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Relocations.cpp
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Relocations.cpp
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//===- Relocations.cpp ----------------------------------------------------===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains platform-independent functions to process relocations.
// I'll describe the overview of this file here.
//
// Simple relocations are easy to handle for the linker. For example,
// for R_X86_64_PC64 relocs, the linker just has to fix up locations
// with the relative offsets to the target symbols. It would just be
// reading records from relocation sections and applying them to output.
//
// But not all relocations are that easy to handle. For example, for
// R_386_GOTOFF relocs, the linker has to create new GOT entries for
// symbols if they don't exist, and fix up locations with GOT entry
// offsets from the beginning of GOT section. So there is more than
// fixing addresses in relocation processing.
//
// ELF defines a large number of complex relocations.
//
// The functions in this file analyze relocations and do whatever needs
// to be done. It includes, but not limited to, the following.
//
// - create GOT/PLT entries
// - create new relocations in .dynsym to let the dynamic linker resolve
// them at runtime (since ELF supports dynamic linking, not all
// relocations can be resolved at link-time)
// - create COPY relocs and reserve space in .bss
// - replace expensive relocs (in terms of runtime cost) with cheap ones
// - error out infeasible combinations such as PIC and non-relative relocs
//
// Note that the functions in this file don't actually apply relocations
// because it doesn't know about the output file nor the output file buffer.
// It instead stores Relocation objects to InputSection's Relocations
// vector to let it apply later in InputSection::writeTo.
//
//===----------------------------------------------------------------------===//
#include "Relocations.h"
#include "Config.h"
#include "LinkerScript.h"
#include "OutputSections.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "Thunks.h"
#include "lld/Common/Memory.h"
#include "lld/Common/Strings.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
using namespace llvm::support::endian;
using namespace lld;
using namespace lld::elf;
// Construct a message in the following format.
//
// >>> defined in /home/alice/src/foo.o
// >>> referenced by bar.c:12 (/home/alice/src/bar.c:12)
// >>> /home/alice/src/bar.o:(.text+0x1)
static std::string getLocation(InputSectionBase &S, const Symbol &Sym,
uint64_t Off) {
std::string Msg =
"\n>>> defined in " + toString(Sym.File) + "\n>>> referenced by ";
std::string Src = S.getSrcMsg(Sym, Off);
if (!Src.empty())
Msg += Src + "\n>>> ";
return Msg + S.getObjMsg(Off);
}
// This function is similar to the `handleTlsRelocation`. MIPS does not
// support any relaxations for TLS relocations so by factoring out MIPS
// handling in to the separate function we can simplify the code and do not
// pollute other `handleTlsRelocation` by MIPS `ifs` statements.
// Mips has a custom MipsGotSection that handles the writing of GOT entries
// without dynamic relocations.
static unsigned handleMipsTlsRelocation(RelType Type, Symbol &Sym,
InputSectionBase &C, uint64_t Offset,
int64_t Addend, RelExpr Expr) {
if (Expr == R_MIPS_TLSLD) {
InX::MipsGot->addTlsIndex(*C.File);
C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return 1;
}
if (Expr == R_MIPS_TLSGD) {
InX::MipsGot->addDynTlsEntry(*C.File, Sym);
C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return 1;
}
return 0;
}
// This function is similar to the `handleMipsTlsRelocation`. ARM also does not
// support any relaxations for TLS relocations. ARM is logically similar to Mips
// in how it handles TLS, but Mips uses its own custom GOT which handles some
// of the cases that ARM uses GOT relocations for.
//
// We look for TLS global dynamic and local dynamic relocations, these may
// require the generation of a pair of GOT entries that have associated
// dynamic relocations. When the results of the dynamic relocations can be
// resolved at static link time we do so. This is necessary for static linking
// as there will be no dynamic loader to resolve them at load-time.
//
// The pair of GOT entries created are of the form
// GOT[e0] Module Index (Used to find pointer to TLS block at run-time)
// GOT[e1] Offset of symbol in TLS block
template <class ELFT>
static unsigned handleARMTlsRelocation(RelType Type, Symbol &Sym,
InputSectionBase &C, uint64_t Offset,
int64_t Addend, RelExpr Expr) {
// The Dynamic TLS Module Index Relocation for a symbol defined in an
// executable is always 1. If the target Symbol is not preemptible then
// we know the offset into the TLS block at static link time.
bool NeedDynId = Sym.IsPreemptible || Config->Shared;
bool NeedDynOff = Sym.IsPreemptible;
auto AddTlsReloc = [&](uint64_t Off, RelType Type, Symbol *Dest, bool Dyn) {
if (Dyn)
InX::RelaDyn->addReloc(Type, InX::Got, Off, Dest);
else
InX::Got->Relocations.push_back({R_ABS, Type, Off, 0, Dest});
};
// Local Dynamic is for access to module local TLS variables, while still
// being suitable for being dynamically loaded via dlopen.
// GOT[e0] is the module index, with a special value of 0 for the current
// module. GOT[e1] is unused. There only needs to be one module index entry.
if (Expr == R_TLSLD_PC && InX::Got->addTlsIndex()) {
AddTlsReloc(InX::Got->getTlsIndexOff(), Target->TlsModuleIndexRel,
NeedDynId ? nullptr : &Sym, NeedDynId);
C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return 1;
}
// Global Dynamic is the most general purpose access model. When we know
// the module index and offset of symbol in TLS block we can fill these in
// using static GOT relocations.
if (Expr == R_TLSGD_PC) {
if (InX::Got->addDynTlsEntry(Sym)) {
uint64_t Off = InX::Got->getGlobalDynOffset(Sym);
AddTlsReloc(Off, Target->TlsModuleIndexRel, &Sym, NeedDynId);
AddTlsReloc(Off + Config->Wordsize, Target->TlsOffsetRel, &Sym,
NeedDynOff);
}
C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return 1;
}
return 0;
}
// Returns the number of relocations processed.
template <class ELFT>
static unsigned
handleTlsRelocation(RelType Type, Symbol &Sym, InputSectionBase &C,
typename ELFT::uint Offset, int64_t Addend, RelExpr Expr) {
if (!Sym.isTls())
return 0;
if (Config->EMachine == EM_ARM)
return handleARMTlsRelocation<ELFT>(Type, Sym, C, Offset, Addend, Expr);
if (Config->EMachine == EM_MIPS)
return handleMipsTlsRelocation(Type, Sym, C, Offset, Addend, Expr);
if (isRelExprOneOf<R_TLSDESC, R_TLSDESC_PAGE, R_TLSDESC_CALL>(Expr) &&
Config->Shared) {
if (InX::Got->addDynTlsEntry(Sym)) {
uint64_t Off = InX::Got->getGlobalDynOffset(Sym);
InX::RelaDyn->addReloc(
{Target->TlsDescRel, InX::Got, Off, !Sym.IsPreemptible, &Sym, 0});
}
if (Expr != R_TLSDESC_CALL)
C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return 1;
}
if (isRelExprOneOf<R_TLSLD_GOT, R_TLSLD_GOT_FROM_END, R_TLSLD_PC,
R_TLSLD_HINT>(Expr)) {
// Local-Dynamic relocs can be relaxed to Local-Exec.
if (!Config->Shared) {
C.Relocations.push_back(
{Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_LD_TO_LE), Type,
Offset, Addend, &Sym});
return Target->TlsGdRelaxSkip;
}
if (Expr == R_TLSLD_HINT)
return 1;
if (InX::Got->addTlsIndex())
InX::RelaDyn->addReloc(Target->TlsModuleIndexRel, InX::Got,
InX::Got->getTlsIndexOff(), nullptr);
C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return 1;
}
// Local-Dynamic relocs can be relaxed to Local-Exec.
if (Expr == R_ABS && !Config->Shared) {
C.Relocations.push_back(
{Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_LD_TO_LE), Type,
Offset, Addend, &Sym});
return 1;
}
// Local-Dynamic sequence where offset of tls variable relative to dynamic
// thread pointer is stored in the got.
if (Expr == R_TLSLD_GOT_OFF) {
// Local-Dynamic relocs can be relaxed to local-exec
if (!Config->Shared) {
C.Relocations.push_back({R_RELAX_TLS_LD_TO_LE, Type, Offset, Addend, &Sym});
return 1;
}
if (!Sym.isInGot()) {
InX::Got->addEntry(Sym);
uint64_t Off = Sym.getGotOffset();
InX::Got->Relocations.push_back({R_ABS, Target->TlsOffsetRel, Off, 0, &Sym});
}
C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return 1;
}
if (isRelExprOneOf<R_TLSDESC, R_TLSDESC_PAGE, R_TLSDESC_CALL, R_TLSGD_GOT,
R_TLSGD_GOT_FROM_END, R_TLSGD_PC>(Expr)) {
if (Config->Shared) {
if (InX::Got->addDynTlsEntry(Sym)) {
uint64_t Off = InX::Got->getGlobalDynOffset(Sym);
InX::RelaDyn->addReloc(Target->TlsModuleIndexRel, InX::Got, Off, &Sym);
// If the symbol is preemptible we need the dynamic linker to write
// the offset too.
uint64_t OffsetOff = Off + Config->Wordsize;
if (Sym.IsPreemptible)
InX::RelaDyn->addReloc(Target->TlsOffsetRel, InX::Got, OffsetOff,
&Sym);
else
InX::Got->Relocations.push_back(
{R_ABS, Target->TlsOffsetRel, OffsetOff, 0, &Sym});
}
C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return 1;
}
// Global-Dynamic relocs can be relaxed to Initial-Exec or Local-Exec
// depending on the symbol being locally defined or not.
if (Sym.IsPreemptible) {
C.Relocations.push_back(
{Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_IE), Type,
Offset, Addend, &Sym});
if (!Sym.isInGot()) {
InX::Got->addEntry(Sym);
InX::RelaDyn->addReloc(Target->TlsGotRel, InX::Got, Sym.getGotOffset(),
&Sym);
}
} else {
C.Relocations.push_back(
{Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_LE), Type,
Offset, Addend, &Sym});
}
return Target->TlsGdRelaxSkip;
}
// Initial-Exec relocs can be relaxed to Local-Exec if the symbol is locally
// defined.
if (isRelExprOneOf<R_GOT, R_GOT_FROM_END, R_GOT_PC, R_GOT_PAGE_PC, R_GOT_OFF,
R_TLSIE_HINT>(Expr) &&
!Config->Shared && !Sym.IsPreemptible) {
C.Relocations.push_back({R_RELAX_TLS_IE_TO_LE, Type, Offset, Addend, &Sym});
return 1;
}
if (Expr == R_TLSIE_HINT)
return 1;
return 0;
}
static RelType getMipsPairType(RelType Type, bool IsLocal) {
switch (Type) {
case R_MIPS_HI16:
return R_MIPS_LO16;
case R_MIPS_GOT16:
// In case of global symbol, the R_MIPS_GOT16 relocation does not
// have a pair. Each global symbol has a unique entry in the GOT
// and a corresponding instruction with help of the R_MIPS_GOT16
// relocation loads an address of the symbol. In case of local
// symbol, the R_MIPS_GOT16 relocation creates a GOT entry to hold
// the high 16 bits of the symbol's value. A paired R_MIPS_LO16
// relocations handle low 16 bits of the address. That allows
// to allocate only one GOT entry for every 64 KBytes of local data.
return IsLocal ? R_MIPS_LO16 : R_MIPS_NONE;
case R_MICROMIPS_GOT16:
return IsLocal ? R_MICROMIPS_LO16 : R_MIPS_NONE;
case R_MIPS_PCHI16:
return R_MIPS_PCLO16;
case R_MICROMIPS_HI16:
return R_MICROMIPS_LO16;
default:
return R_MIPS_NONE;
}
}
// True if non-preemptable symbol always has the same value regardless of where
// the DSO is loaded.
static bool isAbsolute(const Symbol &Sym) {
if (Sym.isUndefWeak())
return true;
if (const auto *DR = dyn_cast<Defined>(&Sym))
return DR->Section == nullptr; // Absolute symbol.
return false;
}
static bool isAbsoluteValue(const Symbol &Sym) {
return isAbsolute(Sym) || Sym.isTls();
}
// Returns true if Expr refers a PLT entry.
static bool needsPlt(RelExpr Expr) {
return isRelExprOneOf<R_PLT_PC, R_PPC_CALL_PLT, R_PLT, R_PLT_PAGE_PC>(Expr);
}
// Returns true if Expr refers a GOT entry. Note that this function
// returns false for TLS variables even though they need GOT, because
// TLS variables uses GOT differently than the regular variables.
static bool needsGot(RelExpr Expr) {
return isRelExprOneOf<R_GOT, R_GOT_OFF, R_MIPS_GOT_LOCAL_PAGE, R_MIPS_GOT_OFF,
R_MIPS_GOT_OFF32, R_GOT_PAGE_PC, R_GOT_PC,
R_GOT_FROM_END>(Expr);
}
// True if this expression is of the form Sym - X, where X is a position in the
// file (PC, or GOT for example).
static bool isRelExpr(RelExpr Expr) {
return isRelExprOneOf<R_PC, R_GOTREL, R_GOTREL_FROM_END, R_MIPS_GOTREL,
R_PPC_CALL, R_PPC_CALL_PLT, R_PAGE_PC,
R_RELAX_GOT_PC>(Expr);
}
// Returns true if a given relocation can be computed at link-time.
//
// For instance, we know the offset from a relocation to its target at
// link-time if the relocation is PC-relative and refers a
// non-interposable function in the same executable. This function
// will return true for such relocation.
//
// If this function returns false, that means we need to emit a
// dynamic relocation so that the relocation will be fixed at load-time.
static bool isStaticLinkTimeConstant(RelExpr E, RelType Type, const Symbol &Sym,
InputSectionBase &S, uint64_t RelOff) {
// These expressions always compute a constant
if (isRelExprOneOf<
R_GOT_FROM_END, R_GOT_OFF, R_TLSLD_GOT_OFF, R_MIPS_GOT_LOCAL_PAGE,
R_MIPS_GOTREL, R_MIPS_GOT_OFF, R_MIPS_GOT_OFF32, R_MIPS_GOT_GP_PC,
R_MIPS_TLSGD, R_GOT_PAGE_PC, R_GOT_PC, R_GOTONLY_PC,
R_GOTONLY_PC_FROM_END, R_PLT_PC, R_TLSGD_GOT, R_TLSGD_GOT_FROM_END,
R_TLSGD_PC, R_PPC_CALL_PLT, R_TLSDESC_CALL, R_TLSDESC_PAGE, R_HINT,
R_TLSLD_HINT, R_TLSIE_HINT>(E))
return true;
// These never do, except if the entire file is position dependent or if
// only the low bits are used.
if (E == R_GOT || E == R_PLT || E == R_TLSDESC)
return Target->usesOnlyLowPageBits(Type) || !Config->Pic;
if (Sym.IsPreemptible)
return false;
if (!Config->Pic)
return true;
// The size of a non preemptible symbol is a constant.
if (E == R_SIZE)
return true;
// For the target and the relocation, we want to know if they are
// absolute or relative.
bool AbsVal = isAbsoluteValue(Sym);
bool RelE = isRelExpr(E);
if (AbsVal && !RelE)
return true;
if (!AbsVal && RelE)
return true;
if (!AbsVal && !RelE)
return Target->usesOnlyLowPageBits(Type);
// Relative relocation to an absolute value. This is normally unrepresentable,
// but if the relocation refers to a weak undefined symbol, we allow it to
// resolve to the image base. This is a little strange, but it allows us to
// link function calls to such symbols. Normally such a call will be guarded
// with a comparison, which will load a zero from the GOT.
// Another special case is MIPS _gp_disp symbol which represents offset
// between start of a function and '_gp' value and defined as absolute just
// to simplify the code.
assert(AbsVal && RelE);
if (Sym.isUndefWeak())
return true;
error("relocation " + toString(Type) + " cannot refer to absolute symbol: " +
toString(Sym) + getLocation(S, Sym, RelOff));
return true;
}
static RelExpr toPlt(RelExpr Expr) {
switch (Expr) {
case R_PPC_CALL:
return R_PPC_CALL_PLT;
case R_PC:
return R_PLT_PC;
case R_PAGE_PC:
return R_PLT_PAGE_PC;
case R_ABS:
return R_PLT;
default:
return Expr;
}
}
static RelExpr fromPlt(RelExpr Expr) {
// We decided not to use a plt. Optimize a reference to the plt to a
// reference to the symbol itself.
switch (Expr) {
case R_PLT_PC:
return R_PC;
case R_PPC_CALL_PLT:
return R_PPC_CALL;
case R_PLT:
return R_ABS;
default:
return Expr;
}
}
// Returns true if a given shared symbol is in a read-only segment in a DSO.
template <class ELFT> static bool isReadOnly(SharedSymbol &SS) {
typedef typename ELFT::Phdr Elf_Phdr;
// Determine if the symbol is read-only by scanning the DSO's program headers.
const SharedFile<ELFT> &File = SS.getFile<ELFT>();
for (const Elf_Phdr &Phdr : check(File.getObj().program_headers()))
if ((Phdr.p_type == ELF::PT_LOAD || Phdr.p_type == ELF::PT_GNU_RELRO) &&
!(Phdr.p_flags & ELF::PF_W) && SS.Value >= Phdr.p_vaddr &&
SS.Value < Phdr.p_vaddr + Phdr.p_memsz)
return true;
return false;
}
// Returns symbols at the same offset as a given symbol, including SS itself.
//
// If two or more symbols are at the same offset, and at least one of
// them are copied by a copy relocation, all of them need to be copied.
// Otherwise, they would refer to different places at runtime.
template <class ELFT>
static SmallSet<SharedSymbol *, 4> getSymbolsAt(SharedSymbol &SS) {
typedef typename ELFT::Sym Elf_Sym;
SharedFile<ELFT> &File = SS.getFile<ELFT>();
SmallSet<SharedSymbol *, 4> Ret;
for (const Elf_Sym &S : File.getGlobalELFSyms()) {
if (S.st_shndx == SHN_UNDEF || S.st_shndx == SHN_ABS ||
S.getType() == STT_TLS || S.st_value != SS.Value)
continue;
StringRef Name = check(S.getName(File.getStringTable()));
Symbol *Sym = Symtab->find(Name);
if (auto *Alias = dyn_cast_or_null<SharedSymbol>(Sym))
Ret.insert(Alias);
}
return Ret;
}
// When a symbol is copy relocated or we create a canonical plt entry, it is
// effectively a defined symbol. In the case of copy relocation the symbol is
// in .bss and in the case of a canonical plt entry it is in .plt. This function
// replaces the existing symbol with a Defined pointing to the appropriate
// location.
static void replaceWithDefined(Symbol &Sym, SectionBase *Sec, uint64_t Value,
uint64_t Size) {
Symbol Old = Sym;
replaceSymbol<Defined>(&Sym, Sym.File, Sym.getName(), Sym.Binding,
Sym.StOther, Sym.Type, Value, Size, Sec);
Sym.PltIndex = Old.PltIndex;
Sym.GotIndex = Old.GotIndex;
Sym.VerdefIndex = Old.VerdefIndex;
Sym.IsPreemptible = true;
Sym.ExportDynamic = true;
Sym.IsUsedInRegularObj = true;
Sym.Used = true;
}
// Reserve space in .bss or .bss.rel.ro for copy relocation.
//
// The copy relocation is pretty much a hack. If you use a copy relocation
// in your program, not only the symbol name but the symbol's size, RW/RO
// bit and alignment become part of the ABI. In addition to that, if the
// symbol has aliases, the aliases become part of the ABI. That's subtle,
// but if you violate that implicit ABI, that can cause very counter-
// intuitive consequences.
//
// So, what is the copy relocation? It's for linking non-position
// independent code to DSOs. In an ideal world, all references to data
// exported by DSOs should go indirectly through GOT. But if object files
// are compiled as non-PIC, all data references are direct. There is no
// way for the linker to transform the code to use GOT, as machine
// instructions are already set in stone in object files. This is where
// the copy relocation takes a role.
//
// A copy relocation instructs the dynamic linker to copy data from a DSO
// to a specified address (which is usually in .bss) at load-time. If the
// static linker (that's us) finds a direct data reference to a DSO
// symbol, it creates a copy relocation, so that the symbol can be
// resolved as if it were in .bss rather than in a DSO.
//
// As you can see in this function, we create a copy relocation for the
// dynamic linker, and the relocation contains not only symbol name but
// various other informtion about the symbol. So, such attributes become a
// part of the ABI.
//
// Note for application developers: I can give you a piece of advice if
// you are writing a shared library. You probably should export only
// functions from your library. You shouldn't export variables.
//
// As an example what can happen when you export variables without knowing
// the semantics of copy relocations, assume that you have an exported
// variable of type T. It is an ABI-breaking change to add new members at
// end of T even though doing that doesn't change the layout of the
// existing members. That's because the space for the new members are not
// reserved in .bss unless you recompile the main program. That means they
// are likely to overlap with other data that happens to be laid out next
// to the variable in .bss. This kind of issue is sometimes very hard to
// debug. What's a solution? Instead of exporting a varaible V from a DSO,
// define an accessor getV().
template <class ELFT> static void addCopyRelSymbol(SharedSymbol &SS) {
// Copy relocation against zero-sized symbol doesn't make sense.
uint64_t SymSize = SS.getSize();
if (SymSize == 0 || SS.Alignment == 0)
fatal("cannot create a copy relocation for symbol " + toString(SS));
// See if this symbol is in a read-only segment. If so, preserve the symbol's
// memory protection by reserving space in the .bss.rel.ro section.
bool IsReadOnly = isReadOnly<ELFT>(SS);
BssSection *Sec = make<BssSection>(IsReadOnly ? ".bss.rel.ro" : ".bss",
SymSize, SS.Alignment);
if (IsReadOnly)
InX::BssRelRo->getParent()->addSection(Sec);
else
InX::Bss->getParent()->addSection(Sec);
// Look through the DSO's dynamic symbol table for aliases and create a
// dynamic symbol for each one. This causes the copy relocation to correctly
// interpose any aliases.
for (SharedSymbol *Sym : getSymbolsAt<ELFT>(SS))
replaceWithDefined(*Sym, Sec, 0, Sym->Size);
InX::RelaDyn->addReloc(Target->CopyRel, Sec, 0, &SS);
}
// MIPS has an odd notion of "paired" relocations to calculate addends.
// For example, if a relocation is of R_MIPS_HI16, there must be a
// R_MIPS_LO16 relocation after that, and an addend is calculated using
// the two relocations.
template <class ELFT, class RelTy>
static int64_t computeMipsAddend(const RelTy &Rel, const RelTy *End,
InputSectionBase &Sec, RelExpr Expr,
bool IsLocal) {
if (Expr == R_MIPS_GOTREL && IsLocal)
return Sec.getFile<ELFT>()->MipsGp0;
// The ABI says that the paired relocation is used only for REL.
// See p. 4-17 at ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
if (RelTy::IsRela)
return 0;
RelType Type = Rel.getType(Config->IsMips64EL);
uint32_t PairTy = getMipsPairType(Type, IsLocal);
if (PairTy == R_MIPS_NONE)
return 0;
const uint8_t *Buf = Sec.Data.data();
uint32_t SymIndex = Rel.getSymbol(Config->IsMips64EL);
// To make things worse, paired relocations might not be contiguous in
// the relocation table, so we need to do linear search. *sigh*
for (const RelTy *RI = &Rel; RI != End; ++RI)
if (RI->getType(Config->IsMips64EL) == PairTy &&
RI->getSymbol(Config->IsMips64EL) == SymIndex)
return Target->getImplicitAddend(Buf + RI->r_offset, PairTy);
warn("can't find matching " + toString(PairTy) + " relocation for " +
toString(Type));
return 0;
}
// Returns an addend of a given relocation. If it is RELA, an addend
// is in a relocation itself. If it is REL, we need to read it from an
// input section.
template <class ELFT, class RelTy>
static int64_t computeAddend(const RelTy &Rel, const RelTy *End,
InputSectionBase &Sec, RelExpr Expr,
bool IsLocal) {
int64_t Addend;
RelType Type = Rel.getType(Config->IsMips64EL);
if (RelTy::IsRela) {
Addend = getAddend<ELFT>(Rel);
} else {
const uint8_t *Buf = Sec.Data.data();
Addend = Target->getImplicitAddend(Buf + Rel.r_offset, Type);
}
if (Config->EMachine == EM_PPC64 && Config->Pic && Type == R_PPC64_TOC)
Addend += getPPC64TocBase();
if (Config->EMachine == EM_MIPS)
Addend += computeMipsAddend<ELFT>(Rel, End, Sec, Expr, IsLocal);
return Addend;
}
// Report an undefined symbol if necessary.
// Returns true if this function printed out an error message.
static bool maybeReportUndefined(Symbol &Sym, InputSectionBase &Sec,
uint64_t Offset) {
if (Sym.isLocal() || !Sym.isUndefined() || Sym.isWeak())
return false;
bool CanBeExternal =
Sym.computeBinding() != STB_LOCAL && Sym.Visibility == STV_DEFAULT;
if (Config->UnresolvedSymbols == UnresolvedPolicy::Ignore && CanBeExternal)
return false;
std::string Msg =
"undefined symbol: " + toString(Sym) + "\n>>> referenced by ";
std::string Src = Sec.getSrcMsg(Sym, Offset);
if (!Src.empty())
Msg += Src + "\n>>> ";
Msg += Sec.getObjMsg(Offset);
if ((Config->UnresolvedSymbols == UnresolvedPolicy::Warn && CanBeExternal) ||
Config->NoinhibitExec) {
warn(Msg);
return false;
}
error(Msg);
return true;
}
// MIPS N32 ABI treats series of successive relocations with the same offset
// as a single relocation. The similar approach used by N64 ABI, but this ABI
// packs all relocations into the single relocation record. Here we emulate
// this for the N32 ABI. Iterate over relocation with the same offset and put
// theirs types into the single bit-set.
template <class RelTy> static RelType getMipsN32RelType(RelTy *&Rel, RelTy *End) {
RelType Type = 0;
uint64_t Offset = Rel->r_offset;
int N = 0;
while (Rel != End && Rel->r_offset == Offset)
Type |= (Rel++)->getType(Config->IsMips64EL) << (8 * N++);
return Type;
}
// .eh_frame sections are mergeable input sections, so their input
// offsets are not linearly mapped to output section. For each input
// offset, we need to find a section piece containing the offset and
// add the piece's base address to the input offset to compute the
// output offset. That isn't cheap.
//
// This class is to speed up the offset computation. When we process
// relocations, we access offsets in the monotonically increasing
// order. So we can optimize for that access pattern.
//
// For sections other than .eh_frame, this class doesn't do anything.
namespace {
class OffsetGetter {
public:
explicit OffsetGetter(InputSectionBase &Sec) {
if (auto *Eh = dyn_cast<EhInputSection>(&Sec))
Pieces = Eh->Pieces;
}
// Translates offsets in input sections to offsets in output sections.
// Given offset must increase monotonically. We assume that Piece is
// sorted by InputOff.
uint64_t get(uint64_t Off) {
if (Pieces.empty())
return Off;
while (I != Pieces.size() && Pieces[I].InputOff + Pieces[I].Size <= Off)
++I;
if (I == Pieces.size())
fatal(".eh_frame: relocation is not in any piece");
// Pieces must be contiguous, so there must be no holes in between.
assert(Pieces[I].InputOff <= Off && "Relocation not in any piece");
// Offset -1 means that the piece is dead (i.e. garbage collected).
if (Pieces[I].OutputOff == -1)
return -1;
return Pieces[I].OutputOff + Off - Pieces[I].InputOff;
}
private:
ArrayRef<EhSectionPiece> Pieces;
size_t I = 0;
};
} // namespace
static void addRelativeReloc(InputSectionBase *IS, uint64_t OffsetInSec,
Symbol *Sym, int64_t Addend, RelExpr Expr,
RelType Type) {
// Add a relative relocation. If RelrDyn section is enabled, and the
// relocation offset is guaranteed to be even, add the relocation to
// the RelrDyn section, otherwise add it to the RelaDyn section.
// RelrDyn sections don't support odd offsets. Also, RelrDyn sections
// don't store the addend values, so we must write it to the relocated
// address.
if (InX::RelrDyn && IS->Alignment >= 2 && OffsetInSec % 2 == 0) {
IS->Relocations.push_back({Expr, Type, OffsetInSec, Addend, Sym});
InX::RelrDyn->Relocs.push_back({IS, OffsetInSec});
return;
}
InX::RelaDyn->addReloc(Target->RelativeRel, IS, OffsetInSec, Sym, Addend,
Expr, Type);
}
template <class ELFT, class GotPltSection>
static void addPltEntry(PltSection *Plt, GotPltSection *GotPlt,
RelocationBaseSection *Rel, RelType Type, Symbol &Sym) {
Plt->addEntry<ELFT>(Sym);
GotPlt->addEntry(Sym);
Rel->addReloc(
{Type, GotPlt, Sym.getGotPltOffset(), !Sym.IsPreemptible, &Sym, 0});
}
template <class ELFT> static void addGotEntry(Symbol &Sym) {
InX::Got->addEntry(Sym);
RelExpr Expr = Sym.isTls() ? R_TLS : R_ABS;
uint64_t Off = Sym.getGotOffset();
// If a GOT slot value can be calculated at link-time, which is now,
// we can just fill that out.
//
// (We don't actually write a value to a GOT slot right now, but we
// add a static relocation to a Relocations vector so that
// InputSection::relocate will do the work for us. We may be able
// to just write a value now, but it is a TODO.)
bool IsLinkTimeConstant =
!Sym.IsPreemptible && (!Config->Pic || isAbsolute(Sym));
if (IsLinkTimeConstant) {
InX::Got->Relocations.push_back({Expr, Target->GotRel, Off, 0, &Sym});
return;
}
// Otherwise, we emit a dynamic relocation to .rel[a].dyn so that
// the GOT slot will be fixed at load-time.
if (!Sym.isTls() && !Sym.IsPreemptible && Config->Pic && !isAbsolute(Sym)) {
addRelativeReloc(InX::Got, Off, &Sym, 0, R_ABS, Target->GotRel);
return;
}
InX::RelaDyn->addReloc(Sym.isTls() ? Target->TlsGotRel : Target->GotRel,
InX::Got, Off, &Sym, 0,
Sym.IsPreemptible ? R_ADDEND : R_ABS, Target->GotRel);
}
// Return true if we can define a symbol in the executable that
// contains the value/function of a symbol defined in a shared
// library.
static bool canDefineSymbolInExecutable(Symbol &Sym) {
// If the symbol has default visibility the symbol defined in the
// executable will preempt it.
// Note that we want the visibility of the shared symbol itself, not
// the visibility of the symbol in the output file we are producing. That is
// why we use Sym.StOther.
if ((Sym.StOther & 0x3) == STV_DEFAULT)
return true;
// If we are allowed to break address equality of functions, defining
// a plt entry will allow the program to call the function in the
// .so, but the .so and the executable will no agree on the address
// of the function. Similar logic for objects.
return ((Sym.isFunc() && Config->IgnoreFunctionAddressEquality) ||
(Sym.isObject() && Config->IgnoreDataAddressEquality));
}
// The reason we have to do this early scan is as follows
// * To mmap the output file, we need to know the size
// * For that, we need to know how many dynamic relocs we will have.
// It might be possible to avoid this by outputting the file with write:
// * Write the allocated output sections, computing addresses.
// * Apply relocations, recording which ones require a dynamic reloc.
// * Write the dynamic relocations.
// * Write the rest of the file.
// This would have some drawbacks. For example, we would only know if .rela.dyn
// is needed after applying relocations. If it is, it will go after rw and rx
// sections. Given that it is ro, we will need an extra PT_LOAD. This
// complicates things for the dynamic linker and means we would have to reserve
// space for the extra PT_LOAD even if we end up not using it.
template <class ELFT, class RelTy>
static void processRelocAux(InputSectionBase &Sec, RelExpr Expr, RelType Type,
uint64_t Offset, Symbol &Sym, const RelTy &Rel,
int64_t Addend) {
if (isStaticLinkTimeConstant(Expr, Type, Sym, Sec, Offset)) {
Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return;
}
bool CanWrite = (Sec.Flags & SHF_WRITE) || !Config->ZText;
if (CanWrite) {
// R_GOT refers to a position in the got, even if the symbol is preemptible.
bool IsPreemptibleValue = Sym.IsPreemptible && Expr != R_GOT;
if (!IsPreemptibleValue) {
addRelativeReloc(&Sec, Offset, &Sym, Addend, Expr, Type);
return;
} else if (RelType Rel = Target->getDynRel(Type)) {
InX::RelaDyn->addReloc(Rel, &Sec, Offset, &Sym, Addend, R_ADDEND, Type);
// MIPS ABI turns using of GOT and dynamic relocations inside out.
// While regular ABI uses dynamic relocations to fill up GOT entries
// MIPS ABI requires dynamic linker to fills up GOT entries using
// specially sorted dynamic symbol table. This affects even dynamic
// relocations against symbols which do not require GOT entries
// creation explicitly, i.e. do not have any GOT-relocations. So if
// a preemptible symbol has a dynamic relocation we anyway have
// to create a GOT entry for it.
// If a non-preemptible symbol has a dynamic relocation against it,
// dynamic linker takes it st_value, adds offset and writes down
// result of the dynamic relocation. In case of preemptible symbol
// dynamic linker performs symbol resolution, writes the symbol value
// to the GOT entry and reads the GOT entry when it needs to perform
// a dynamic relocation.
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf p.4-19
if (Config->EMachine == EM_MIPS)
InX::MipsGot->addEntry(*Sec.File, Sym, Addend, Expr);
return;
}
}
// If the relocation is to a weak undef, and we are producing
// executable, give up on it and produce a non preemptible 0.
if (!Config->Shared && Sym.isUndefWeak()) {
Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return;
}
if (!CanWrite && (Config->Pic && !isRelExpr(Expr))) {
error(
"can't create dynamic relocation " + toString(Type) + " against " +
(Sym.getName().empty() ? "local symbol" : "symbol: " + toString(Sym)) +
" in readonly segment; recompile object files with -fPIC "
"or pass '-Wl,-z,notext' to allow text relocations in the output" +
getLocation(Sec, Sym, Offset));
return;
}
// Copy relocations are only possible if we are creating an executable.
if (Config->Shared) {
errorOrWarn("relocation " + toString(Type) +
" cannot be used against symbol " + toString(Sym) +
"; recompile with -fPIC" + getLocation(Sec, Sym, Offset));
return;
}
// If the symbol is undefined we already reported any relevant errors.
if (Sym.isUndefined())
return;
if (!canDefineSymbolInExecutable(Sym)) {
error("cannot preempt symbol: " + toString(Sym) +
getLocation(Sec, Sym, Offset));
return;
}
if (Sym.isObject()) {
// Produce a copy relocation.
if (auto *SS = dyn_cast<SharedSymbol>(&Sym)) {
if (!Config->ZCopyreloc)
error("unresolvable relocation " + toString(Type) +
" against symbol '" + toString(*SS) +
"'; recompile with -fPIC or remove '-z nocopyreloc'" +
getLocation(Sec, Sym, Offset));
addCopyRelSymbol<ELFT>(*SS);
}
Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return;
}
if (Sym.isFunc()) {
// This handles a non PIC program call to function in a shared library. In
// an ideal world, we could just report an error saying the relocation can
// overflow at runtime. In the real world with glibc, crt1.o has a
// R_X86_64_PC32 pointing to libc.so.
//
// The general idea on how to handle such cases is to create a PLT entry and
// use that as the function value.
//
// For the static linking part, we just return a plt expr and everything
// else will use the PLT entry as the address.
//
// The remaining problem is making sure pointer equality still works. We
// need the help of the dynamic linker for that. We let it know that we have
// a direct reference to a so symbol by creating an undefined symbol with a
// non zero st_value. Seeing that, the dynamic linker resolves the symbol to
// the value of the symbol we created. This is true even for got entries, so
// pointer equality is maintained. To avoid an infinite loop, the only entry
// that points to the real function is a dedicated got entry used by the
// plt. That is identified by special relocation types (R_X86_64_JUMP_SLOT,
// R_386_JMP_SLOT, etc).
// For position independent executable on i386, the plt entry requires ebx
// to be set. This causes two problems:
// * If some code has a direct reference to a function, it was probably
// compiled without -fPIE/-fPIC and doesn't maintain ebx.
// * If a library definition gets preempted to the executable, it will have
// the wrong ebx value.
if (Config->Pie && Config->EMachine == EM_386)
errorOrWarn("symbol '" + toString(Sym) +
"' cannot be preempted; recompile with -fPIE" +
getLocation(Sec, Sym, Offset));
if (!Sym.isInPlt())
addPltEntry<ELFT>(InX::Plt, InX::GotPlt, InX::RelaPlt, Target->PltRel,
Sym);
if (!Sym.isDefined())
replaceWithDefined(Sym, InX::Plt, Sym.getPltOffset(), 0);
Sym.NeedsPltAddr = true;
Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return;
}
errorOrWarn("symbol '" + toString(Sym) + "' has no type" +
getLocation(Sec, Sym, Offset));
}
template <class ELFT, class RelTy>
static void scanReloc(InputSectionBase &Sec, OffsetGetter &GetOffset, RelTy *&I,
RelTy *End) {
const RelTy &Rel = *I;
Symbol &Sym = Sec.getFile<ELFT>()->getRelocTargetSym(Rel);
RelType Type;
// Deal with MIPS oddity.
if (Config->MipsN32Abi) {
Type = getMipsN32RelType(I, End);
} else {
Type = Rel.getType(Config->IsMips64EL);
++I;
}
// Get an offset in an output section this relocation is applied to.
uint64_t Offset = GetOffset.get(Rel.r_offset);
if (Offset == uint64_t(-1))
return;
// Skip if the target symbol is an erroneous undefined symbol.
if (maybeReportUndefined(Sym, Sec, Rel.r_offset))
return;
const uint8_t *RelocatedAddr = Sec.Data.begin() + Rel.r_offset;
RelExpr Expr = Target->getRelExpr(Type, Sym, RelocatedAddr);
// Ignore "hint" relocations because they are only markers for relaxation.
if (isRelExprOneOf<R_HINT, R_NONE>(Expr))
return;
// Strenghten or relax relocations.
//
// GNU ifunc symbols must be accessed via PLT because their addresses
// are determined by runtime.
//
// On the other hand, if we know that a PLT entry will be resolved within
// the same ELF module, we can skip PLT access and directly jump to the
// destination function. For example, if we are linking a main exectuable,
// all dynamic symbols that can be resolved within the executable will
// actually be resolved that way at runtime, because the main exectuable
// is always at the beginning of a search list. We can leverage that fact.
if (Sym.isGnuIFunc())
Expr = toPlt(Expr);
else if (!Sym.IsPreemptible && Expr == R_GOT_PC && !isAbsoluteValue(Sym))
Expr = Target->adjustRelaxExpr(Type, RelocatedAddr, Expr);
else if (!Sym.IsPreemptible)
Expr = fromPlt(Expr);
// This relocation does not require got entry, but it is relative to got and
// needs it to be created. Here we request for that.
if (isRelExprOneOf<R_GOTONLY_PC, R_GOTONLY_PC_FROM_END, R_GOTREL,
R_GOTREL_FROM_END, R_PPC_TOC>(Expr))
InX::Got->HasGotOffRel = true;
// Read an addend.
int64_t Addend = computeAddend<ELFT>(Rel, End, Sec, Expr, Sym.isLocal());
// Process some TLS relocations, including relaxing TLS relocations.
// Note that this function does not handle all TLS relocations.