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espindolaespindola
committedFeb 9, 2016
Use the plt entry as the address of some symbols.
This is the function equivalent of a copy relocation. Since functions are expected to change sizes, we cannot use copy relocations. In situations where one would be needed, what is done instead is: * Create a plt entry * Output an undefined symbol whose addr is the plt entry. The dynamic linker makes sure any shared library uses the plt entry as the function address. llvm-svn: 260224
·
llvmorg-21-initllvmorg-3.9.0-rc1
1 parent ad32937 commit a0a65f9

9 files changed

+83
-49
lines changed
 

‎lld/ELF/OutputSections.cpp

+1-1
Original file line numberDiff line numberDiff line change
@@ -1328,7 +1328,7 @@ void SymbolTableSection<ELFT>::writeGlobalSymbols(uint8_t *Buf) {
13281328
OutSec = Out<ELFT>::Bss;
13291329
break;
13301330
case SymbolBody::SharedKind: {
1331-
if (cast<SharedSymbol<ELFT>>(Body)->NeedsCopy)
1331+
if (cast<SharedSymbol<ELFT>>(Body)->needsCopy())
13321332
OutSec = Out<ELFT>::Bss;
13331333
break;
13341334
}

‎lld/ELF/Symbols.cpp

+5-2
Original file line numberDiff line numberDiff line change
@@ -56,9 +56,12 @@ typename ELFFile<ELFT>::uintX_t SymbolBody::getVA() const {
5656
return Out<ELFT>::Bss->getVA() + cast<DefinedCommon>(this)->OffsetInBss;
5757
case SharedKind: {
5858
auto *SS = cast<SharedSymbol<ELFT>>(this);
59-
if (SS->NeedsCopy)
59+
if (!SS->NeedsCopyOrPltAddr)
60+
return 0;
61+
if (SS->IsFunc)
62+
return getPltVA<ELFT>();
63+
else
6064
return Out<ELFT>::Bss->getVA() + SS->OffsetInBss;
61-
return 0;
6265
}
6366
case UndefinedElfKind:
6467
case UndefinedKind:

‎lld/ELF/Symbols.h

+10-4
Original file line numberDiff line numberDiff line change
@@ -130,7 +130,8 @@ class SymbolBody {
130130
SymbolBody(Kind K, StringRef Name, bool IsWeak, uint8_t Visibility,
131131
bool IsTls, bool IsFunc)
132132
: SymbolKind(K), IsWeak(IsWeak), Visibility(Visibility),
133-
MustBeInDynSym(false), IsTls(IsTls), IsFunc(IsFunc), Name(Name) {
133+
MustBeInDynSym(false), NeedsCopyOrPltAddr(false), IsTls(IsTls),
134+
IsFunc(IsFunc), Name(Name) {
134135
IsUsedInRegularObj = K != SharedKind && K != LazyKind;
135136
}
136137

@@ -148,9 +149,14 @@ class SymbolBody {
148149
// If true, the symbol is added to .dynsym symbol table.
149150
unsigned MustBeInDynSym : 1;
150151

152+
// True if the linker has to generate a copy relocation for this shared
153+
// symbol or if the symbol should point to its plt entry.
154+
unsigned NeedsCopyOrPltAddr : 1;
155+
151156
protected:
152157
unsigned IsTls : 1;
153158
unsigned IsFunc : 1;
159+
154160
StringRef Name;
155161
Symbol *Backref = nullptr;
156162
};
@@ -280,10 +286,10 @@ template <class ELFT> class SharedSymbol : public DefinedElf<ELFT> {
280286

281287
SharedFile<ELFT> *File;
282288

283-
// True if the linker has to generate a copy relocation for this shared
284-
// symbol. OffsetInBss is significant only when NeedsCopy is true.
285-
bool NeedsCopy = false;
289+
// OffsetInBss is significant only when needsCopy() is true.
286290
uintX_t OffsetInBss = 0;
291+
292+
bool needsCopy() const { return this->NeedsCopyOrPltAddr && !this->IsFunc; }
287293
};
288294

289295
// This class represents a symbol defined in an archive file. It is

‎lld/ELF/Target.cpp

+34-36
Original file line numberDiff line numberDiff line change
@@ -90,8 +90,8 @@ class X86TargetInfo final : public TargetInfo {
9090
int32_t Index, unsigned RelOff) const override;
9191
bool needsCopyRel(uint32_t Type, const SymbolBody &S) const override;
9292
bool needsDynRelative(unsigned Type) const override;
93-
bool needsGot(uint32_t Type, const SymbolBody &S) const override;
94-
bool needsPlt(uint32_t Type, const SymbolBody &S) const override;
93+
bool needsGot(uint32_t Type, SymbolBody &S) const override;
94+
bool needsPlt(uint32_t Type, SymbolBody &S) const override;
9595
void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t P,
9696
uint64_t SA, uint64_t ZA = 0,
9797
uint8_t *PairedLoc = nullptr) const override;
@@ -121,8 +121,8 @@ class X86_64TargetInfo final : public TargetInfo {
121121
void writePlt(uint8_t *Buf, uint64_t GotEntryAddr, uint64_t PltEntryAddr,
122122
int32_t Index, unsigned RelOff) const override;
123123
bool needsCopyRel(uint32_t Type, const SymbolBody &S) const override;
124-
bool needsGot(uint32_t Type, const SymbolBody &S) const override;
125-
bool needsPlt(uint32_t Type, const SymbolBody &S) const override;
124+
bool needsGot(uint32_t Type, SymbolBody &S) const override;
125+
bool needsPlt(uint32_t Type, SymbolBody &S) const override;
126126
void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t P,
127127
uint64_t SA, uint64_t ZA = 0,
128128
uint8_t *PairedLoc = nullptr) const override;
@@ -157,8 +157,8 @@ class PPC64TargetInfo final : public TargetInfo {
157157
PPC64TargetInfo();
158158
void writePlt(uint8_t *Buf, uint64_t GotEntryAddr, uint64_t PltEntryAddr,
159159
int32_t Index, unsigned RelOff) const override;
160-
bool needsGot(uint32_t Type, const SymbolBody &S) const override;
161-
bool needsPlt(uint32_t Type, const SymbolBody &S) const override;
160+
bool needsGot(uint32_t Type, SymbolBody &S) const override;
161+
bool needsPlt(uint32_t Type, SymbolBody &S) const override;
162162
void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t P,
163163
uint64_t SA, uint64_t ZA = 0,
164164
uint8_t *PairedLoc = nullptr) const override;
@@ -176,8 +176,8 @@ class AArch64TargetInfo final : public TargetInfo {
176176
unsigned getTlsGotRel(unsigned Type = -1) const override;
177177
bool isTlsDynRel(unsigned Type, const SymbolBody &S) const override;
178178
bool needsCopyRel(uint32_t Type, const SymbolBody &S) const override;
179-
bool needsGot(uint32_t Type, const SymbolBody &S) const override;
180-
bool needsPlt(uint32_t Type, const SymbolBody &S) const override;
179+
bool needsGot(uint32_t Type, SymbolBody &S) const override;
180+
bool needsPlt(uint32_t Type, SymbolBody &S) const override;
181181
void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t P,
182182
uint64_t SA, uint64_t ZA = 0,
183183
uint8_t *PairedLoc = nullptr) const override;
@@ -197,8 +197,8 @@ template <class ELFT> class MipsTargetInfo final : public TargetInfo {
197197
unsigned getDynRel(unsigned Type) const override;
198198
void writeGotHeader(uint8_t *Buf) const override;
199199
bool needsCopyRel(uint32_t Type, const SymbolBody &S) const override;
200-
bool needsGot(uint32_t Type, const SymbolBody &S) const override;
201-
bool needsPlt(uint32_t Type, const SymbolBody &S) const override;
200+
bool needsGot(uint32_t Type, SymbolBody &S) const override;
201+
bool needsPlt(uint32_t Type, SymbolBody &S) const override;
202202
void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t P,
203203
uint64_t SA, uint64_t ZA = 0,
204204
uint8_t *PairedLoc = nullptr) const override;
@@ -263,13 +263,9 @@ bool TargetInfo::isHintRel(uint32_t Type) const { return false; }
263263
bool TargetInfo::isRelRelative(uint32_t Type) const { return true; }
264264
bool TargetInfo::isSizeRel(uint32_t Type) const { return false; }
265265

266-
bool TargetInfo::needsGot(uint32_t Type, const SymbolBody &S) const {
267-
return false;
268-
}
266+
bool TargetInfo::needsGot(uint32_t Type, SymbolBody &S) const { return false; }
269267

270-
bool TargetInfo::needsPlt(uint32_t Type, const SymbolBody &S) const {
271-
return false;
272-
}
268+
bool TargetInfo::needsPlt(uint32_t Type, SymbolBody &S) const { return false; }
273269

274270
unsigned TargetInfo::relaxTls(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type,
275271
uint64_t P, uint64_t SA,
@@ -376,15 +372,15 @@ bool X86TargetInfo::needsCopyRel(uint32_t Type, const SymbolBody &S) const {
376372
return false;
377373
}
378374

379-
bool X86TargetInfo::needsGot(uint32_t Type, const SymbolBody &S) const {
375+
bool X86TargetInfo::needsGot(uint32_t Type, SymbolBody &S) const {
380376
if (S.isTls() && Type == R_386_TLS_GD)
381377
return Target->canRelaxTls(Type, &S) && canBePreempted(&S, true);
382378
if (Type == R_386_TLS_GOTIE || Type == R_386_TLS_IE)
383379
return !canRelaxTls(Type, &S);
384380
return Type == R_386_GOT32 || needsPlt(Type, S);
385381
}
386382

387-
bool X86TargetInfo::needsPlt(uint32_t Type, const SymbolBody &S) const {
383+
bool X86TargetInfo::needsPlt(uint32_t Type, SymbolBody &S) const {
388384
return isGnuIFunc<ELF32LE>(S) ||
389385
(Type == R_386_PLT32 && canBePreempted(&S, true)) ||
390386
(Type == R_386_PC32 && S.isShared());
@@ -646,7 +642,7 @@ bool X86_64TargetInfo::needsCopyRel(uint32_t Type, const SymbolBody &S) const {
646642
return false;
647643
}
648644

649-
bool X86_64TargetInfo::needsGot(uint32_t Type, const SymbolBody &S) const {
645+
bool X86_64TargetInfo::needsGot(uint32_t Type, SymbolBody &S) const {
650646
if (Type == R_X86_64_TLSGD)
651647
return Target->canRelaxTls(Type, &S) && canBePreempted(&S, true);
652648
if (Type == R_X86_64_GOTTPOFF)
@@ -658,7 +654,7 @@ bool X86_64TargetInfo::isTlsDynRel(unsigned Type, const SymbolBody &S) const {
658654
return Type == R_X86_64_GOTTPOFF || Type == R_X86_64_TLSGD;
659655
}
660656

661-
bool X86_64TargetInfo::needsPlt(uint32_t Type, const SymbolBody &S) const {
657+
bool X86_64TargetInfo::needsPlt(uint32_t Type, SymbolBody &S) const {
662658
if (needsCopyRel(Type, S))
663659
return false;
664660
if (isGnuIFunc<ELF64LE>(S))
@@ -684,17 +680,19 @@ bool X86_64TargetInfo::needsPlt(uint32_t Type, const SymbolBody &S) const {
684680
// For the static linking part, we just return true and everything else
685681
// will use the the PLT entry as the address.
686682
//
687-
// The remaining (unimplemented) problem is making sure pointer equality
688-
// still works. We need the help of the dynamic linker for that. We
689-
// let it know that we have a direct reference to a so symbol by creating
690-
// an undefined symbol with a non zero st_value. Seeing that, the
691-
// dynamic linker resolves the symbol to the value of the symbol we created.
692-
// This is true even for got entries, so pointer equality is maintained.
693-
// To avoid an infinite loop, the only entry that points to the
694-
// real function is a dedicated got entry used by the plt. That is
695-
// identified by special relocation types (R_X86_64_JUMP_SLOT,
683+
// The remaining problem is making sure pointer equality still works. We
684+
// need the help of the dynamic linker for that. We let it know that we have
685+
// a direct reference to a so symbol by creating an undefined symbol with a
686+
// non zero st_value. Seeing that, the dynamic linker resolves the symbol to
687+
// the value of the symbol we created. This is true even for got entries, so
688+
// pointer equality is maintained. To avoid an infinite loop, the only entry
689+
// that points to the real function is a dedicated got entry used by the
690+
// plt. That is identified by special relocation types (R_X86_64_JUMP_SLOT,
696691
// R_386_JMP_SLOT, etc).
697-
return S.isShared();
692+
if (!S.isShared())
693+
return false;
694+
S.NeedsCopyOrPltAddr = true;
695+
return true;
698696
case R_X86_64_PLT32:
699697
return canBePreempted(&S, true);
700698
}
@@ -989,7 +987,7 @@ void PPC64TargetInfo::writePlt(uint8_t *Buf, uint64_t GotEntryAddr,
989987
write32be(Buf + 28, 0x4e800420); // bctr
990988
}
991989

992-
bool PPC64TargetInfo::needsGot(uint32_t Type, const SymbolBody &S) const {
990+
bool PPC64TargetInfo::needsGot(uint32_t Type, SymbolBody &S) const {
993991
if (needsPlt(Type, S))
994992
return true;
995993

@@ -1005,7 +1003,7 @@ bool PPC64TargetInfo::needsGot(uint32_t Type, const SymbolBody &S) const {
10051003
}
10061004
}
10071005

1008-
bool PPC64TargetInfo::needsPlt(uint32_t Type, const SymbolBody &S) const {
1006+
bool PPC64TargetInfo::needsPlt(uint32_t Type, SymbolBody &S) const {
10091007
// These are function calls that need to be redirected through a PLT stub.
10101008
return Type == R_PPC64_REL24 && canBePreempted(&S, false);
10111009
}
@@ -1240,7 +1238,7 @@ bool AArch64TargetInfo::needsCopyRel(uint32_t Type, const SymbolBody &S) const {
12401238
}
12411239
}
12421240

1243-
bool AArch64TargetInfo::needsGot(uint32_t Type, const SymbolBody &S) const {
1241+
bool AArch64TargetInfo::needsGot(uint32_t Type, SymbolBody &S) const {
12441242
switch (Type) {
12451243
case R_AARCH64_TLSIE_LD64_GOTTPREL_LO12_NC:
12461244
case R_AARCH64_TLSIE_ADR_GOTTPREL_PAGE21:
@@ -1252,7 +1250,7 @@ bool AArch64TargetInfo::needsGot(uint32_t Type, const SymbolBody &S) const {
12521250
}
12531251
}
12541252

1255-
bool AArch64TargetInfo::needsPlt(uint32_t Type, const SymbolBody &S) const {
1253+
bool AArch64TargetInfo::needsPlt(uint32_t Type, SymbolBody &S) const {
12561254
if (isGnuIFunc<ELF64LE>(S))
12571255
return true;
12581256
switch (Type) {
@@ -1439,12 +1437,12 @@ bool MipsTargetInfo<ELFT>::needsCopyRel(uint32_t Type,
14391437
}
14401438

14411439
template <class ELFT>
1442-
bool MipsTargetInfo<ELFT>::needsGot(uint32_t Type, const SymbolBody &S) const {
1440+
bool MipsTargetInfo<ELFT>::needsGot(uint32_t Type, SymbolBody &S) const {
14431441
return Type == R_MIPS_GOT16 || Type == R_MIPS_CALL16;
14441442
}
14451443

14461444
template <class ELFT>
1447-
bool MipsTargetInfo<ELFT>::needsPlt(uint32_t Type, const SymbolBody &S) const {
1445+
bool MipsTargetInfo<ELFT>::needsPlt(uint32_t Type, SymbolBody &S) const {
14481446
return false;
14491447
}
14501448

‎lld/ELF/Target.h

+2-2
Original file line numberDiff line numberDiff line change
@@ -54,8 +54,8 @@ class TargetInfo {
5454

5555
virtual bool isSizeRel(uint32_t Type) const;
5656
virtual bool needsDynRelative(unsigned Type) const { return false; }
57-
virtual bool needsGot(uint32_t Type, const SymbolBody &S) const;
58-
virtual bool needsPlt(uint32_t Type, const SymbolBody &S) const;
57+
virtual bool needsGot(uint32_t Type, SymbolBody &S) const;
58+
virtual bool needsPlt(uint32_t Type, SymbolBody &S) const;
5959
virtual void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type,
6060
uint64_t P, uint64_t SA, uint64_t ZA = 0,
6161
uint8_t *PairedLoc = nullptr) const = 0;

‎lld/ELF/Writer.cpp

+3-3
Original file line numberDiff line numberDiff line change
@@ -302,10 +302,10 @@ void Writer<ELFT>::scanRelocs(
302302
// If a symbol in a DSO is referenced directly instead of through GOT,
303303
// we need to create a copy relocation for the symbol.
304304
if (auto *B = dyn_cast_or_null<SharedSymbol<ELFT>>(Body)) {
305-
if (B->NeedsCopy)
305+
if (B->needsCopy())
306306
continue;
307307
if (Target->needsCopyRel(Type, *B)) {
308-
B->NeedsCopy = true;
308+
B->NeedsCopyOrPltAddr = true;
309309
Out<ELFT>::RelaDyn->addReloc(
310310
{Target->CopyRel, DynamicReloc<ELFT>::Off_Bss, B});
311311
continue;
@@ -968,7 +968,7 @@ template <class ELFT> bool Writer<ELFT>::createSections() {
968968
if (auto *C = dyn_cast<DefinedCommon>(Body))
969969
CommonSymbols.push_back(C);
970970
if (auto *SC = dyn_cast<SharedSymbol<ELFT>>(Body))
971-
if (SC->NeedsCopy)
971+
if (SC->needsCopy())
972972
CopyRelSymbols.push_back(SC);
973973

974974
if (!includeInSymtab<ELFT>(*Body))

‎lld/test/ELF/Inputs/undef-with-plt-addr.s

+4
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,4 @@
1+
.globl set_data
2+
.type set_data,@function
3+
set_data:
4+
retq

‎lld/test/ELF/symbol-override.s

+1-1
Original file line numberDiff line numberDiff line change
@@ -43,4 +43,4 @@ nop
4343
.text
4444
.globl _start
4545
_start:
46-
callq do
46+
callq do@plt

‎lld/test/ELF/undef-with-plt-addr.s

+23
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,23 @@
1+
// REQUIRES: x86
2+
// RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
3+
// RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %p/Inputs/undef-with-plt-addr.s -o %t2.o
4+
// RUN: ld.lld %t2.o -o %t2.so -shared
5+
// RUN: ld.lld %t.o %t2.so -o %t3
6+
// RUN: llvm-readobj -t -s %t3 | FileCheck %s
7+
8+
.globl _start
9+
_start:
10+
movabsq $set_data, %rax
11+
12+
// Test that set_data has an address in the .plt
13+
14+
// CHECK: Name: .plt
15+
// CHECK-NEXT: Type: SHT_PROGBITS
16+
// CHECK-NEXT: Flags [
17+
// CHECK-NEXT: SHF_ALLOC
18+
// CHECK-NEXT: SHF_EXECINSTR
19+
// CHECK-NEXT: ]
20+
// CHECK-NEXT: Address: 0x11010
21+
22+
// CHECK: Name: set_data
23+
// CHECK-NEXT: Value: 0x11020

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