[lld] r260224 - Use the plt entry as the address of some symbols.
Rui Ueyama via llvm-commits
llvm-commits at lists.llvm.org
Tue Feb 9 16:18:50 PST 2016
On Tue, Feb 9, 2016 at 7:11 AM, Rafael Espindola via llvm-commits <
llvm-commits at lists.llvm.org> wrote:
> Author: rafael
> Date: Tue Feb 9 09:11:01 2016
> New Revision: 260224
>
> URL: http://llvm.org/viewvc/llvm-project?rev=260224&view=rev
> Log:
> 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.
>
> Added:
> lld/trunk/test/ELF/Inputs/undef-with-plt-addr.s
> lld/trunk/test/ELF/undef-with-plt-addr.s
> Modified:
> lld/trunk/ELF/OutputSections.cpp
> lld/trunk/ELF/Symbols.cpp
> lld/trunk/ELF/Symbols.h
> lld/trunk/ELF/Target.cpp
> lld/trunk/ELF/Target.h
> lld/trunk/ELF/Writer.cpp
> lld/trunk/test/ELF/symbol-override.s
>
> Modified: lld/trunk/ELF/OutputSections.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/OutputSections.cpp?rev=260224&r1=260223&r2=260224&view=diff
>
> ==============================================================================
> --- lld/trunk/ELF/OutputSections.cpp (original)
> +++ lld/trunk/ELF/OutputSections.cpp Tue Feb 9 09:11:01 2016
> @@ -1328,7 +1328,7 @@ void SymbolTableSection<ELFT>::writeGlob
> OutSec = Out<ELFT>::Bss;
> break;
> case SymbolBody::SharedKind: {
> - if (cast<SharedSymbol<ELFT>>(Body)->NeedsCopy)
> + if (cast<SharedSymbol<ELFT>>(Body)->needsCopy())
> OutSec = Out<ELFT>::Bss;
> break;
> }
>
> Modified: lld/trunk/ELF/Symbols.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Symbols.cpp?rev=260224&r1=260223&r2=260224&view=diff
>
> ==============================================================================
> --- lld/trunk/ELF/Symbols.cpp (original)
> +++ lld/trunk/ELF/Symbols.cpp Tue Feb 9 09:11:01 2016
> @@ -56,9 +56,12 @@ typename ELFFile<ELFT>::uintX_t SymbolBo
> return Out<ELFT>::Bss->getVA() +
> cast<DefinedCommon>(this)->OffsetInBss;
> case SharedKind: {
> auto *SS = cast<SharedSymbol<ELFT>>(this);
> - if (SS->NeedsCopy)
> + if (!SS->NeedsCopyOrPltAddr)
> + return 0;
> + if (SS->IsFunc)
> + return getPltVA<ELFT>();
> + else
> return Out<ELFT>::Bss->getVA() + SS->OffsetInBss;
> - return 0;
> }
> case UndefinedElfKind:
> case UndefinedKind:
>
> Modified: lld/trunk/ELF/Symbols.h
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Symbols.h?rev=260224&r1=260223&r2=260224&view=diff
>
> ==============================================================================
> --- lld/trunk/ELF/Symbols.h (original)
> +++ lld/trunk/ELF/Symbols.h Tue Feb 9 09:11:01 2016
> @@ -130,7 +130,8 @@ protected:
> SymbolBody(Kind K, StringRef Name, bool IsWeak, uint8_t Visibility,
> bool IsTls, bool IsFunc)
> : SymbolKind(K), IsWeak(IsWeak), Visibility(Visibility),
> - MustBeInDynSym(false), IsTls(IsTls), IsFunc(IsFunc), Name(Name) {
> + MustBeInDynSym(false), NeedsCopyOrPltAddr(false), IsTls(IsTls),
> + IsFunc(IsFunc), Name(Name) {
> IsUsedInRegularObj = K != SharedKind && K != LazyKind;
> }
>
> @@ -148,9 +149,14 @@ public:
> // If true, the symbol is added to .dynsym symbol table.
> unsigned MustBeInDynSym : 1;
>
> + // True if the linker has to generate a copy relocation for this shared
> + // symbol or if the symbol should point to its plt entry.
> + unsigned NeedsCopyOrPltAddr : 1;
> +
> protected:
> unsigned IsTls : 1;
> unsigned IsFunc : 1;
> +
> StringRef Name;
> Symbol *Backref = nullptr;
> };
> @@ -280,10 +286,10 @@ public:
>
> SharedFile<ELFT> *File;
>
> - // True if the linker has to generate a copy relocation for this shared
> - // symbol. OffsetInBss is significant only when NeedsCopy is true.
> - bool NeedsCopy = false;
> + // OffsetInBss is significant only when needsCopy() is true.
> uintX_t OffsetInBss = 0;
> +
> + bool needsCopy() const { return this->NeedsCopyOrPltAddr &&
> !this->IsFunc; }
> };
>
> // This class represents a symbol defined in an archive file. It is
>
> Modified: lld/trunk/ELF/Target.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Target.cpp?rev=260224&r1=260223&r2=260224&view=diff
>
> ==============================================================================
> --- lld/trunk/ELF/Target.cpp (original)
> +++ lld/trunk/ELF/Target.cpp Tue Feb 9 09:11:01 2016
> @@ -90,8 +90,8 @@ public:
> int32_t Index, unsigned RelOff) const override;
> bool needsCopyRel(uint32_t Type, const SymbolBody &S) const override;
> bool needsDynRelative(unsigned Type) const override;
> - bool needsGot(uint32_t Type, const SymbolBody &S) const override;
> - bool needsPlt(uint32_t Type, const SymbolBody &S) const override;
> + bool needsGot(uint32_t Type, SymbolBody &S) const override;
> + bool needsPlt(uint32_t Type, SymbolBody &S) const override;
> void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t
> P,
> uint64_t SA, uint64_t ZA = 0,
> uint8_t *PairedLoc = nullptr) const override;
> @@ -121,8 +121,8 @@ public:
> void writePlt(uint8_t *Buf, uint64_t GotEntryAddr, uint64_t
> PltEntryAddr,
> int32_t Index, unsigned RelOff) const override;
> bool needsCopyRel(uint32_t Type, const SymbolBody &S) const override;
> - bool needsGot(uint32_t Type, const SymbolBody &S) const override;
> - bool needsPlt(uint32_t Type, const SymbolBody &S) const override;
> + bool needsGot(uint32_t Type, SymbolBody &S) const override;
> + bool needsPlt(uint32_t Type, SymbolBody &S) const override;
> void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t
> P,
> uint64_t SA, uint64_t ZA = 0,
> uint8_t *PairedLoc = nullptr) const override;
> @@ -157,8 +157,8 @@ public:
> PPC64TargetInfo();
> void writePlt(uint8_t *Buf, uint64_t GotEntryAddr, uint64_t
> PltEntryAddr,
> int32_t Index, unsigned RelOff) const override;
> - bool needsGot(uint32_t Type, const SymbolBody &S) const override;
> - bool needsPlt(uint32_t Type, const SymbolBody &S) const override;
> + bool needsGot(uint32_t Type, SymbolBody &S) const override;
> + bool needsPlt(uint32_t Type, SymbolBody &S) const override;
> void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t
> P,
> uint64_t SA, uint64_t ZA = 0,
> uint8_t *PairedLoc = nullptr) const override;
> @@ -176,8 +176,8 @@ public:
> unsigned getTlsGotRel(unsigned Type = -1) const override;
> bool isTlsDynRel(unsigned Type, const SymbolBody &S) const override;
> bool needsCopyRel(uint32_t Type, const SymbolBody &S) const override;
> - bool needsGot(uint32_t Type, const SymbolBody &S) const override;
> - bool needsPlt(uint32_t Type, const SymbolBody &S) const override;
> + bool needsGot(uint32_t Type, SymbolBody &S) const override;
> + bool needsPlt(uint32_t Type, SymbolBody &S) const override;
> void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t
> P,
> uint64_t SA, uint64_t ZA = 0,
> uint8_t *PairedLoc = nullptr) const override;
> @@ -197,8 +197,8 @@ public:
> unsigned getDynRel(unsigned Type) const override;
> void writeGotHeader(uint8_t *Buf) const override;
> bool needsCopyRel(uint32_t Type, const SymbolBody &S) const override;
> - bool needsGot(uint32_t Type, const SymbolBody &S) const override;
> - bool needsPlt(uint32_t Type, const SymbolBody &S) const override;
> + bool needsGot(uint32_t Type, SymbolBody &S) const override;
> + bool needsPlt(uint32_t Type, SymbolBody &S) const override;
> void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t
> P,
> uint64_t SA, uint64_t ZA = 0,
> uint8_t *PairedLoc = nullptr) const override;
> @@ -263,13 +263,9 @@ bool TargetInfo::isHintRel(uint32_t Type
> bool TargetInfo::isRelRelative(uint32_t Type) const { return true; }
> bool TargetInfo::isSizeRel(uint32_t Type) const { return false; }
>
> -bool TargetInfo::needsGot(uint32_t Type, const SymbolBody &S) const {
> - return false;
> -}
> +bool TargetInfo::needsGot(uint32_t Type, SymbolBody &S) const { return
> false; }
>
> -bool TargetInfo::needsPlt(uint32_t Type, const SymbolBody &S) const {
> - return false;
> -}
> +bool TargetInfo::needsPlt(uint32_t Type, SymbolBody &S) const { return
> false; }
>
> unsigned TargetInfo::relaxTls(uint8_t *Loc, uint8_t *BufEnd, uint32_t
> Type,
> uint64_t P, uint64_t SA,
> @@ -376,7 +372,7 @@ bool X86TargetInfo::needsCopyRel(uint32_
> return false;
> }
>
> -bool X86TargetInfo::needsGot(uint32_t Type, const SymbolBody &S) const {
> +bool X86TargetInfo::needsGot(uint32_t Type, SymbolBody &S) const {
> if (S.isTls() && Type == R_386_TLS_GD)
> return Target->canRelaxTls(Type, &S) && canBePreempted(&S, true);
> if (Type == R_386_TLS_GOTIE || Type == R_386_TLS_IE)
> @@ -384,7 +380,7 @@ bool X86TargetInfo::needsGot(uint32_t Ty
> return Type == R_386_GOT32 || needsPlt(Type, S);
> }
>
> -bool X86TargetInfo::needsPlt(uint32_t Type, const SymbolBody &S) const {
> +bool X86TargetInfo::needsPlt(uint32_t Type, SymbolBody &S) const {
> return isGnuIFunc<ELF32LE>(S) ||
> (Type == R_386_PLT32 && canBePreempted(&S, true)) ||
> (Type == R_386_PC32 && S.isShared());
> @@ -646,7 +642,7 @@ bool X86_64TargetInfo::needsCopyRel(uint
> return false;
> }
>
> -bool X86_64TargetInfo::needsGot(uint32_t Type, const SymbolBody &S) const
> {
> +bool X86_64TargetInfo::needsGot(uint32_t Type, SymbolBody &S) const {
> if (Type == R_X86_64_TLSGD)
> return Target->canRelaxTls(Type, &S) && canBePreempted(&S, true);
> if (Type == R_X86_64_GOTTPOFF)
> @@ -658,7 +654,7 @@ bool X86_64TargetInfo::isTlsDynRel(unsig
> return Type == R_X86_64_GOTTPOFF || Type == R_X86_64_TLSGD;
> }
>
> -bool X86_64TargetInfo::needsPlt(uint32_t Type, const SymbolBody &S) const
> {
> +bool X86_64TargetInfo::needsPlt(uint32_t Type, SymbolBody &S) const {
> if (needsCopyRel(Type, S))
> return false;
> if (isGnuIFunc<ELF64LE>(S))
> @@ -684,17 +680,19 @@ bool X86_64TargetInfo::needsPlt(uint32_t
> // For the static linking part, we just return true and everything
> else
> // will use the the PLT entry as the address.
> //
> - // The remaining (unimplemented) 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,
> + // 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).
> - return S.isShared();
> + if (!S.isShared())
> + return false;
> + S.NeedsCopyOrPltAddr = true;
>
Can you move this to a caller of needsPlt (probably to scanRelocs)?
needsPlt is, by its name implies, a predicate, so it is not expected to
mutate a given object.
+ return true;
> case R_X86_64_PLT32:
> return canBePreempted(&S, true);
> }
> @@ -989,7 +987,7 @@ void PPC64TargetInfo::writePlt(uint8_t *
> write32be(Buf + 28, 0x4e800420); // bctr
> }
>
> -bool PPC64TargetInfo::needsGot(uint32_t Type, const SymbolBody &S) const {
> +bool PPC64TargetInfo::needsGot(uint32_t Type, SymbolBody &S) const {
> if (needsPlt(Type, S))
> return true;
>
> @@ -1005,7 +1003,7 @@ bool PPC64TargetInfo::needsGot(uint32_t
> }
> }
>
> -bool PPC64TargetInfo::needsPlt(uint32_t Type, const SymbolBody &S) const {
> +bool PPC64TargetInfo::needsPlt(uint32_t Type, SymbolBody &S) const {
> // These are function calls that need to be redirected through a PLT
> stub.
> return Type == R_PPC64_REL24 && canBePreempted(&S, false);
> }
> @@ -1240,7 +1238,7 @@ bool AArch64TargetInfo::needsCopyRel(uin
> }
> }
>
> -bool AArch64TargetInfo::needsGot(uint32_t Type, const SymbolBody &S)
> const {
> +bool AArch64TargetInfo::needsGot(uint32_t Type, SymbolBody &S) const {
> switch (Type) {
> case R_AARCH64_TLSIE_LD64_GOTTPREL_LO12_NC:
> case R_AARCH64_TLSIE_ADR_GOTTPREL_PAGE21:
> @@ -1252,7 +1250,7 @@ bool AArch64TargetInfo::needsGot(uint32_
> }
> }
>
> -bool AArch64TargetInfo::needsPlt(uint32_t Type, const SymbolBody &S)
> const {
> +bool AArch64TargetInfo::needsPlt(uint32_t Type, SymbolBody &S) const {
> if (isGnuIFunc<ELF64LE>(S))
> return true;
> switch (Type) {
> @@ -1439,12 +1437,12 @@ bool MipsTargetInfo<ELFT>::needsCopyRel(
> }
>
> template <class ELFT>
> -bool MipsTargetInfo<ELFT>::needsGot(uint32_t Type, const SymbolBody &S)
> const {
> +bool MipsTargetInfo<ELFT>::needsGot(uint32_t Type, SymbolBody &S) const {
> return Type == R_MIPS_GOT16 || Type == R_MIPS_CALL16;
> }
>
> template <class ELFT>
> -bool MipsTargetInfo<ELFT>::needsPlt(uint32_t Type, const SymbolBody &S)
> const {
> +bool MipsTargetInfo<ELFT>::needsPlt(uint32_t Type, SymbolBody &S) const {
> return false;
> }
>
>
> Modified: lld/trunk/ELF/Target.h
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Target.h?rev=260224&r1=260223&r2=260224&view=diff
>
> ==============================================================================
> --- lld/trunk/ELF/Target.h (original)
> +++ lld/trunk/ELF/Target.h Tue Feb 9 09:11:01 2016
> @@ -54,8 +54,8 @@ public:
>
> virtual bool isSizeRel(uint32_t Type) const;
> virtual bool needsDynRelative(unsigned Type) const { return false; }
> - virtual bool needsGot(uint32_t Type, const SymbolBody &S) const;
> - virtual bool needsPlt(uint32_t Type, const SymbolBody &S) const;
> + virtual bool needsGot(uint32_t Type, SymbolBody &S) const;
> + virtual bool needsPlt(uint32_t Type, SymbolBody &S) const;
> virtual void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type,
> uint64_t P, uint64_t SA, uint64_t ZA = 0,
> uint8_t *PairedLoc = nullptr) const = 0;
>
> Modified: lld/trunk/ELF/Writer.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Writer.cpp?rev=260224&r1=260223&r2=260224&view=diff
>
> ==============================================================================
> --- lld/trunk/ELF/Writer.cpp (original)
> +++ lld/trunk/ELF/Writer.cpp Tue Feb 9 09:11:01 2016
> @@ -302,10 +302,10 @@ void Writer<ELFT>::scanRelocs(
> // If a symbol in a DSO is referenced directly instead of through GOT,
> // we need to create a copy relocation for the symbol.
> if (auto *B = dyn_cast_or_null<SharedSymbol<ELFT>>(Body)) {
> - if (B->NeedsCopy)
> + if (B->needsCopy())
> continue;
> if (Target->needsCopyRel(Type, *B)) {
> - B->NeedsCopy = true;
> + B->NeedsCopyOrPltAddr = true;
> Out<ELFT>::RelaDyn->addReloc(
> {Target->CopyRel, DynamicReloc<ELFT>::Off_Bss, B});
> continue;
> @@ -968,7 +968,7 @@ template <class ELFT> bool Writer<ELFT>:
> if (auto *C = dyn_cast<DefinedCommon>(Body))
> CommonSymbols.push_back(C);
> if (auto *SC = dyn_cast<SharedSymbol<ELFT>>(Body))
> - if (SC->NeedsCopy)
> + if (SC->needsCopy())
> CopyRelSymbols.push_back(SC);
>
> if (!includeInSymtab<ELFT>(*Body))
>
> Added: lld/trunk/test/ELF/Inputs/undef-with-plt-addr.s
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/Inputs/undef-with-plt-addr.s?rev=260224&view=auto
>
> ==============================================================================
> --- lld/trunk/test/ELF/Inputs/undef-with-plt-addr.s (added)
> +++ lld/trunk/test/ELF/Inputs/undef-with-plt-addr.s Tue Feb 9 09:11:01
> 2016
> @@ -0,0 +1,4 @@
> + .globl set_data
> + .type set_data, at function
> +set_data:
> + retq
>
> Modified: lld/trunk/test/ELF/symbol-override.s
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/symbol-override.s?rev=260224&r1=260223&r2=260224&view=diff
>
> ==============================================================================
> --- lld/trunk/test/ELF/symbol-override.s (original)
> +++ lld/trunk/test/ELF/symbol-override.s Tue Feb 9 09:11:01 2016
> @@ -43,4 +43,4 @@ nop
> .text
> .globl _start
> _start:
> -callq do
> +callq do at plt
>
> Added: lld/trunk/test/ELF/undef-with-plt-addr.s
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/undef-with-plt-addr.s?rev=260224&view=auto
>
> ==============================================================================
> --- lld/trunk/test/ELF/undef-with-plt-addr.s (added)
> +++ lld/trunk/test/ELF/undef-with-plt-addr.s Tue Feb 9 09:11:01 2016
> @@ -0,0 +1,23 @@
> +// REQUIRES: x86
> +// RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
> +// RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux
> %p/Inputs/undef-with-plt-addr.s -o %t2.o
> +// RUN: ld.lld %t2.o -o %t2.so -shared
> +// RUN: ld.lld %t.o %t2.so -o %t3
> +// RUN: llvm-readobj -t -s %t3 | FileCheck %s
> +
> +.globl _start
> +_start:
> +movabsq $set_data, %rax
> +
> +// Test that set_data has an address in the .plt
> +
> +// CHECK: Name: .plt
> +// CHECK-NEXT: Type: SHT_PROGBITS
> +// CHECK-NEXT: Flags [
> +// CHECK-NEXT: SHF_ALLOC
> +// CHECK-NEXT: SHF_EXECINSTR
> +// CHECK-NEXT: ]
> +// CHECK-NEXT: Address: 0x11010
> +
> +// CHECK: Name: set_data
> +// CHECK-NEXT: Value: 0x11020
>
>
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