32T(uint8_t *Off, uint32_t V) {
uint16_t ImmW = readMOV(Off); // read MOVW operand
uint16_t ImmT = readMOV(Off + 4); // read MOVT operand
uint32_t Imm = ImmW | (ImmT << 16);
V += Imm; // add the immediate offset
applyMOV(Off, V); // set MOVW operand
applyMOV(Off + 4, V >> 16); // set MOVT operand
}
static void applyBranch20T(uint8_t *Off, int32_t V) {
uint32_t S = V < 0 ? 1 : 0;
uint32_t J1 = (V >> 19) & 1;
uint32_t J2 = (V >> 18) & 1;
or16(Off, (S << 10) | ((V >> 12) & 0x3f));
or16(Off + 2, (J1 << 13) | (J2 << 11) | ((V >> 1) & 0x7ff));
}
void applyBranch24T(uint8_t *Off, int32_t V) {
if (!isInt<25>(V))
fatal("relocation out of range");
uint32_t S = V < 0 ? 1 : 0;
uint32_t J1 = ((~V >> 23) & 1) ^ S;
uint32_t J2 = ((~V >> 22) & 1) ^ S;
or16(Off, (S << 10) | ((V >> 12) & 0x3ff));
// Clear out the J1 and J2 bits which may be set.
write16le(Off + 2, (read16le(Off + 2) & 0xd000) | (J1 << 13) | (J2 << 11) | ((V >> 1) & 0x7ff));
}
void SectionChunk::applyRelARM(uint8_t *Off, uint16_t Type, OutputSection *OS,
uint64_t S, uint64_t P) const {
// Pointer to thumb code must have the LSB set.
uint64_t SX = S;
if (OS && (OS->getPermissions() & IMAGE_SCN_MEM_EXECUTE))
SX |= 1;
switch (Type) {
case IMAGE_REL_ARM_ADDR32: add32(Off, SX + Config->ImageBase); break;
case IMAGE_REL_ARM_ADDR32NB: add32(Off, SX); break;
case IMAGE_REL_ARM_MOV32T: applyMOV32T(Off, SX + Config->ImageBase); break;
case IMAGE_REL_ARM_BRANCH20T: applyBranch20T(Off, SX - P - 4); break;
case IMAGE_REL_ARM_BRANCH24T: applyBranch24T(Off, SX - P - 4); break;
case IMAGE_REL_ARM_BLX23T: applyBranch24T(Off, SX - P - 4); break;
case IMAGE_REL_ARM_SECTION: applySecIdx(Off, OS); break;
case IMAGE_REL_ARM_SECREL: applySecRel(this, Off, OS, S); break;
default:
fatal("unsupported relocation type 0x" + Twine::utohexstr(Type));
}
}
// Interpret the existing immediate value as a byte offset to the
// target symbol, then update the instruction with the immediate as
// the page offset from the current instruction to the target.
static void applyArm64Addr(uint8_t *Off, uint64_t S, uint64_t P) {
uint32_t Orig = read32le(Off);
uint64_t Imm = ((Orig >> 29) & 0x3) | ((Orig >> 3) & 0x1FFFFC);
S += Imm;
Imm = (S >> 12) - (P >> 12);
uint32_t ImmLo = (Imm & 0x3) << 29;
uint32_t ImmHi = (Imm & 0x1FFFFC) << 3;
uint64_t Mask = (0x3 << 29) | (0x1FFFFC << 3);
write32le(Off, (Orig & ~Mask) | ImmLo | ImmHi);
}
// Update the immediate field in a AARCH64 ldr, str, and add instruction.
// Optionally limit the range of the written immediate by one or more bits
// (RangeLimit).
static void applyArm64Imm(uint8_t *Off, uint64_t Imm, uint32_t RangeLimit) {
uint32_t Orig = read32le(Off);
Imm += (Orig >> 10) & 0xFFF;
Orig &= ~(0xFFF << 10);
write32le(Off, Orig | ((Imm & (0xFFF >> RangeLimit)) << 10));
}
// Add the 12 bit page offset to the existing immediate.
// Ldr/str instructions store the opcode immediate scaled
// by the load/store size (giving a larger range for larger
// loads/stores). The immediate is always (both before and after
// fixing up the relocation) stored scaled similarly.
// Even if larger loads/stores have a larger range, limit the
// effective offset to 12 bit, since it is intended to be a
// page offset.
static void applyArm64Ldr(uint8_t *Off, uint64_t Imm) {
uint32_t Orig = read32le(Off);
uint32_t Size = Orig >> 30;
// 0x04000000 indicates SIMD/FP registers
// 0x00800000 indicates 128 bit
if ((Orig & 0x4800000) == 0x4800000)
Size += 4;
if ((Imm & ((1 << Size) - 1)) != 0)
fatal("misaligned ldr/str offset");
applyArm64Imm(Off, Imm >> Size, Size);
}
void SectionChunk::applyRelARM64(uint8_t *Off, uint16_t Type, OutputSection *OS,
uint64_t S, uint64_t P) const {
switch (Type) {
case IMAGE_REL_ARM64_PAGEBASE_REL21: applyArm64Addr(Off, S, P); break;
case IMAGE_REL_ARM64_PAGEOFFSET_12A: applyArm64Imm(Off, S & 0xfff, 0); break;
case IMAGE_REL_ARM64_PAGEOFFSET_12L: applyArm64Ldr(Off, S & 0xfff); break;
case IMAGE_REL_ARM64_BRANCH26: or32(Off, ((S - P) & 0x0FFFFFFC) >> 2); break;
case IMAGE_REL_ARM64_ADDR32: add32(Off, S + Config->ImageBase); break;
case IMAGE_REL_ARM64_ADDR64: add64(Off, S + Config->ImageBase); break;
default:
fatal("unsupported relocation type 0x" + Twine::utohexstr(Type));
}
}
void SectionChunk::writeTo(uint8_t *Buf) const {
if (!hasData())
return;
// Copy section contents from source object file to output file.
ArrayRef<uint8_t> A = getContents();
memcpy(Buf + OutputSectionOff, A.data(), A.size());
// Apply relocations.
size_t InputSize = getSize();
for (const coff_relocation &Rel : Relocs) {
// Check for an invalid relocation offset. This check isn't perfect, because
// we don't have the relocation size, which is only known after checking the
// machine and relocation type. As a result, a relocation may overwrite the
// beginning of the following input section.
if (Rel.VirtualAddress >= InputSize)
fatal("relocation points beyond the end of its parent section");
uint8_t *Off = Buf + OutputSectionOff + Rel.VirtualAddress;
// Get the output section of the symbol for this relocation. The output
// section is needed to compute SECREL and SECTION relocations used in debug
// info.
SymbolBody *Body = File->getSymbolBody(Rel.SymbolTableIndex);
Defined *Sym = cast<Defined>(Body);
Chunk *C = Sym->getChunk();
OutputSection *OS = C ? C->getOutputSection() : nullptr;
// Only absolute and __ImageBase symbols lack an output section. For any
// other symbol, this indicates that the chunk was discarded. Normally
// relocations against discarded sections are an error. However, debug info
// sections are not GC roots and can end up with these kinds of relocations.
// Skip these relocations.
if (!OS && !isa<DefinedAbsolute>(Sym) && !isa<DefinedSynthetic>(Sym)) {
if (isCodeView() || isDWARF())
continue;
fatal("relocation against symbol in discarded section: " +
Sym->getName());
}
uint64_t S = Sym->getRVA();
// Compute the RVA of the relocation for relative relocations.
uint64_t P = RVA + Rel.VirtualAddress;
switch (Config->Machine) {
case AMD64:
applyRelX64(Off, Rel.Type, OS, S, P);
break;
case I386:
applyRelX86(Off, Rel.Type, OS, S, P);
break;
case ARMNT:
applyRelARM(Off, Rel.Type, OS, S, P);
break;
case ARM64:
applyRelARM64(Off, Rel.Type, OS, S, P);
break;
default:
llvm_unreachable("unknown machine type");
}
}
}
void SectionChunk::addAssociative(SectionChunk *Child) {
AssocChildren.push_back(Child);
}
static uint8_t getBaserelType(const coff_relocation &Rel) {
switch (Config->Machine) {
case AMD64:
if (Rel.Type == IMAGE_REL_AMD64_ADDR64)
return IMAGE_REL_BASED_DIR64;
return IMAGE_REL_BASED_ABSOLUTE;
case I386:
if (Rel.Type == IMAGE_REL_I386_DIR32)
return IMAGE_REL_BASED_HIGHLOW;
return IMAGE_REL_BASED_ABSOLUTE;
case ARMNT:
if (Rel.Type == IMAGE_REL_ARM_ADDR32)
return IMAGE_REL_BASED_HIGHLOW;
if (Rel.Type == IMAGE_REL_ARM_MOV32T)
return IMAGE_REL_BASED_ARM_MOV32T;
return IMAGE_REL_BASED_ABSOLUTE;
case ARM64:
if (Rel.Type == IMAGE_REL_ARM64_ADDR64)
return IMAGE_REL_BASED_DIR64;
return IMAGE_REL_BASED_ABSOLUTE;
default:
llvm_unreachable("unknown machine type");
}
}
// Windows-specific.
// Collect all locations that contain absolute addresses, which need to be
// fixed by the loader if load-time relocation is needed.
// Only called when base relocation is enabled.
void SectionChunk::getBaserels(std::vector<Baserel> *Res) {
for (const coff_relocation &Rel : Relocs) {
uint8_t Ty = getBaserelType(Rel);
if (Ty == IMAGE_REL_BASED_ABSOLUTE)
continue;
SymbolBody *Body = File->getSymbolBody(Rel.SymbolTableIndex);
if (isa<DefinedAbsolute>(Body))
continue;
Res->emplace_back(RVA + Rel.VirtualAddress, Ty);
}
}
bool SectionChunk::hasData() const {
return !(Header->Characteristics & IMAGE_SCN_CNT_UNINITIALIZED_DATA);
}
uint32_t SectionChunk::getPermissions() const {
return Header->Characteristics & PermMask;
}
bool SectionChunk::isCOMDAT() const {
return Header->Characteristics & IMAGE_SCN_LNK_COMDAT;
}
void SectionChunk::printDiscardedMessage() const {
// Removed by dead-stripping. If it's removed by ICF, ICF already
// printed out the name, so don't repeat that here.
if (Sym && this == Repl) {
if (Discarded)
message("Discarded comdat symbol " + Sym->getName());
else if (!Live)
message("Discarded " + Sym->getName());
}
}
StringRef SectionChunk::getDebugName() {
if (Sym)
return Sym->getName();
return "";
}
ArrayRef<uint8_t> SectionChunk::getContents() const {
ArrayRef<uint8_t> A;
File->getCOFFObj()->getSectionContents(Header, A);
return A;
}
void SectionChunk::replace(SectionChunk *Other) {
Other->Repl = Repl;
Other->Live = false;
}
CommonChunk::CommonChunk(const COFFSymbolRef S) : Sym(S) {
// Common symbols are aligned on natural boundaries up to 32 bytes.
// This is what MSVC link.exe does.
Alignment = std::min(uint64_t(32), PowerOf2Ceil(Sym.getValue()));
}
uint32_t CommonChunk::getPermissions() const {
return IMAGE_SCN_CNT_UNINITIALIZED_DATA | IMAGE_SCN_MEM_READ |
IMAGE_SCN_MEM_WRITE;
}
void StringChunk::writeTo(uint8_t *Buf) const {
memcpy(Buf + OutputSectionOff, Str.data(), Str.size());
}
ImportThunkChunkX64::ImportThunkChunkX64(Defined *S) : ImpSymbol(S) {
// Intel Optimization Manual says that all branch targets
// should be 16-byte aligned. MSVC linker does this too.
Alignment = 16;
}
void ImportThunkChunkX64::writeTo(uint8_t *Buf) const {
memcpy(Buf + OutputSectionOff, ImportThunkX86, sizeof(ImportThunkX86));
// The first two bytes is a JMP instruction. Fill its operand.
write32le(Buf + OutputSectionOff + 2, ImpSymbol->getRVA() - RVA - getSize());
}
void ImportThunkChunkX86::getBaserels(std::vector<Baserel> *Res) {
Res->emplace_back(getRVA() + 2);
}
void ImportThunkChunkX86::writeTo(uint8_t *Buf) const {
memcpy(Buf + OutputSectionOff, ImportThunkX86, sizeof(ImportThunkX86));
// The first two bytes is a JMP instruction. Fill its operand.
write32le(Buf + OutputSectionOff + 2,
ImpSymbol->getRVA() + Config->ImageBase);
}
void ImportThunkChunkARM::getBaserels(std::vector<Baserel> *Res) {
Res->emplace_back(getRVA(), IMAGE_REL_BASED_ARM_MOV32T);
}
void ImportThunkChunkARM::writeTo(uint8_t *Buf) const {
memcpy(Buf + OutputSectionOff, ImportThunkARM, sizeof(ImportThunkARM));
// Fix mov.w and mov.t operands.
applyMOV32T(Buf + OutputSectionOff, ImpSymbol->getRVA() + Config->ImageBase);
}
void ImportThunkChunkARM64::writeTo(uint8_t *Buf) const {
int64_t Off = ImpSymbol->getRVA() & 0xfff;
memcpy(Buf + OutputSectionOff, ImportThunkARM64, sizeof(ImportThunkARM64));
applyArm64Addr(Buf + OutputSectionOff, ImpSymbol->getRVA(), RVA);
applyArm64Ldr(Buf + OutputSectionOff + 4, Off);
}
void LocalImportChunk::getBaserels(std::vector<Baserel> *Res) {
Res->emplace_back(getRVA());
}
size_t LocalImportChunk::getSize() const {
return Config->is64() ? 8 : 4;
}
void LocalImportChunk::writeTo(uint8_t *Buf) const {
if (Config->is64()) {
write64le(Buf + OutputSectionOff, Sym->getRVA() + Config->ImageBase);
} else {
write32le(Buf + OutputSectionOff, Sym->getRVA() + Config->ImageBase);
}
}
void SEHTableChunk::writeTo(uint8_t *Buf) const {
ulittle32_t *Begin = reinterpret_cast<ulittle32_t *>(Buf + OutputSectionOff);
size_t Cnt = 0;
for (Defined *D : Syms)
Begin[Cnt++] = D->getRVA();
std::sort(Begin, Begin + Cnt);
}
// Windows-specific. This class represents a block in .reloc section.
// The format is described here.
//
// On Windows, each DLL is linked against a fixed base address and
// usually loaded to that address. However, if there's already another
// DLL that overlaps, the loader has to relocate it. To do that, DLLs
// contain .reloc sections which contain offsets that need to be fixed
// up at runtime. If the loader finds that a DLL cannot be loaded to its
// desired base address, it loads it to somewhere else, and add <actual
// base address> - <desired base address> to each offset that is
// specified by the .reloc section. In ELF terms, .reloc sections
// contain relative relocations in REL format (as opposed to RELA.)
//
// This already significantly reduces the size of relocations compared
// to ELF .rel.dyn, but Windows does more to reduce it (probably because
// it was invented for PCs in the late '80s or early '90s.) Offsets in
// .reloc are grouped by page where the page size is 12 bits, and
// offsets sharing the same page address are stored consecutively to
// represent them with less space. This is very similar to the page
// table which is grouped by (multiple stages of) pages.
//
// For example, let's say we have 0x00030, 0x00500, 0x00700, 0x00A00,
// 0x20004, and 0x20008 in a .reloc section for x64. The uppermost 4
// bits have a type IMAGE_REL_BASED_DIR64 or 0xA. In the section, they
// are represented like this:
//
// 0x00000 -- page address (4 bytes)
// 16 -- size of this block (4 bytes)
// 0xA030 -- entries (2 bytes each)
// 0xA500
// 0xA700
// 0xAA00
// 0x20000 -- page address (4 bytes)
// 12 -- size of this block (4 bytes)
// 0xA004 -- entries (2 bytes each)
// 0xA008
//
// Usually we have a lot of relocations for each page, so the number of
// bytes for one .reloc entry is close to 2 bytes on average.
BaserelChunk::BaserelChunk(uint32_t Page, Baserel *Begin, Baserel *End) {
// Block header consists of 4 byte page RVA and 4 byte block size.
// Each entry is 2 byte. Last entry may be padding.
Data.resize(alignTo((End - Begin) * 2 + 8, 4));
uint8_t *P = Data.data();
write32le(P, Page);
write32le(P + 4, Data.size());
P += 8;
for (Baserel *I = Begin; I != End; ++I) {
write16le(P, (I->Type << 12) | (I->RVA - Page));
P += 2;
}
}
void BaserelChunk::writeTo(uint8_t *Buf) const {
memcpy(Buf + OutputSectionOff, Data.data(), Data.size());
}
uint8_t Baserel::getDefaultType() {
switch (Config->Machine) {
case AMD64:
return IMAGE_REL_BASED_DIR64;
case I386:
case ARMNT:
return IMAGE_REL_BASED_HIGHLOW;
default:
llvm_unreachable("unknown machine type");
}
}
} // namespace coff
} // namespace lld
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