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llvm-project/llvm/lib/Target/X86/MCTargetDesc/X86MachObjectWriter.cpp

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//===-- X86MachObjectWriter.cpp - X86 Mach-O Writer -----------------------===//
//
// 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 "MCTargetDesc/X86FixupKinds.h"
#include "MCTargetDesc/X86MCTargetDesc.h"
#include "llvm/ADT/Twine.h"
#include "llvm/BinaryFormat/MachO.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCAsmLayout.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCMachObjectWriter.h"
#include "llvm/MC/MCSectionMachO.h"
#include "llvm/MC/MCValue.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/Format.h"
using namespace llvm;
namespace {
class X86MachObjectWriter : public MCMachObjectTargetWriter {
bool recordScatteredRelocation(MachObjectWriter *Writer,
const MCAssembler &Asm,
const MCAsmLayout &Layout,
const MCFragment *Fragment,
const MCFixup &Fixup,
MCValue Target,
unsigned Log2Size,
uint64_t &FixedValue);
void recordTLVPRelocation(MachObjectWriter *Writer,
const MCAssembler &Asm,
const MCAsmLayout &Layout,
const MCFragment *Fragment,
const MCFixup &Fixup,
MCValue Target,
uint64_t &FixedValue);
void RecordX86Relocation(MachObjectWriter *Writer,
const MCAssembler &Asm,
const MCAsmLayout &Layout,
const MCFragment *Fragment,
const MCFixup &Fixup,
MCValue Target,
uint64_t &FixedValue);
void RecordX86_64Relocation(MachObjectWriter *Writer, MCAssembler &Asm,
const MCAsmLayout &Layout,
const MCFragment *Fragment, const MCFixup &Fixup,
MCValue Target, uint64_t &FixedValue);
public:
X86MachObjectWriter(bool Is64Bit, uint32_t CPUType, uint32_t CPUSubtype)
: MCMachObjectTargetWriter(Is64Bit, CPUType, CPUSubtype) {}
void recordRelocation(MachObjectWriter *Writer, MCAssembler &Asm,
const MCAsmLayout &Layout, const MCFragment *Fragment,
const MCFixup &Fixup, MCValue Target,
uint64_t &FixedValue) override {
if (Writer->is64Bit())
RecordX86_64Relocation(Writer, Asm, Layout, Fragment, Fixup, Target,
FixedValue);
else
RecordX86Relocation(Writer, Asm, Layout, Fragment, Fixup, Target,
FixedValue);
}
};
} // namespace
static bool isFixupKindRIPRel(unsigned Kind) {
return Kind == X86::reloc_riprel_4byte ||
Kind == X86::reloc_riprel_4byte_movq_load ||
Kind == X86::reloc_riprel_4byte_relax ||
Kind == X86::reloc_riprel_4byte_relax_rex;
}
static unsigned getFixupKindLog2Size(unsigned Kind) {
switch (Kind) {
default:
llvm_unreachable("invalid fixup kind!");
case FK_PCRel_1:
case FK_Data_1: return 0;
case FK_PCRel_2:
case FK_Data_2: return 1;
case FK_PCRel_4:
// FIXME: Remove these!!!
case X86::reloc_riprel_4byte:
case X86::reloc_riprel_4byte_relax:
case X86::reloc_riprel_4byte_relax_rex:
case X86::reloc_riprel_4byte_movq_load:
case X86::reloc_signed_4byte:
case X86::reloc_signed_4byte_relax:
case X86::reloc_branch_4byte_pcrel:
case FK_Data_4: return 2;
case FK_Data_8: return 3;
}
}
void X86MachObjectWriter::RecordX86_64Relocation(
MachObjectWriter *Writer, MCAssembler &Asm, const MCAsmLayout &Layout,
const MCFragment *Fragment, const MCFixup &Fixup, MCValue Target,
uint64_t &FixedValue) {
unsigned IsPCRel = Writer->isFixupKindPCRel(Asm, Fixup.getKind());
unsigned IsRIPRel = isFixupKindRIPRel(Fixup.getKind());
unsigned Log2Size = getFixupKindLog2Size(Fixup.getKind());
// See <reloc.h>.
uint32_t FixupOffset =
Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
uint32_t FixupAddress =
Writer->getFragmentAddress(Fragment, Layout) + Fixup.getOffset();
int64_t Value = 0;
unsigned Index = 0;
unsigned IsExtern = 0;
unsigned Type = 0;
const MCSymbol *RelSymbol = nullptr;
Value = Target.getConstant();
if (IsPCRel) {
// Compensate for the relocation offset, Darwin x86_64 relocations only have
// the addend and appear to have attempted to define it to be the actual
// expression addend without the PCrel bias. However, instructions with data
// following the relocation are not accommodated for (see comment below
// regarding SIGNED{1,2,4}), so it isn't exactly that either.
Value += 1LL << Log2Size;
}
if (Target.isAbsolute()) { // constant
// SymbolNum of 0 indicates the absolute section.
Type = MachO::X86_64_RELOC_UNSIGNED;
// FIXME: I believe this is broken, I don't think the linker can understand
// it. I think it would require a local relocation, but I'm not sure if that
// would work either. The official way to get an absolute PCrel relocation
// is to use an absolute symbol (which we don't support yet).
if (IsPCRel) {
IsExtern = 1;
Type = MachO::X86_64_RELOC_BRANCH;
}
} else if (Target.getSymB()) { // A - B + constant
const MCSymbol *A = &Target.getSymA()->getSymbol();
if (A->isTemporary())
A = &Writer->findAliasedSymbol(*A);
const MCSymbol *A_Base = Asm.getAtom(*A);
const MCSymbol *B = &Target.getSymB()->getSymbol();
if (B->isTemporary())
B = &Writer->findAliasedSymbol(*B);
const MCSymbol *B_Base = Asm.getAtom(*B);
// Neither symbol can be modified.
if (Target.getSymA()->getKind() != MCSymbolRefExpr::VK_None) {
Asm.getContext().reportError(Fixup.getLoc(),
"unsupported relocation of modified symbol");
return;
}
// We don't support PCrel relocations of differences. Darwin 'as' doesn't
// implement most of these correctly.
if (IsPCRel) {
Asm.getContext().reportError(
Fixup.getLoc(), "unsupported pc-relative relocation of difference");
return;
}
// The support for the situation where one or both of the symbols would
// require a local relocation is handled just like if the symbols were
// external. This is certainly used in the case of debug sections where the
// section has only temporary symbols and thus the symbols don't have base
// symbols. This is encoded using the section ordinal and non-extern
// relocation entries.
// Darwin 'as' doesn't emit correct relocations for this (it ends up with a
// single SIGNED relocation); reject it for now. Except the case where both
// symbols don't have a base, equal but both NULL.
if (A_Base == B_Base && A_Base) {
Asm.getContext().reportError(
Fixup.getLoc(), "unsupported relocation with identical base");
return;
}
// A subtraction expression where either symbol is undefined is a
// non-relocatable expression.
if (A->isUndefined() || B->isUndefined()) {
StringRef Name = A->isUndefined() ? A->getName() : B->getName();
Asm.getContext().reportError(Fixup.getLoc(),
"unsupported relocation with subtraction expression, symbol '" +
Name + "' can not be undefined in a subtraction expression");
return;
}
Value += Writer->getSymbolAddress(*A, Layout) -
(!A_Base ? 0 : Writer->getSymbolAddress(*A_Base, Layout));
Value -= Writer->getSymbolAddress(*B, Layout) -
(!B_Base ? 0 : Writer->getSymbolAddress(*B_Base, Layout));
if (!A_Base)
Index = A->getFragment()->getParent()->getOrdinal() + 1;
Type = MachO::X86_64_RELOC_UNSIGNED;
MachO::any_relocation_info MRE;
MRE.r_word0 = FixupOffset;
MRE.r_word1 =
(Index << 0) | (IsPCRel << 24) | (Log2Size << 25) | (Type << 28);
Writer->addRelocation(A_Base, Fragment->getParent(), MRE);
if (B_Base)
RelSymbol = B_Base;
else
Index = B->getFragment()->getParent()->getOrdinal() + 1;
Type = MachO::X86_64_RELOC_SUBTRACTOR;
} else {
const MCSymbol *Symbol = &Target.getSymA()->getSymbol();
if (Symbol->isTemporary() && Value) {
const MCSection &Sec = Symbol->getSection();
if (!Asm.getContext().getAsmInfo()->isSectionAtomizableBySymbols(Sec))
Symbol->setUsedInReloc();
}
RelSymbol = Asm.getAtom(*Symbol);
// Relocations inside debug sections always use local relocations when
// possible. This seems to be done because the debugger doesn't fully
// understand x86_64 relocation entries, and expects to find values that
// have already been fixed up.
if (Symbol->isInSection()) {
const MCSectionMachO &Section =
static_cast<const MCSectionMachO &>(*Fragment->getParent());
if (Section.hasAttribute(MachO::S_ATTR_DEBUG))
RelSymbol = nullptr;
}
// x86_64 almost always uses external relocations, except when there is no
// symbol to use as a base address (a local symbol with no preceding
// non-local symbol).
if (RelSymbol) {
// Add the local offset, if needed.
if (RelSymbol != Symbol)
Value += Layout.getSymbolOffset(*Symbol) -
Layout.getSymbolOffset(*RelSymbol);
} else if (Symbol->isInSection() && !Symbol->isVariable()) {
// The index is the section ordinal (1-based).
Index = Symbol->getFragment()->getParent()->getOrdinal() + 1;
Value += Writer->getSymbolAddress(*Symbol, Layout);
if (IsPCRel)
Value -= FixupAddress + (1 << Log2Size);
} else if (Symbol->isVariable()) {
const MCExpr *Value = Symbol->getVariableValue();
int64_t Res;
bool isAbs = Value->evaluateAsAbsolute(Res, Layout,
Writer->getSectionAddressMap());
if (isAbs) {
FixedValue = Res;
return;
} else {
Asm.getContext().reportError(Fixup.getLoc(),
"unsupported relocation of variable '" +
Symbol->getName() + "'");
return;
}
} else {
Asm.getContext().reportError(
Fixup.getLoc(), "unsupported relocation of undefined symbol '" +
Symbol->getName() + "'");
return;
}
MCSymbolRefExpr::VariantKind Modifier = Target.getSymA()->getKind();
if (IsPCRel) {
if (IsRIPRel) {
if (Modifier == MCSymbolRefExpr::VK_GOTPCREL) {
// x86_64 distinguishes movq foo@GOTPCREL so that the linker can
// rewrite the movq to an leaq at link time if the symbol ends up in
// the same linkage unit.
if (Fixup.getTargetKind() == X86::reloc_riprel_4byte_movq_load)
Type = MachO::X86_64_RELOC_GOT_LOAD;
else
Type = MachO::X86_64_RELOC_GOT;
} else if (Modifier == MCSymbolRefExpr::VK_TLVP) {
Type = MachO::X86_64_RELOC_TLV;
} else if (Modifier != MCSymbolRefExpr::VK_None) {
Asm.getContext().reportError(
Fixup.getLoc(), "unsupported symbol modifier in relocation");
return;
} else {
Type = MachO::X86_64_RELOC_SIGNED;
// The Darwin x86_64 relocation format has a problem where it cannot
// encode an address (L<foo> + <constant>) which is outside the atom
// containing L<foo>. Generally, this shouldn't occur but it does
// happen when we have a RIPrel instruction with data following the
// relocation entry (e.g., movb $012, L0(%rip)). Even with the PCrel
// adjustment Darwin x86_64 uses, the offset is still negative and the
// linker has no way to recognize this.
//
// To work around this, Darwin uses several special relocation types
// to indicate the offsets. However, the specification or
// implementation of these seems to also be incomplete; they should
// adjust the addend as well based on the actual encoded instruction
// (the additional bias), but instead appear to just look at the final
// offset.
switch (-(Target.getConstant() + (1LL << Log2Size))) {
case 1: Type = MachO::X86_64_RELOC_SIGNED_1; break;
case 2: Type = MachO::X86_64_RELOC_SIGNED_2; break;
case 4: Type = MachO::X86_64_RELOC_SIGNED_4; break;
}
}
} else {
if (Modifier != MCSymbolRefExpr::VK_None) {
Asm.getContext().reportError(
Fixup.getLoc(),
"unsupported symbol modifier in branch relocation");
return;
}
Type = MachO::X86_64_RELOC_BRANCH;
}
} else {
if (Modifier == MCSymbolRefExpr::VK_GOT) {
Type = MachO::X86_64_RELOC_GOT;
} else if (Modifier == MCSymbolRefExpr::VK_GOTPCREL) {
// GOTPCREL is allowed as a modifier on non-PCrel instructions, in which
// case all we do is set the PCrel bit in the relocation entry; this is
// used with exception handling, for example. The source is required to
// include any necessary offset directly.
Type = MachO::X86_64_RELOC_GOT;
IsPCRel = 1;
} else if (Modifier == MCSymbolRefExpr::VK_TLVP) {
Asm.getContext().reportError(
Fixup.getLoc(), "TLVP symbol modifier should have been rip-rel");
return;
} else if (Modifier != MCSymbolRefExpr::VK_None) {
Asm.getContext().reportError(
Fixup.getLoc(), "unsupported symbol modifier in relocation");
return;
} else {
Type = MachO::X86_64_RELOC_UNSIGNED;
if (Fixup.getTargetKind() == X86::reloc_signed_4byte) {
Asm.getContext().reportError(
Fixup.getLoc(),
"32-bit absolute addressing is not supported in 64-bit mode");
return;
}
}
}
}
// x86_64 always writes custom values into the fixups.
FixedValue = Value;
// struct relocation_info (8 bytes)
MachO::any_relocation_info MRE;
MRE.r_word0 = FixupOffset;
MRE.r_word1 = (Index << 0) | (IsPCRel << 24) | (Log2Size << 25) |
(IsExtern << 27) | (Type << 28);
Writer->addRelocation(RelSymbol, Fragment->getParent(), MRE);
}
bool X86MachObjectWriter::recordScatteredRelocation(MachObjectWriter *Writer,
const MCAssembler &Asm,
const MCAsmLayout &Layout,
const MCFragment *Fragment,
const MCFixup &Fixup,
MCValue Target,
unsigned Log2Size,
uint64_t &FixedValue) {
uint64_t OriginalFixedValue = FixedValue;
uint32_t FixupOffset = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
unsigned IsPCRel = Writer->isFixupKindPCRel(Asm, Fixup.getKind());
unsigned Type = MachO::GENERIC_RELOC_VANILLA;
// See <reloc.h>.
const MCSymbol *A = &Target.getSymA()->getSymbol();
if (!A->getFragment()) {
Asm.getContext().reportError(
Fixup.getLoc(),
"symbol '" + A->getName() +
"' can not be undefined in a subtraction expression");
return false;
}
uint32_t Value = Writer->getSymbolAddress(*A, Layout);
uint64_t SecAddr = Writer->getSectionAddress(A->getFragment()->getParent());
FixedValue += SecAddr;
uint32_t Value2 = 0;
if (const MCSymbolRefExpr *B = Target.getSymB()) {
const MCSymbol *SB = &B->getSymbol();
if (!SB->getFragment()) {
Asm.getContext().reportError(
Fixup.getLoc(),
"symbol '" + SB->getName() +
"' can not be undefined in a subtraction expression");
return false;
}
// Select the appropriate difference relocation type.
//
// Note that there is no longer any semantic difference between these two
// relocation types from the linkers point of view, this is done solely for
// pedantic compatibility with 'as'.
Type = A->isExternal() ? (unsigned)MachO::GENERIC_RELOC_SECTDIFF
: (unsigned)MachO::GENERIC_RELOC_LOCAL_SECTDIFF;
Value2 = Writer->getSymbolAddress(*SB, Layout);
FixedValue -= Writer->getSectionAddress(SB->getFragment()->getParent());
}
// Relocations are written out in reverse order, so the PAIR comes first.
if (Type == MachO::GENERIC_RELOC_SECTDIFF ||
Type == MachO::GENERIC_RELOC_LOCAL_SECTDIFF) {
// If the offset is too large to fit in a scattered relocation,
// we're hosed. It's an unfortunate limitation of the MachO format.
if (FixupOffset > 0xffffff) {
char Buffer[32];
format("0x%x", FixupOffset).print(Buffer, sizeof(Buffer));
Asm.getContext().reportError(Fixup.getLoc(),
Twine("Section too large, can't encode "
"r_address (") + Buffer +
") into 24 bits of scattered "
"relocation entry.");
return false;
}
MachO::any_relocation_info MRE;
MRE.r_word0 = ((0 << 0) | // r_address
(MachO::GENERIC_RELOC_PAIR << 24) | // r_type
(Log2Size << 28) |
(IsPCRel << 30) |
MachO::R_SCATTERED);
MRE.r_word1 = Value2;
Writer->addRelocation(nullptr, Fragment->getParent(), MRE);
} else {
// If the offset is more than 24-bits, it won't fit in a scattered
// relocation offset field, so we fall back to using a non-scattered
// relocation. This is a bit risky, as if the offset reaches out of
// the block and the linker is doing scattered loading on this
// symbol, things can go badly.
//
// Required for 'as' compatibility.
if (FixupOffset > 0xffffff) {
FixedValue = OriginalFixedValue;
return false;
}
}
MachO::any_relocation_info MRE;
MRE.r_word0 = ((FixupOffset << 0) |
(Type << 24) |
(Log2Size << 28) |
(IsPCRel << 30) |
MachO::R_SCATTERED);
MRE.r_word1 = Value;
Writer->addRelocation(nullptr, Fragment->getParent(), MRE);
return true;
}
void X86MachObjectWriter::recordTLVPRelocation(MachObjectWriter *Writer,
const MCAssembler &Asm,
const MCAsmLayout &Layout,
const MCFragment *Fragment,
const MCFixup &Fixup,
MCValue Target,
uint64_t &FixedValue) {
const MCSymbolRefExpr *SymA = Target.getSymA();
assert(SymA->getKind() == MCSymbolRefExpr::VK_TLVP && !is64Bit() &&
"Should only be called with a 32-bit TLVP relocation!");
unsigned Log2Size = getFixupKindLog2Size(Fixup.getKind());
uint32_t Value = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
unsigned IsPCRel = 0;
// We're only going to have a second symbol in pic mode and it'll be a
// subtraction from the picbase. For 32-bit pic the addend is the difference
// between the picbase and the next address. For 32-bit static the addend is
// zero.
if (auto *SymB = Target.getSymB()) {
// If this is a subtraction then we're pcrel.
uint32_t FixupAddress =
Writer->getFragmentAddress(Fragment, Layout) + Fixup.getOffset();
IsPCRel = 1;
FixedValue = FixupAddress -
Writer->getSymbolAddress(SymB->getSymbol(), Layout) +
Target.getConstant();
FixedValue += 1ULL << Log2Size;
} else {
FixedValue = 0;
}
// struct relocation_info (8 bytes)
MachO::any_relocation_info MRE;
MRE.r_word0 = Value;
MRE.r_word1 =
(IsPCRel << 24) | (Log2Size << 25) | (MachO::GENERIC_RELOC_TLV << 28);
Writer->addRelocation(&SymA->getSymbol(), Fragment->getParent(), MRE);
}
void X86MachObjectWriter::RecordX86Relocation(MachObjectWriter *Writer,
const MCAssembler &Asm,
const MCAsmLayout &Layout,
const MCFragment *Fragment,
const MCFixup &Fixup,
MCValue Target,
uint64_t &FixedValue) {
unsigned IsPCRel = Writer->isFixupKindPCRel(Asm, Fixup.getKind());
unsigned Log2Size = getFixupKindLog2Size(Fixup.getKind());
// If this is a 32-bit TLVP reloc it's handled a bit differently.
if (Target.getSymA() &&
Target.getSymA()->getKind() == MCSymbolRefExpr::VK_TLVP) {
recordTLVPRelocation(Writer, Asm, Layout, Fragment, Fixup, Target,
FixedValue);
return;
}
// If this is a difference or a defined symbol plus an offset, then we need a
// scattered relocation entry. Differences always require scattered
// relocations.
if (Target.getSymB()) {
recordScatteredRelocation(Writer, Asm, Layout, Fragment, Fixup,
Target, Log2Size, FixedValue);
return;
}
// Get the symbol data, if any.
const MCSymbol *A = nullptr;
if (Target.getSymA())
A = &Target.getSymA()->getSymbol();
// If this is an internal relocation with an offset, it also needs a scattered
// relocation entry.
uint32_t Offset = Target.getConstant();
if (IsPCRel)
Offset += 1 << Log2Size;
// Try to record the scattered relocation if needed. Fall back to non
// scattered if necessary (see comments in recordScatteredRelocation()
// for details).
if (Offset && A && !Writer->doesSymbolRequireExternRelocation(*A) &&
recordScatteredRelocation(Writer, Asm, Layout, Fragment, Fixup, Target,
Log2Size, FixedValue))
return;
// See <reloc.h>.
uint32_t FixupOffset = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
unsigned Index = 0;
unsigned Type = 0;
const MCSymbol *RelSymbol = nullptr;
if (Target.isAbsolute()) { // constant
// SymbolNum of 0 indicates the absolute section.
//
// FIXME: Currently, these are never generated (see code below). I cannot
// find a case where they are actually emitted.
Type = MachO::GENERIC_RELOC_VANILLA;
} else {
// Resolve constant variables.
if (A->isVariable()) {
int64_t Res;
if (A->getVariableValue()->evaluateAsAbsolute(
Res, Layout, Writer->getSectionAddressMap())) {
FixedValue = Res;
return;
}
}
// Check whether we need an external or internal relocation.
if (Writer->doesSymbolRequireExternRelocation(*A)) {
RelSymbol = A;
// For external relocations, make sure to offset the fixup value to
// compensate for the addend of the symbol address, if it was
// undefined. This occurs with weak definitions, for example.
if (!A->isUndefined())
FixedValue -= Layout.getSymbolOffset(*A);
} else {
// The index is the section ordinal (1-based).
const MCSection &Sec = A->getSection();
Index = Sec.getOrdinal() + 1;
FixedValue += Writer->getSectionAddress(&Sec);
}
if (IsPCRel)
FixedValue -= Writer->getSectionAddress(Fragment->getParent());
Type = MachO::GENERIC_RELOC_VANILLA;
}
// struct relocation_info (8 bytes)
MachO::any_relocation_info MRE;
MRE.r_word0 = FixupOffset;
MRE.r_word1 =
(Index << 0) | (IsPCRel << 24) | (Log2Size << 25) | (Type << 28);
Writer->addRelocation(RelSymbol, Fragment->getParent(), MRE);
}
std::unique_ptr<MCObjectTargetWriter>
llvm::createX86MachObjectWriter(bool Is64Bit, uint32_t CPUType,
uint32_t CPUSubtype) {
return std::make_unique<X86MachObjectWriter>(Is64Bit, CPUType, CPUSubtype);
}
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