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llvm-project/lld/ELF/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 "Memory.h"
#include "OutputSections.h"
#include "Strings.h"
#include "SymbolTable.h"
#include "SyntheticSections.h"
#include "Target.h"
#include "Thunks.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)
template <class ELFT>
static std::string getLocation(InputSectionBase &S, const SymbolBody &Sym,
uint64_t Off) {
std::string Msg =
"\n>>> defined in " + toString(Sym.File) + "\n>>> referenced by ";
std::string Src = S.getSrcMsg<ELFT>(Off);
if (!Src.empty())
Msg += Src + "\n>>> ";
return Msg + S.getObjMsg<ELFT>(Off);
}
static bool isPreemptible(const SymbolBody &Body, uint32_t Type) {
// In case of MIPS GP-relative relocations always resolve to a definition
// in a regular input file, ignoring the one-definition rule. So we,
// for example, should not attempt to create a dynamic relocation even
// if the target symbol is preemptible. There are two two MIPS GP-relative
// relocations R_MIPS_GPREL16 and R_MIPS_GPREL32. But only R_MIPS_GPREL16
// can be against a preemptible symbol.
// To get MIPS relocation type we apply 0xff mask. In case of O32 ABI all
// relocation types occupy eight bit. In case of N64 ABI we extract first
// relocation from 3-in-1 packet because only the first relocation can
// be against a real symbol.
if (Config->EMachine == EM_MIPS && (Type & 0xff) == R_MIPS_GPREL16)
return false;
return Body.isPreemptible();
}
// 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.
template <class ELFT>
static unsigned handleMipsTlsRelocation(uint32_t Type, SymbolBody &Body,
InputSectionBase &C, uint64_t Offset,
int64_t Addend, RelExpr Expr) {
if (Expr == R_MIPS_TLSLD) {
if (InX::MipsGot->addTlsIndex() && Config->Pic)
In<ELFT>::RelaDyn->addReloc({Target->TlsModuleIndexRel, InX::MipsGot,
InX::MipsGot->getTlsIndexOff(), false,
nullptr, 0});
C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
return 1;
}
if (Expr == R_MIPS_TLSGD) {
if (InX::MipsGot->addDynTlsEntry(Body) && Body.isPreemptible()) {
uint64_t Off = InX::MipsGot->getGlobalDynOffset(Body);
In<ELFT>::RelaDyn->addReloc(
{Target->TlsModuleIndexRel, InX::MipsGot, Off, false, &Body, 0});
if (Body.isPreemptible())
In<ELFT>::RelaDyn->addReloc({Target->TlsOffsetRel, InX::MipsGot,
Off + Config->Wordsize, false, &Body, 0});
}
C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
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(uint32_t Type, SymbolBody &Body,
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 preemtible then
// we know the offset into the TLS block at static link time.
bool NeedDynId = Body.isPreemptible() || Config->Shared;
bool NeedDynOff = Body.isPreemptible();
auto AddTlsReloc = [&](uint64_t Off, uint32_t Type, SymbolBody *Dest,
bool Dyn) {
if (Dyn)
In<ELFT>::RelaDyn->addReloc({Type, InX::Got, Off, false, Dest, 0});
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 : &Body, NeedDynId);
C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
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(Body)) {
uint64_t Off = InX::Got->getGlobalDynOffset(Body);
AddTlsReloc(Off, Target->TlsModuleIndexRel, &Body, NeedDynId);
AddTlsReloc(Off + Config->Wordsize, Target->TlsOffsetRel, &Body,
NeedDynOff);
}
C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
return 1;
}
return 0;
}
// Returns the number of relocations processed.
template <class ELFT>
static unsigned
handleTlsRelocation(uint32_t Type, SymbolBody &Body, InputSectionBase &C,
typename ELFT::uint Offset, int64_t Addend, RelExpr Expr) {
if (!(C.Flags & SHF_ALLOC))
return 0;
if (!Body.isTls())
return 0;
if (Config->EMachine == EM_ARM)
return handleARMTlsRelocation<ELFT>(Type, Body, C, Offset, Addend, Expr);
if (Config->EMachine == EM_MIPS)
return handleMipsTlsRelocation<ELFT>(Type, Body, C, Offset, Addend, Expr);
bool IsPreemptible = isPreemptible(Body, Type);
if (isRelExprOneOf<R_TLSDESC, R_TLSDESC_PAGE, R_TLSDESC_CALL>(Expr) &&
Config->Shared) {
if (InX::Got->addDynTlsEntry(Body)) {
uint64_t Off = InX::Got->getGlobalDynOffset(Body);
In<ELFT>::RelaDyn->addReloc(
{Target->TlsDescRel, InX::Got, Off, !IsPreemptible, &Body, 0});
}
if (Expr != R_TLSDESC_CALL)
C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
return 1;
}
if (isRelExprOneOf<R_TLSLD_PC, R_TLSLD>(Expr)) {
// 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, &Body});
return 2;
}
if (InX::Got->addTlsIndex())
In<ELFT>::RelaDyn->addReloc({Target->TlsModuleIndexRel, InX::Got,
InX::Got->getTlsIndexOff(), false, nullptr,
0});
C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
return 1;
}
// Local-Dynamic relocs can be relaxed to Local-Exec.
if (isRelExprOneOf<R_ABS, R_TLSLD, R_TLSLD_PC>(Expr) && !Config->Shared) {
C.Relocations.push_back(
{R_RELAX_TLS_LD_TO_LE, Type, Offset, Addend, &Body});
return 1;
}
if (isRelExprOneOf<R_TLSDESC, R_TLSDESC_PAGE, R_TLSDESC_CALL, R_TLSGD,
R_TLSGD_PC>(Expr)) {
if (Config->Shared) {
if (InX::Got->addDynTlsEntry(Body)) {
uint64_t Off = InX::Got->getGlobalDynOffset(Body);
In<ELFT>::RelaDyn->addReloc(
{Target->TlsModuleIndexRel, InX::Got, Off, false, &Body, 0});
// If the symbol is preemptible we need the dynamic linker to write
// the offset too.
uint64_t OffsetOff = Off + Config->Wordsize;
if (IsPreemptible)
In<ELFT>::RelaDyn->addReloc(
{Target->TlsOffsetRel, InX::Got, OffsetOff, false, &Body, 0});
else
InX::Got->Relocations.push_back(
{R_ABS, Target->TlsOffsetRel, OffsetOff, 0, &Body});
}
C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
return 1;
}
// Global-Dynamic relocs can be relaxed to Initial-Exec or Local-Exec
// depending on the symbol being locally defined or not.
if (IsPreemptible) {
C.Relocations.push_back(
{Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_IE), Type,
Offset, Addend, &Body});
if (!Body.isInGot()) {
InX::Got->addEntry(Body);
In<ELFT>::RelaDyn->addReloc({Target->TlsGotRel, InX::Got,
Body.getGotOffset(), false, &Body, 0});
}
} else {
C.Relocations.push_back(
{Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_LE), Type,
Offset, Addend, &Body});
}
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>(Expr) &&
!Config->Shared && !IsPreemptible) {
C.Relocations.push_back(
{R_RELAX_TLS_IE_TO_LE, Type, Offset, Addend, &Body});
return 1;
}
if (Expr == R_TLSDESC_CALL)
return 1;
return 0;
}
static uint32_t getMipsPairType(uint32_t Type, const SymbolBody &Sym) {
switch (Type) {
case R_MIPS_HI16:
return R_MIPS_LO16;
case R_MIPS_GOT16:
return Sym.isLocal() ? R_MIPS_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 SymbolBody &Body) {
if (Body.isUndefined())
return !Body.isLocal() && Body.symbol()->isWeak();
if (const auto *DR = dyn_cast<DefinedRegular>(&Body))
return DR->Section == nullptr; // Absolute symbol.
return false;
}
static bool isAbsoluteValue(const SymbolBody &Body) {
return isAbsolute(Body) || Body.isTls();
}
// Returns true if Expr refers a PLT entry.
static bool needsPlt(RelExpr Expr) {
return isRelExprOneOf<R_PLT_PC, R_PPC_PLT_OPD, 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_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.
template <class ELFT>
static bool isStaticLinkTimeConstant(RelExpr E, uint32_t Type,
const SymbolBody &Body,
InputSectionBase &S, uint64_t RelOff) {
// These expressions always compute a constant
if (isRelExprOneOf<R_SIZE, R_GOT_FROM_END, R_GOT_OFF, R_MIPS_GOT_LOCAL_PAGE,
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_PC, R_TLSGD,
R_PPC_PLT_OPD, R_TLSDESC_CALL, R_TLSDESC_PAGE, R_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 (isPreemptible(Body, Type))
return false;
if (!Config->Pic)
return true;
// For the target and the relocation, we want to know if they are
// absolute or relative.
bool AbsVal = isAbsoluteValue(Body);
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 (Body.isUndefined() && !Body.isLocal() && Body.symbol()->isWeak())
return true;
error("relocation " + toString(Type) + " cannot refer to absolute symbol: " +
toString(Body) + getLocation<ELFT>(S, Body, RelOff));
return true;
}
static RelExpr toPlt(RelExpr Expr) {
if (Expr == R_PPC_OPD)
return R_PPC_PLT_OPD;
if (Expr == R_PC)
return R_PLT_PC;
if (Expr == R_PAGE_PC)
return R_PLT_PAGE_PC;
if (Expr == R_ABS)
return R_PLT;
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.
if (Expr == R_PLT_PC)
return R_PC;
if (Expr == R_PPC_PLT_OPD)
return R_PPC_OPD;
if (Expr == R_PLT)
return R_ABS;
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;
uint64_t Value = SS->getValue<ELFT>();
// Determine if the symbol is read-only by scanning the DSO's program headers.
auto *File = cast<SharedFile<ELFT>>(SS->File);
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) && Value >= Phdr.p_vaddr &&
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 different places at runtime.
template <class ELFT>
static std::vector<SharedSymbol *> getSymbolsAt(SharedSymbol *SS) {
typedef typename ELFT::Sym Elf_Sym;
auto *File = cast<SharedFile<ELFT>>(SS->File);
uint64_t Shndx = SS->getShndx<ELFT>();
uint64_t Value = SS->getValue<ELFT>();
std::vector<SharedSymbol *> Ret;
for (const Elf_Sym &S : File->getGlobalSymbols()) {
if (S.st_shndx != Shndx || S.st_value != Value)
continue;
StringRef Name = check(S.getName(File->getStringTable()));
SymbolBody *Sym = Symtab<ELFT>::X->find(Name);
if (auto *Alias = dyn_cast_or_null<SharedSymbol>(Sym))
Ret.push_back(Alias);
}
return Ret;
}
// 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->template getSize<ELFT>();
if (SymSize == 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 = IsReadOnly ? InX::BssRelRo : InX::Bss;
uint64_t Off = Sec->reserveSpace(SymSize, SS->getAlignment<ELFT>());
// 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)) {
Sym->NeedsCopy = true;
Sym->CopyRelSec = Sec;
Sym->CopyRelSecOff = Off;
Sym->symbol()->IsUsedInRegularObj = true;
}
In<ELFT>::RelaDyn->addReloc({Target->CopyRel, Sec, Off, false, SS, 0});
}
static void errorOrWarn(const Twine &Msg) {
if (!Config->NoinhibitExec)
error(Msg);
else
warn(Msg);
}
template <class ELFT>
static RelExpr adjustExpr(SymbolBody &Body, RelExpr Expr, uint32_t Type,
const uint8_t *Data, InputSectionBase &S,
typename ELFT::uint RelOff) {
if (Body.isGnuIFunc()) {
Expr = toPlt(Expr);
} else if (!isPreemptible(Body, Type)) {
if (needsPlt(Expr))
Expr = fromPlt(Expr);
if (Expr == R_GOT_PC && !isAbsoluteValue(Body))
Expr = Target->adjustRelaxExpr(Type, Data, Expr);
}
bool IsWrite = !Config->ZText || (S.Flags & SHF_WRITE);
if (IsWrite || isStaticLinkTimeConstant<ELFT>(Expr, Type, Body, S, RelOff))
return Expr;
// This relocation would require the dynamic linker to write a value to read
// only memory. We can hack around it if we are producing an executable and
// the refered symbol can be preemepted to refer to the executable.
if (Config->Shared || (Config->Pic && !isRelExpr(Expr))) {
error("can't create dynamic relocation " + toString(Type) + " against " +
(Body.getName().empty() ? "local symbol"
: "symbol: " + toString(Body)) +
" in readonly segment" + getLocation<ELFT>(S, Body, RelOff));
return Expr;
}
if (Body.getVisibility() != STV_DEFAULT) {
error("cannot preempt symbol: " + toString(Body) +
getLocation<ELFT>(S, Body, RelOff));
return Expr;
}
if (Body.isObject()) {
// Produce a copy relocation.
auto *B = cast<SharedSymbol>(&Body);
if (!B->NeedsCopy) {
if (Config->ZNocopyreloc)
error("unresolvable relocation " + toString(Type) +
" against symbol '" + toString(*B) +
"'; recompile with -fPIC or remove '-z nocopyreloc'" +
getLocation<ELFT>(S, Body, RelOff));
addCopyRelSymbol<ELFT>(B);
}
return Expr;
}
if (Body.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 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).
Body.NeedsPltAddr = true;
return toPlt(Expr);
}
errorOrWarn("symbol '" + toString(Body) + "' defined in " +
toString(Body.File) + " has no type");
return Expr;
}
// 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 uint8_t *Buf) {
uint32_t Type = Rel.getType(Config->IsMips64EL);
int64_t A = RelTy::IsRela
? getAddend<ELFT>(Rel)
: Target->getImplicitAddend(Buf + Rel.r_offset, Type);
if (Config->EMachine == EM_PPC64 && Config->Pic && Type == R_PPC64_TOC)
A += getPPC64TocBase();
return A;
}
// 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, InputSectionBase &Sec,
RelExpr Expr, SymbolBody &Body,
const RelTy *End) {
if (Expr == R_MIPS_GOTREL && Body.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;
uint32_t Type = Rel.getType(Config->IsMips64EL);
uint32_t PairTy = getMipsPairType(Type, Body);
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)
continue;
if (RI->getSymbol(Config->IsMips64EL) != SymIndex)
continue;
endianness E = Config->Endianness;
int32_t Hi = (read32(Buf + Rel.r_offset, E) & 0xffff) << 16;
int32_t Lo = SignExtend32<16>(read32(Buf + RI->r_offset, E));
return Hi + Lo;
}
warn("can't find matching " + toString(PairTy) + " relocation for " +
toString(Type));
return 0;
}
template <class ELFT>
static void reportUndefined(SymbolBody &Sym, InputSectionBase &S,
uint64_t Offset) {
if (Config->UnresolvedSymbols == UnresolvedPolicy::IgnoreAll)
return;
bool CanBeExternal = Sym.symbol()->computeBinding() != STB_LOCAL &&
Sym.getVisibility() == STV_DEFAULT;
if (Config->UnresolvedSymbols == UnresolvedPolicy::Ignore && CanBeExternal)
return;
std::string Msg =
"undefined symbol: " + toString(Sym) + "\n>>> referenced by ";
std::string Src = S.getSrcMsg<ELFT>(Offset);
if (!Src.empty())
Msg += Src + "\n>>> ";
Msg += S.getObjMsg<ELFT>(Offset);
if (Config->UnresolvedSymbols == UnresolvedPolicy::Warn && CanBeExternal)
warn(Msg);
else
errorOrWarn(Msg);
}
template <class RelTy>
static std::pair<uint32_t, uint32_t>
mergeMipsN32RelTypes(uint32_t Type, uint32_t Offset, RelTy *I, RelTy *E) {
// 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.
uint32_t Processed = 0;
for (; I != E && Offset == I->r_offset; ++I) {
++Processed;
Type |= I->getType(Config->IsMips64EL) << (8 * Processed);
}
return std::make_pair(Type, Processed);
}
// .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)) {
P = Eh->Pieces;
Size = Eh->Pieces.size();
}
}
// Translates offsets in input sections to offsets in output sections.
// Given offset must increase monotonically. We assume that P is
// sorted by InputOff.
uint64_t get(uint64_t Off) {
if (P.empty())
return Off;
while (I != Size && P[I].InputOff + P[I].size() <= Off)
++I;
if (I == Size)
return Off;
// P must be contiguous, so there must be no holes in between.
assert(P[I].InputOff <= Off && "Relocation not in any piece");
// Offset -1 means that the piece is dead (i.e. garbage collected).
if (P[I].OutputOff == -1)
return -1;
return P[I].OutputOff + Off - P[I].InputOff;
}
private:
ArrayRef<EhSectionPiece> P;
size_t I = 0;
size_t Size;
};
} // namespace
template <class ELFT, class GotPltSection>
static void addPltEntry(PltSection *Plt, GotPltSection *GotPlt,
RelocationSection<ELFT> *Rel, uint32_t Type,
SymbolBody &Sym, bool UseSymVA) {
Plt->addEntry<ELFT>(Sym);
GotPlt->addEntry(Sym);
Rel->addReloc({Type, GotPlt, Sym.getGotPltOffset(), UseSymVA, &Sym, 0});
}
template <class ELFT>
static void addGotEntry(SymbolBody &Sym, bool Preemptible) {
InX::Got->addEntry(Sym);
uint64_t Off = Sym.getGotOffset();
uint32_t DynType;
RelExpr Expr = R_ABS;
if (Sym.isTls()) {
DynType = Target->TlsGotRel;
Expr = R_TLS;
} else if (!Preemptible && Config->Pic && !isAbsolute(Sym)) {
DynType = Target->RelativeRel;
} else {
DynType = Target->GotRel;
}
bool Constant = !Preemptible && !(Config->Pic && !isAbsolute(Sym));
if (!Constant)
In<ELFT>::RelaDyn->addReloc(
{DynType, InX::Got, Off, !Preemptible, &Sym, 0});
if (Constant || (!Config->IsRela && !Preemptible))
InX::Got->Relocations.push_back({Expr, DynType, Off, 0, &Sym});
}
// 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 scanRelocs(InputSectionBase &Sec, ArrayRef<RelTy> Rels) {
OffsetGetter GetOffset(Sec);
for (auto I = Rels.begin(), End = Rels.end(); I != End; ++I) {
const RelTy &Rel = *I;
SymbolBody &Body = Sec.getFile<ELFT>()->getRelocTargetSym(Rel);
uint32_t Type = Rel.getType(Config->IsMips64EL);
if (Config->MipsN32Abi) {
uint32_t Processed;
std::tie(Type, Processed) =
mergeMipsN32RelTypes(Type, Rel.r_offset, I + 1, End);
I += Processed;
}
// Compute the offset of this section in the output section.
uint64_t Offset = GetOffset.get(Rel.r_offset);
if (Offset == uint64_t(-1))
continue;
// Report undefined symbols. The fact that we report undefined
// symbols here means that we report undefined symbols only when
// they have relocations pointing to them. We don't care about
// undefined symbols that are in dead-stripped sections.
if (!Body.isLocal() && Body.isUndefined() && !Body.symbol()->isWeak())
reportUndefined<ELFT>(Body, Sec, Rel.r_offset);
RelExpr Expr =
Target->getRelExpr(Type, Body, Sec.Data.begin() + Rel.r_offset);
// Ignore "hint" relocations because they are only markers for relaxation.
if (isRelExprOneOf<R_HINT, R_NONE>(Expr))
continue;
bool Preemptible = isPreemptible(Body, Type);
Expr = adjustExpr<ELFT>(Body, Expr, Type, Sec.Data.data() + Rel.r_offset,
Sec, Rel.r_offset);
if (ErrorCount)
continue;
// 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, Sec.Data.data());
if (Config->EMachine == EM_MIPS)
Addend += computeMipsAddend<ELFT>(Rel, Sec, Expr, Body, End);
// Process some TLS relocations, including relaxing TLS relocations.
// Note that this function does not handle all TLS relocations.
if (unsigned Processed =
handleTlsRelocation<ELFT>(Type, Body, Sec, Offset, Addend, Expr)) {
I += (Processed - 1);
continue;
}
// If a relocation needs PLT, we create PLT and GOTPLT slots for the symbol.
if (needsPlt(Expr) && !Body.isInPlt()) {
if (Body.isGnuIFunc() && !Preemptible)
addPltEntry(InX::Iplt, InX::IgotPlt, In<ELFT>::RelaIplt,
Target->IRelativeRel, Body, true);
else
addPltEntry(InX::Plt, InX::GotPlt, In<ELFT>::RelaPlt, Target->PltRel,
Body, !Preemptible);
}
// Create a GOT slot if a relocation needs GOT.
if (needsGot(Expr)) {
if (Config->EMachine == EM_MIPS) {
// MIPS ABI has special rules to process GOT entries and doesn't
// require relocation entries for them. A special case is TLS
// relocations. In that case dynamic loader applies dynamic
// relocations to initialize TLS GOT entries.
// See "Global Offset Table" in Chapter 5 in the following document
// for detailed description:
// ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
InX::MipsGot->addEntry(Body, Addend, Expr);
if (Body.isTls() && Body.isPreemptible())
In<ELFT>::RelaDyn->addReloc({Target->TlsGotRel, InX::MipsGot,
Body.getGotOffset(), false, &Body, 0});
} else if (!Body.isInGot()) {
addGotEntry<ELFT>(Body, Preemptible);
}
}
if (!needsPlt(Expr) && !needsGot(Expr) && isPreemptible(Body, Type)) {
// We don't know anything about the finaly symbol. Just ask the dynamic
// linker to handle the relocation for us.
if (!Target->isPicRel(Type))
errorOrWarn(
"relocation " + toString(Type) +
" cannot be used against shared object; recompile with -fPIC" +
getLocation<ELFT>(Sec, Body, Offset));
In<ELFT>::RelaDyn->addReloc(
{Target->getDynRel(Type), &Sec, Offset, false, &Body, Addend});
// 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(Body, Addend, Expr);
continue;
}
// If the relocation points to something in the file, we can process it.
bool IsConstant =
isStaticLinkTimeConstant<ELFT>(Expr, Type, Body, Sec, Rel.r_offset);
// The size is not going to change, so we fold it in here.
if (Expr == R_SIZE)
Addend += Body.getSize<ELFT>();
// If the output being produced is position independent, the final value
// is still not known. In that case we still need some help from the
// dynamic linker. We can however do better than just copying the incoming
// relocation. We can process some of it and and just ask the dynamic
// linker to add the load address.
if (!IsConstant)
In<ELFT>::RelaDyn->addReloc(
{Target->RelativeRel, &Sec, Offset, true, &Body, Addend});
// If the produced value is a constant, we just remember to write it
// when outputting this section. We also have to do it if the format
// uses Elf_Rel, since in that case the written value is the addend.
if (IsConstant || !RelTy::IsRela)
Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
}
}
template <class ELFT> void elf::scanRelocations(InputSectionBase &S) {
if (S.AreRelocsRela)
scanRelocs<ELFT>(S, S.relas<ELFT>());
else
scanRelocs<ELFT>(S, S.rels<ELFT>());
}
// Insert the Thunks for OutputSection OS into their designated place
// in the Sections vector, and recalculate the InputSection output section
// offsets.
// This may invalidate any output section offsets stored outside of InputSection
void ThunkCreator::mergeThunks() {
for (auto &KV : ThunkSections) {
std::vector<InputSection *> *ISR = KV.first;
std::vector<ThunkSection *> &Thunks = KV.second;
// Order Thunks in ascending OutSecOff
auto ThunkCmp = [](const ThunkSection *A, const ThunkSection *B) {
return A->OutSecOff < B->OutSecOff;
};
std::stable_sort(Thunks.begin(), Thunks.end(), ThunkCmp);
// Merge sorted vectors of Thunks and InputSections by OutSecOff
std::vector<InputSection *> Tmp;
Tmp.reserve(ISR->size() + Thunks.size());
auto MergeCmp = [](const InputSection *A, const InputSection *B) {
// std::merge requires a strict weak ordering.
if (A->OutSecOff < B->OutSecOff)
return true;
if (A->OutSecOff == B->OutSecOff)
// Check if Thunk is immediately before any specific Target InputSection
// for example Mips LA25 Thunks.
if (auto *TA = dyn_cast<ThunkSection>(A))
if (TA && TA->getTargetInputSection() == B)
return true;
return false;
};
std::merge(ISR->begin(), ISR->end(), Thunks.begin(), Thunks.end(),
std::back_inserter(Tmp), MergeCmp);
*ISR = std::move(Tmp);
}
}
static uint32_t findEndOfFirstNonExec(OutputSectionCommand &Cmd) {
for (BaseCommand *Base : Cmd.Commands)
if (auto *ISD = dyn_cast<InputSectionDescription>(Base))
for (auto *IS : ISD->Sections)
if ((IS->Flags & SHF_EXECINSTR) == 0)
return IS->OutSecOff + IS->getSize();
return 0;
}
ThunkSection *ThunkCreator::getOSThunkSec(OutputSectionCommand *Cmd,
std::vector<InputSection *> *ISR) {
if (CurTS == nullptr) {
uint32_t Off = findEndOfFirstNonExec(*Cmd);
CurTS = addThunkSection(Cmd->Sec, ISR, Off);
}
return CurTS;
}
ThunkSection *ThunkCreator::getISThunkSec(InputSection *IS, OutputSection *OS) {
ThunkSection *TS = ThunkedSections.lookup(IS);
if (TS)
return TS;
auto *TOS = IS->getParent();
// Find InputSectionRange within TOS that IS is in
OutputSectionCommand *C = Script->getCmd(TOS);
std::vector<InputSection *> *Range = nullptr;
for (BaseCommand *BC : C->Commands)
if (auto *ISD = dyn_cast<InputSectionDescription>(BC)) {
InputSection *first = ISD->Sections.front();
InputSection *last = ISD->Sections.back();
if (IS->OutSecOff >= first->OutSecOff &&
IS->OutSecOff <= last->OutSecOff) {
Range = &ISD->Sections;
break;
}
}
TS = addThunkSection(TOS, Range, IS->OutSecOff);
ThunkedSections[IS] = TS;
return TS;
}
ThunkSection *ThunkCreator::addThunkSection(OutputSection *OS,
std::vector<InputSection *> *ISR,
uint64_t Off) {
auto *TS = make<ThunkSection>(OS, Off);
ThunkSections[ISR].push_back(TS);
return TS;
}
std::pair<Thunk *, bool> ThunkCreator::getThunk(SymbolBody &Body,
uint32_t Type) {
auto Res = ThunkedSymbols.insert({&Body, std::vector<Thunk *>()});
if (!Res.second) {
// Check existing Thunks for Body to see if they can be reused
for (Thunk *ET : Res.first->second)
if (ET->isCompatibleWith(Type))
return std::make_pair(ET, false);
}
// No existing compatible Thunk in range, create a new one
Thunk *T = addThunk(Type, Body);
Res.first->second.push_back(T);
return std::make_pair(T, true);
}
// Call Fn on every executable InputSection accessed via the linker script
// InputSectionDescription::Sections.
void ThunkCreator::forEachExecInputSection(
ArrayRef<OutputSectionCommand *> OutputSections,
std::function<void(OutputSectionCommand *, std::vector<InputSection *> *,
InputSection *)>
Fn) {
for (OutputSectionCommand *Cmd : OutputSections) {
OutputSection *OS = Cmd->Sec;
if (!(OS->Flags & SHF_ALLOC) || !(OS->Flags & SHF_EXECINSTR))
continue;
for (BaseCommand *BC : Cmd->Commands)
if (auto *ISD = dyn_cast<InputSectionDescription>(BC)) {
CurTS = nullptr;
for (InputSection *IS : ISD->Sections)
Fn(Cmd, &ISD->Sections, IS);
}
}
}
// Process all relocations from the InputSections that have been assigned
// to OutputSections and redirect through Thunks if needed.
//
// createThunks must be called after scanRelocs has created the Relocations for
// each InputSection. It must be called before the static symbol table is
// finalized. If any Thunks are added to an OutputSection the output section
// offsets of the InputSections will change.
//
// FIXME: All Thunks are assumed to be in range of the relocation. Range
// extension Thunks are not yet supported.
bool ThunkCreator::createThunks(
ArrayRef<OutputSectionCommand *> OutputSections) {
if (Pass > 0)
ThunkSections.clear();
// Create all the Thunks and insert them into synthetic ThunkSections. The
// ThunkSections are later inserted back into the OutputSection.
// We separate the creation of ThunkSections from the insertion of the
// ThunkSections back into the OutputSection as ThunkSections are not always
// inserted into the same OutputSection as the caller.
forEachExecInputSection(OutputSections, [&](OutputSectionCommand *Cmd,
std::vector<InputSection *> *ISR,
InputSection *IS) {
for (Relocation &Rel : IS->Relocations) {
SymbolBody &Body = *Rel.Sym;
if (Thunks.find(&Body) != Thunks.end() ||
!Target->needsThunk(Rel.Expr, Rel.Type, IS->File, Body))
continue;
Thunk *T;
bool IsNew;
std::tie(T, IsNew) = getThunk(Body, Rel.Type);
if (IsNew) {
// Find or create a ThunkSection for the new Thunk
ThunkSection *TS;
if (auto *TIS = T->getTargetInputSection())
TS = getISThunkSec(TIS, Cmd->Sec);
else
TS = getOSThunkSec(Cmd, ISR);
TS->addThunk(T);
Thunks[T->ThunkSym] = T;
}
// Redirect relocation to Thunk, we never go via the PLT to a Thunk
Rel.Sym = T->ThunkSym;
Rel.Expr = fromPlt(Rel.Expr);
}
});
// Merge all created synthetic ThunkSections back into OutputSection
mergeThunks();
++Pass;
return !ThunkSections.empty();
}
template void elf::scanRelocations<ELF32LE>(InputSectionBase &);
template void elf::scanRelocations<ELF32BE>(InputSectionBase &);
template void elf::scanRelocations<ELF64LE>(InputSectionBase &);
template void elf::scanRelocations<ELF64BE>(InputSectionBase &);
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