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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 "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/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.
template <class ELFT>
static unsigned handleMipsTlsRelocation(RelType Type, Symbol &Sym,
InputSectionBase &C, uint64_t Offset,
int64_t Addend, RelExpr Expr) {
if (Expr == R_MIPS_TLSLD) {
if (InX::MipsGot->addTlsIndex() && Config->Pic)
InX::RelaDyn->addReloc(Target->TlsModuleIndexRel, InX::MipsGot,
InX::MipsGot->getTlsIndexOff(), nullptr);
C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return 1;
}
if (Expr == R_MIPS_TLSGD) {
if (InX::MipsGot->addDynTlsEntry(Sym) && Sym.IsPreemptible) {
uint64_t Off = InX::MipsGot->getGlobalDynOffset(Sym);
InX::RelaDyn->addReloc(Target->TlsModuleIndexRel, InX::MipsGot, Off,
&Sym);
if (Sym.IsPreemptible)
InX::RelaDyn->addReloc(Target->TlsOffsetRel, InX::MipsGot,
Off + Config->Wordsize, &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 (!(C.Flags & SHF_ALLOC))
return 0;
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<ELFT>(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_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, &Sym});
return 2;
}
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 (isRelExprOneOf<R_ABS, R_TLSLD, R_TLSLD_PC>(Expr) && !Config->Shared) {
C.Relocations.push_back({R_RELAX_TLS_LD_TO_LE, Type, Offset, Addend, &Sym});
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(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>(Expr) &&
!Config->Shared && !Sym.IsPreemptible) {
C.Relocations.push_back({R_RELAX_TLS_IE_TO_LE, Type, Offset, Addend, &Sym});
return 1;
}
if (Expr == R_TLSDESC_CALL)
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_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.
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_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_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 (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_OPD:
return R_PPC_PLT_OPD;
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_PLT_OPD:
return R_PPC_OPD;
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 different places at runtime.
template <class ELFT>
static std::vector<SharedSymbol *> getSymbolsAt(SharedSymbol &SS) {
typedef typename ELFT::Sym Elf_Sym;
SharedFile<ELFT> &File = SS.getFile<ELFT>();
std::vector<SharedSymbol *> Ret;
for (const Elf_Sym &S : File.getGlobalELFSyms()) {
if (S.st_shndx == SHN_UNDEF || S.st_shndx == SHN_ABS ||
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.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.getSize();
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 = 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)) {
Sym->CopyRelSec = Sec;
Sym->IsUsedInRegularObj = true;
Sym->Used = true;
}
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 (Config->UnresolvedSymbols == UnresolvedPolicy::IgnoreAll)
return false;
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())
return Off;
// 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
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.
RelType Type;
if (Sym.isTls())
Type = Target->TlsGotRel;
else if (!Sym.IsPreemptible && Config->Pic && !isAbsolute(Sym))
Type = Target->RelativeRel;
else
Type = Target->GotRel;
InX::RelaDyn->addReloc(Type, 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 RelExpr 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 Expr;
}
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) {
InX::RelaDyn->addReloc(Target->RelativeRel, &Sec, Offset, &Sym, Addend,
Expr, Type);
return Expr;
} else if (Target->isPicRel(Type)) {
InX::RelaDyn->addReloc(Target->getDynRel(Type), &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(Sym, Addend, Expr);
return Expr;
}
}
// 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 Expr;
}
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 Expr;
}
// 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 Expr;
}
// If the symbol is undefined we already reported any relevant errors.
if (!Sym.isShared()) {
assert(Sym.isUndefined());
return Expr;
}
if (!canDefineSymbolInExecutable(Sym)) {
error("cannot preempt symbol: " + toString(Sym) +
getLocation(Sec, Sym, Offset));
return Expr;
}
if (Sym.isObject()) {
// Produce a copy relocation.
auto &SS = cast<SharedSymbol>(Sym);
if (!SS.CopyRelSec) {
if (Config->ZNocopyreloc)
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 Expr;
}
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));
Sym.NeedsPltAddr = true;
Expr = toPlt(Expr);
Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
return Expr;
}
errorOrWarn("symbol '" + toString(Sym) + "' has no type" +
getLocation(Sec, Sym, Offset));
return Expr;
}
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.
if (unsigned Processed =
handleTlsRelocation<ELFT>(Type, Sym, Sec, Offset, Addend, Expr)) {
I += (Processed - 1);
return;
}
Expr = processRelocAux<ELFT>(Sec, Expr, Type, Offset, Sym, Rel, Addend);
// If a relocation needs PLT, we create PLT and GOTPLT slots for the symbol.
if (needsPlt(Expr) && !Sym.isInPlt()) {
if (Sym.isGnuIFunc() && !Sym.IsPreemptible)
addPltEntry<ELFT>(InX::Iplt, InX::IgotPlt, InX::RelaIplt,
Target->IRelativeRel, Sym);
else
addPltEntry<ELFT>(InX::Plt, InX::GotPlt, InX::RelaPlt, Target->PltRel,
Sym);
}
// 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(Sym, Addend, Expr);
if (Sym.isTls() && Sym.IsPreemptible)
InX::RelaDyn->addReloc(Target->TlsGotRel, InX::MipsGot,
Sym.getGotOffset(), &Sym);
} else if (!Sym.isInGot()) {
addGotEntry<ELFT>(Sym);
}
}
}
template <class ELFT, class RelTy>
static void scanRelocs(InputSectionBase &Sec, ArrayRef<RelTy> Rels) {
OffsetGetter GetOffset(Sec);
// Not all relocations end up in Sec.Relocations, but a lot do.
Sec.Relocations.reserve(Rels.size());
for (auto I = Rels.begin(), End = Rels.end(); I != End;)
scanReloc<ELFT>(Sec, GetOffset, I, End);
}
template <class ELFT> void elf::scanRelocations(InputSectionBase &S) {
if (S.AreRelocsRela)
scanRelocs<ELFT>(S, S.relas<ELFT>());
else
scanRelocs<ELFT>(S, S.rels<ELFT>());
}
// Thunk Implementation
//
// Thunks (sometimes called stubs, veneers or branch islands) are small pieces
// of code that the linker inserts inbetween a caller and a callee. The thunks
// are added at link time rather than compile time as the decision on whether
// a thunk is needed, such as the caller and callee being out of range, can only
// be made at link time.
//
// It is straightforward to tell given the current state of the program when a
// thunk is needed for a particular call. The more difficult part is that
// the thunk needs to be placed in the program such that the caller can reach
// the thunk and the thunk can reach the callee; furthermore, adding thunks to
// the program alters addresses, which can mean more thunks etc.
//
// In lld we have a synthetic ThunkSection that can hold many Thunks.
// The decision to have a ThunkSection act as a container means that we can
// more easily handle the most common case of a single block of contiguous
// Thunks by inserting just a single ThunkSection.
//
// The implementation of Thunks in lld is split across these areas
// Relocations.cpp : Framework for creating and placing thunks
// Thunks.cpp : The code generated for each supported thunk
// Target.cpp : Target specific hooks that the framework uses to decide when
// a thunk is used
// Synthetic.cpp : Implementation of ThunkSection
// Writer.cpp : Iteratively call framework until no more Thunks added
//
// Thunk placement requirements:
// Mips LA25 thunks. These must be placed immediately before the callee section
// We can assume that the caller is in range of the Thunk. These are modelled
// by Thunks that return the section they must precede with
// getTargetInputSection().
//
// ARM interworking and range extension thunks. These thunks must be placed
// within range of the caller. All implemented ARM thunks can always reach the
// callee as they use an indirect jump via a register that has no range
// restrictions.
//
// Thunk placement algorithm:
// For Mips LA25 ThunkSections; the placement is explicit, it has to be before
// getTargetInputSection().
//
// For thunks that must be placed within range of the caller there are many
// possible choices given that the maximum range from the caller is usually
// much larger than the average InputSection size. Desirable properties include:
// - Maximize reuse of thunks by multiple callers
// - Minimize number of ThunkSections to simplify insertion
// - Handle impact of already added Thunks on addresses
// - Simple to understand and implement
//
// In lld for the first pass, we pre-create one or more ThunkSections per
// InputSectionDescription at Target specific intervals. A ThunkSection is
// placed so that the estimated end of the ThunkSection is within range of the
// start of the InputSectionDescription or the previous ThunkSection. For
// example:
// InputSectionDescription
// Section 0
// ...
// Section N
// ThunkSection 0
// Section N + 1
// ...
// Section N + K
// Thunk Section 1
//
// The intention is that we can add a Thunk to a ThunkSection that is well
// spaced enough to service a number of callers without having to do a lot
// of work. An important principle is that it is not an error if a Thunk cannot
// be placed in a pre-created ThunkSection; when this happens we create a new
// ThunkSection placed next to the caller. This allows us to handle the vast
// majority of thunks simply, but also handle rare cases where the branch range
// is smaller than the target specific spacing.
//
// The algorithm is expected to create all the thunks that are needed in a
// single pass, with a small number of programs needing a second pass due to
// the insertion of thunks in the first pass increasing the offset between
// callers and callees that were only just in range.
//
// A consequence of allowing new ThunkSections to be created outside of the
// pre-created ThunkSections is that in rare cases calls to Thunks that were in
// range in pass K, are out of range in some pass > K due to the insertion of
// more Thunks in between the caller and callee. When this happens we retarget
// the relocation back to the original target and create another Thunk.
// Remove ThunkSections that are empty, this should only be the initial set
// precreated on pass 0.
// 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(ArrayRef<OutputSection *> OutputSections) {
forEachInputSectionDescription(
OutputSections, [&](OutputSection *OS, InputSectionDescription *ISD) {
if (ISD->ThunkSections.empty())
return;
// Remove any zero sized precreated Thunks.
llvm::erase_if(ISD->ThunkSections,
[](const std::pair<ThunkSection *, uint32_t> &TS) {
return TS.first->getSize() == 0;
});
// ISD->ThunkSections contains all created ThunkSections, including
// those inserted in previous passes. Extract the Thunks created this
// pass and order them in ascending OutSecOff.
std::vector<ThunkSection *> NewThunks;
for (const std::pair<ThunkSection *, uint32_t> TS : ISD->ThunkSections)
if (TS.second == Pass)
NewThunks.push_back(TS.first);
std::stable_sort(NewThunks.begin(), NewThunks.end(),
[](const ThunkSection *A, const ThunkSection *B) {
return A->OutSecOff < B->OutSecOff;
});
// Merge sorted vectors of Thunks and InputSections by OutSecOff
std::vector<InputSection *> Tmp;
Tmp.reserve(ISD->Sections.size() + NewThunks.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) {
auto *TA = dyn_cast<ThunkSection>(A);
auto *TB = dyn_cast<ThunkSection>(B);
// Check if Thunk is immediately before any specific Target
// InputSection for example Mips LA25 Thunks.
if (TA && TA->getTargetInputSection() == B)
return true;
if (TA && !TB && !TA->getTargetInputSection())
// Place Thunk Sections without specific targets before
// non-Thunk Sections.
return true;
}
return false;
};
std::merge(ISD->Sections.begin(), ISD->Sections.end(),
NewThunks.begin(), NewThunks.end(), std::back_inserter(Tmp),
MergeCmp);
ISD->Sections = std::move(Tmp);
});
}
// Find or create a ThunkSection within the InputSectionDescription (ISD) that
// is in range of Src. An ISD maps to a range of InputSections described by a
// linker script section pattern such as { .text .text.* }.
ThunkSection *ThunkCreator::getISDThunkSec(OutputSection *OS, InputSection *IS,
InputSectionDescription *ISD,
uint32_t Type, uint64_t Src) {
for (std::pair<ThunkSection *, uint32_t> TP : ISD->ThunkSections) {
ThunkSection *TS = TP.first;
uint64_t TSBase = OS->Addr + TS->OutSecOff;
uint64_t TSLimit = TSBase + TS->getSize();
if (Target->inBranchRange(Type, Src, (Src > TSLimit) ? TSBase : TSLimit))
return TS;
}
// No suitable ThunkSection exists. This can happen when there is a branch
// with lower range than the ThunkSection spacing or when there are too
// many Thunks. Create a new ThunkSection as close to the InputSection as
// possible. Error if InputSection is so large we cannot place ThunkSection
// anywhere in Range.
uint64_t ThunkSecOff = IS->OutSecOff;
if (!Target->inBranchRange(Type, Src, OS->Addr + ThunkSecOff)) {
ThunkSecOff = IS->OutSecOff + IS->getSize();
if (!Target->inBranchRange(Type, Src, OS->Addr + ThunkSecOff))
fatal("InputSection too large for range extension thunk " +
IS->getObjMsg(Src - (OS->Addr + IS->OutSecOff)));
}
return addThunkSection(OS, ISD, ThunkSecOff);
}
// Add a Thunk that needs to be placed in a ThunkSection that immediately
// precedes its Target.
ThunkSection *ThunkCreator::getISThunkSec(InputSection *IS) {
ThunkSection *TS = ThunkedSections.lookup(IS);
if (TS)
return TS;
// Find InputSectionRange within Target Output Section (TOS) that the
// InputSection (IS) that we need to precede is in.
OutputSection *TOS = IS->getParent();
for (BaseCommand *BC : TOS->SectionCommands)
if (auto *ISD = dyn_cast<InputSectionDescription>(BC)) {
if (ISD->Sections.empty())
continue;
InputSection *first = ISD->Sections.front();
InputSection *last = ISD->Sections.back();
if (IS->OutSecOff >= first->OutSecOff &&
IS->OutSecOff <= last->OutSecOff) {
TS = addThunkSection(TOS, ISD, IS->OutSecOff);
ThunkedSections[IS] = TS;
break;
}
}
return TS;
}
// Create one or more ThunkSections per OS that can be used to place Thunks.
// We attempt to place the ThunkSections using the following desirable
// properties:
// - Within range of the maximum number of callers
// - Minimise the number of ThunkSections
//
// We follow a simple but conservative heuristic to place ThunkSections at
// offsets that are multiples of a Target specific branch range.
// For an InputSectionDescription that is smaller than the range, a single
// ThunkSection at the end of the range will do.
//
// For an InputSectionDescription that is more than twice the size of the range,
// we place the last ThunkSection at range bytes from the end of the
// InputSectionDescription in order to increase the likelihood that the
// distance from a thunk to its target will be sufficiently small to
// allow for the creation of a short thunk.
void ThunkCreator::createInitialThunkSections(
ArrayRef<OutputSection *> OutputSections) {
forEachInputSectionDescription(
OutputSections, [&](OutputSection *OS, InputSectionDescription *ISD) {
if (ISD->Sections.empty())
return;
uint32_t ISDBegin = ISD->Sections.front()->OutSecOff;
uint32_t ISDEnd =
ISD->Sections.back()->OutSecOff + ISD->Sections.back()->getSize();
uint32_t LastThunkLowerBound = -1;
if (ISDEnd - ISDBegin > Target->ThunkSectionSpacing * 2)
LastThunkLowerBound = ISDEnd - Target->ThunkSectionSpacing;
uint32_t ISLimit;
uint32_t PrevISLimit = ISDBegin;
uint32_t ThunkUpperBound = ISDBegin + Target->ThunkSectionSpacing;
for (const InputSection *IS : ISD->Sections) {
ISLimit = IS->OutSecOff + IS->getSize();
if (ISLimit > ThunkUpperBound) {
addThunkSection(OS, ISD, PrevISLimit);
ThunkUpperBound = PrevISLimit + Target->ThunkSectionSpacing;
}
if (ISLimit > LastThunkLowerBound)
break;
PrevISLimit = ISLimit;
}
addThunkSection(OS, ISD, ISLimit);
});
}
ThunkSection *ThunkCreator::addThunkSection(OutputSection *OS,
InputSectionDescription *ISD,
uint64_t Off) {
auto *TS = make<ThunkSection>(OS, Off);
ISD->ThunkSections.push_back(std::make_pair(TS, Pass));
return TS;
}
std::pair<Thunk *, bool> ThunkCreator::getThunk(Symbol &Sym, RelType Type,
uint64_t Src) {
std::vector<Thunk *> *ThunkVec = nullptr;
// We use (section, offset) pair to find the thunk position if possible so
// that we create only one thunk for aliased symbols or ICFed sections.
if (auto *D = dyn_cast<Defined>(&Sym))
if (!D->isInPlt() && D->Section)
ThunkVec = &ThunkedSymbolsBySection[{D->Section->Repl, D->Value}];
if (!ThunkVec)
ThunkVec = &ThunkedSymbols[&Sym];
// Check existing Thunks for Sym to see if they can be reused
for (Thunk *ET : *ThunkVec)
if (ET->isCompatibleWith(Type) &&
Target->inBranchRange(Type, Src, ET->getThunkTargetSym()->getVA()))
return std::make_pair(ET, false);
// No existing compatible Thunk in range, create a new one
Thunk *T = addThunk(Type, Sym);
ThunkVec->push_back(T);
return std::make_pair(T, true);
}
// Call Fn on every executable InputSection accessed via the linker script
// InputSectionDescription::Sections.
void ThunkCreator::forEachInputSectionDescription(
ArrayRef<OutputSection *> OutputSections,
std::function<void(OutputSection *, InputSectionDescription *)> Fn) {
for (OutputSection *OS : OutputSections) {
if (!(OS->Flags & SHF_ALLOC) || !(OS->Flags & SHF_EXECINSTR))
continue;
for (BaseCommand *BC : OS->SectionCommands)
if (auto *ISD = dyn_cast<InputSectionDescription>(BC))
Fn(OS, ISD);
}
}
// Return true if the relocation target is an in range Thunk.
// Return false if the relocation is not to a Thunk. If the relocation target
// was originally to a Thunk, but is no longer in range we revert the
// relocation back to its original non-Thunk target.
bool ThunkCreator::normalizeExistingThunk(Relocation &Rel, uint64_t Src) {
if (Thunk *ET = Thunks.lookup(Rel.Sym)) {
if (Target->inBranchRange(Rel.Type, Src, Rel.Sym->getVA()))
return true;
Rel.Sym = &ET->Destination;
if (Rel.Sym->isInPlt())
Rel.Expr = toPlt(Rel.Expr);
}
return false;
}
// Process all relocations from the InputSections that have been assigned
// to InputSectionDescriptions and redirect through Thunks if needed. The
// function should be called iteratively until it returns false.
//
// PreConditions:
// All InputSections that may need a Thunk are reachable from
// OutputSectionCommands.
//
// All OutputSections have an address and all InputSections have an offset
// within the OutputSection.
//
// The offsets between caller (relocation place) and callee
// (relocation target) will not be modified outside of createThunks().
//
// PostConditions:
// If return value is true then ThunkSections have been inserted into
// OutputSections. All relocations that needed a Thunk based on the information
// available to createThunks() on entry have been redirected to a Thunk. Note
// that adding Thunks changes offsets between caller and callee so more Thunks
// may be required.
//
// If return value is false then no more Thunks are needed, and createThunks has
// made no changes. If the target requires range extension thunks, currently
// ARM, then any future change in offset between caller and callee risks a
// relocation out of range error.
bool ThunkCreator::createThunks(ArrayRef<OutputSection *> OutputSections) {
bool AddressesChanged = false;
if (Pass == 0 && Target->ThunkSectionSpacing)
createInitialThunkSections(OutputSections);
else if (Pass == 10)
// With Thunk Size much smaller than branch range we expect to
// converge quickly; if we get to 10 something has gone wrong.
fatal("thunk creation not converged");
// Create all the Thunks and insert them into synthetic ThunkSections. The
// ThunkSections are later inserted back into InputSectionDescriptions.
// We separate the creation of ThunkSections from the insertion of the
// ThunkSections as ThunkSections are not always inserted into the same
// InputSectionDescription as the caller.
forEachInputSectionDescription(
OutputSections, [&](OutputSection *OS, InputSectionDescription *ISD) {
for (InputSection *IS : ISD->Sections)
for (Relocation &Rel : IS->Relocations) {
uint64_t Src = IS->getVA(Rel.Offset);
// If we are a relocation to an existing Thunk, check if it is
// still in range. If not then Rel will be altered to point to its
// original target so another Thunk can be generated.
if (Pass > 0 && normalizeExistingThunk(Rel, Src))
continue;
if (!Target->needsThunk(Rel.Expr, Rel.Type, IS->File, Src,
*Rel.Sym))
continue;
Thunk *T;
bool IsNew;
std::tie(T, IsNew) = getThunk(*Rel.Sym, Rel.Type, Src);
if (IsNew) {
// Find or create a ThunkSection for the new Thunk
ThunkSection *TS;
if (auto *TIS = T->getTargetInputSection())
TS = getISThunkSec(TIS);
else
TS = getISDThunkSec(OS, IS, ISD, Rel.Type, Src);
TS->addThunk(T);
Thunks[T->getThunkTargetSym()] = T;
}
// Redirect relocation to Thunk, we never go via the PLT to a Thunk
Rel.Sym = T->getThunkTargetSym();
Rel.Expr = fromPlt(Rel.Expr);
}
for (auto &P : ISD->ThunkSections)
AddressesChanged |= P.first->assignOffsets();
});
for (auto &P : ThunkedSections)
AddressesChanged |= P.second->assignOffsets();
// Merge all created synthetic ThunkSections back into OutputSection
mergeThunks(OutputSections);
++Pass;
return AddressesChanged;
}
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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