[ELF][I386] .got.plt entries for i386 should use the VA of the ifunc resolver function

[peter.smith]

Summary

The i386 ld.so expects the .got.slot entry that is relocated by a R_386_IRELATIVE relocation to point directly at the ifunc resolver and not the address of the PLT entry + 6 (thus entering the lazy resolver). This is also the case for ARM and I suspect it is because these use REL relocations and can't use the addend field to store the address of the ifunc resolver. If the lazy resolver is used we get an error message stating that only R_386_JUMP_SLOT is supported.

As ARM and i386 share the same code, I've removed the ARM specific test and added a writeIgotPlt() function that by default calls writeGotPlt(). ARM and i386 override this to write the address of the ifunc resolver.

[ruiu]

LGTM. Nice!

[joerg]

Please adjust the summary before commit to mention which ld.so you mean. Just to avoid having to guess from the history later...

[peter.smith]

I've altered the commit message to say glibc ld.so. Hope that's ok. R289198

ELF/SyntheticSections.cpp

 }
 
 template <class ELFT> size_t IgotPltSection<ELFT>::getSize() const {
   return Entries.size() * Target->GotPltEntrySize;
 }
 
 template <class ELFT> void IgotPltSection<ELFT>::writeTo(uint8_t *Buf) {
   for (const SymbolBody *B : Entries) {
-    if (Config->EMachine == EM_ARM)
-      // On ARM we are actually part of the Got and not GotPlt.
-      write32le(Buf, B->getVA<ELFT>());
-    else
-      Target->writeGotPlt(Buf, *B);
+    Target->writeIgotPlt(Buf, *B);
     Buf += sizeof(uintX_t);
   }
 }
 
 template <class ELFT>
 StringTableSection<ELFT>::StringTableSection(StringRef Name, bool Dynamic)
     : SyntheticSection<ELFT>(Dynamic ? (uintX_t)SHF_ALLOC : 0, SHT_STRTAB, 1,
                              Name),

ELF/Target.h

 public:
   virtual bool isTlsInitialExecRel(uint32_t Type) const;
   virtual bool isTlsLocalDynamicRel(uint32_t Type) const;
   virtual bool isTlsGlobalDynamicRel(uint32_t Type) const;
   virtual bool isPicRel(uint32_t Type) const { return true; }
   virtual uint32_t getDynRel(uint32_t Type) const { return Type; }
   virtual void writeGotPltHeader(uint8_t *Buf) const {}
   virtual void writeGotPlt(uint8_t *Buf, const SymbolBody &S) const {};
+  virtual void writeIgotPlt(uint8_t *Buf, const SymbolBody &S) const;
   virtual uint64_t getImplicitAddend(const uint8_t *Buf, uint32_t Type) const;
 
   // If lazy binding is supported, the first entry of the PLT has code
   // to call the dynamic linker to resolve PLT entries the first time
   // they are called. This function writes that code.
   virtual void writePltHeader(uint8_t *Buf) const {}
 
   virtual void writePlt(uint8_t *Buf, uint64_t GotEntryAddr,

ELF/Target.cpp

public:
   RelExpr getRelExpr(uint32_t Type, const SymbolBody &S) const override;
   uint64_t getImplicitAddend(const uint8_t *Buf, uint32_t Type) const override;
   void writeGotPltHeader(uint8_t *Buf) const override;
   uint32_t getDynRel(uint32_t Type) const override;
   bool isTlsLocalDynamicRel(uint32_t Type) const override;
   bool isTlsGlobalDynamicRel(uint32_t Type) const override;
   bool isTlsInitialExecRel(uint32_t Type) const override;
   void writeGotPlt(uint8_t *Buf, const SymbolBody &S) const override;
+  void writeIgotPlt(uint8_t *Buf, const SymbolBody &S) const override;
   void writePltHeader(uint8_t *Buf) const override;
   void writePlt(uint8_t *Buf, uint64_t GotEntryAddr, uint64_t PltEntryAddr,
                 int32_t Index, unsigned RelOff) const override;
   void relocateOne(uint8_t *Loc, uint32_t Type, uint64_t Val) const override;
 
   RelExpr adjustRelaxExpr(uint32_t Type, const uint8_t *Data,
                           RelExpr Expr) const override;
   void relaxTlsGdToIe(uint8_t *Loc, uint32_t Type, uint64_t Val) const override;
public:
   RelExpr getRelExpr(uint32_t Type, const SymbolBody &S) const override;
   bool isPicRel(uint32_t Type) const override;
   uint32_t getDynRel(uint32_t Type) const override;
   uint64_t getImplicitAddend(const uint8_t *Buf, uint32_t Type) const override;
   bool isTlsLocalDynamicRel(uint32_t Type) const override;
   bool isTlsGlobalDynamicRel(uint32_t Type) const override;
   bool isTlsInitialExecRel(uint32_t Type) const override;
   void writeGotPlt(uint8_t *Buf, const SymbolBody &S) const override;
+  void writeIgotPlt(uint8_t *Buf, const SymbolBody &S) const override;
   void writePltHeader(uint8_t *Buf) const override;
   void writePlt(uint8_t *Buf, uint64_t GotEntryAddr, uint64_t PltEntryAddr,
                 int32_t Index, unsigned RelOff) const override;
   RelExpr getThunkExpr(RelExpr Expr, uint32_t RelocType, const InputFile &File,
                        const SymbolBody &S) const override;
   void relocateOne(uint8_t *Loc, uint32_t Type, uint64_t Val) const override;
 };
 

 }
 
 bool TargetInfo::isTlsInitialExecRel(uint32_t Type) const { return false; }
 
 bool TargetInfo::isTlsLocalDynamicRel(uint32_t Type) const { return false; }
 
 bool TargetInfo::isTlsGlobalDynamicRel(uint32_t Type) const { return false; }
 
+void TargetInfo::writeIgotPlt(uint8_t *Buf, const SymbolBody &S) const {
+  writeGotPlt(Buf, S);
+}
+
 RelExpr TargetInfo::adjustRelaxExpr(uint32_t Type, const uint8_t *Data,
                                     RelExpr Expr) const {
   return Expr;
 }
 
 void TargetInfo::relaxGot(uint8_t *Loc, uint64_t Val) const {
   llvm_unreachable("Should not have claimed to be relaxable");
 }

 }
 
 void X86TargetInfo::writeGotPlt(uint8_t *Buf, const SymbolBody &S) const {
   // Entries in .got.plt initially points back to the corresponding
   // PLT entries with a fixed offset to skip the first instruction.
   write32le(Buf, S.getPltVA<ELF32LE>() + 6);
 }
 
+void X86TargetInfo::writeIgotPlt(uint8_t *Buf, const SymbolBody &S) const {
+  // An x86 entry is the address of the ifunc resolver function.
+  write32le(Buf, S.getVA<ELF32LE>());
+}
+
 uint32_t X86TargetInfo::getDynRel(uint32_t Type) const {
   if (Type == R_386_TLS_LE)
     return R_386_TLS_TPOFF;
   if (Type == R_386_TLS_LE_32)
     return R_386_TLS_TPOFF32;
   return Type;
 }
 
uint32_t ARMTargetInfo::getDynRel(uint32_t Type) const {
   // Keep it going with a dummy value so that we can find more reloc errors.
   return R_ARM_ABS32;
 }
 
 void ARMTargetInfo::writeGotPlt(uint8_t *Buf, const SymbolBody &) const {
   write32le(Buf, In<ELF32LE>::Plt->getVA());
 }
 
+void ARMTargetInfo::writeIgotPlt(uint8_t *Buf, const SymbolBody &S) const {
+  // An ARM entry is the address of the ifunc resolver function.
+  write32le(Buf, S.getVA<ELF32LE>());
+}
+
 void ARMTargetInfo::writePltHeader(uint8_t *Buf) const {
   const uint8_t PltData[] = {
       0x04, 0xe0, 0x2d, 0xe5, //     str lr, [sp,#-4]!
       0x04, 0xe0, 0x9f, 0xe5, //     ldr lr, L2
       0x0e, 0xe0, 0x8f, 0xe0, // L1: add lr, pc, lr
       0x08, 0xf0, 0xbe, 0xe5, //     ldr pc, [lr, #8]
       0x00, 0x00, 0x00, 0x00, // L2: .word   &(.got.plt) - L1 - 8
   };

test/ELF/gnu-ifunc-plt-i386.s

+// RUN: llvm-mc -filetype=obj -triple=i686-pc-linux %S/Inputs/shared2-x86-64.s -o %t1.o
+// RUN: ld.lld %t1.o --shared -o %t.so
+// RUN: llvm-mc -filetype=obj -triple=i686-pc-linux %s -o %t.o
+// RUN: ld.lld %t.so %t.o -o %tout
+// RUN: llvm-objdump -d %tout | FileCheck %s --check-prefix=DISASM
+// RUN: llvm-objdump -s %tout | FileCheck %s --check-prefix=GOTPLT
+// RUN: llvm-readobj -r -dynamic-table %tout | FileCheck %s
+// REQUIRES: x86
+
+// Check that the IRELATIVE relocations are after the JUMP_SLOT in the plt
+// CHECK: Relocations [
+// CHECK-NEXT:   Section (4) .rel.plt {
+// CHECK-NEXT:     0x1300C R_386_JUMP_SLOT bar2
+// CHECK-NEXT:     0x13010 R_386_JUMP_SLOT zed2
+// CHECK-NEXT:     0x13014 R_386_IRELATIVE
+// CHECK-NEXT:     0x13018 R_386_IRELATIVE
+
+// Check that IRELATIVE .got.plt entries point to ifunc resolver and not
+// back to the plt entry + 6.
+// GOTPLT: Contents of section .got.plt:
+// GOTPLT:       13000 00200100 00000000 00000000 36100100
+// GOTPLT-NEXT:  13010 46100100 00100100 01100100
+
+// Check that the PLTRELSZ tag includes the IRELATIVE relocations
+// CHECK: DynamicSection [
+// CHECK:  0x00000002 PLTRELSZ             32 (bytes)
+
+// Check that a PLT header is written and the ifunc entries appear last
+// DISASM: Disassembly of section .text:
+// DISASM-NEXT: foo:
+// DISASM-NEXT:    11000:       c3      retl
+// DISASM:      bar:
+// DISASM-NEXT:    11001:       c3      retl
+// DISASM:      _start:
+// DISASM-NEXT:    11002:       e8 49 00 00 00          calll   73
+// DISASM-NEXT:    11007:       e8 54 00 00 00          calll   84
+// DISASM-NEXT:    1100c:       e8 1f 00 00 00          calll   31
+// DISASM-NEXT:    11011:       e8 2a 00 00 00          calll   42
+// DISASM-NEXT: Disassembly of section .plt:
+// DISASM-NEXT: .plt:
+// DISASM-NEXT:    11020:       ff 35 04 30 01 00       pushl   77828
+// DISASM-NEXT:    11026:       ff 25 08 30 01 00       jmpl    *77832
+// DISASM-NEXT:    1102c:       90      nop
+// DISASM-NEXT:    1102d:       90      nop
+// DISASM-NEXT:    1102e:       90      nop
+// DISASM-NEXT:    1102f:       90      nop
+// DISASM-NEXT:    11030:       ff 25 0c 30 01 00       jmpl    *77836
+// DISASM-NEXT:    11036:       68 00 00 00 00          pushl   $0
+// DISASM-NEXT:    1103b:       e9 e0 ff ff ff          jmp     -32 <.plt>
+// DISASM-NEXT:    11040:       ff 25 10 30 01 00       jmpl    *77840
+// DISASM-NEXT:    11046:       68 08 00 00 00          pushl   $8
+// DISASM-NEXT:    1104b:       e9 d0 ff ff ff          jmp     -48 <.plt>
+// DISASM-NEXT:    11050:       ff 25 14 30 01 00       jmpl    *77844
+// DISASM-NEXT:    11056:       68 30 00 00 00          pushl   $48
+// DISASM-NEXT:    1105b:       e9 e0 ff ff ff          jmp     -32 <.plt+0x20>
+// DISASM-NEXT:    11060:       ff 25 18 30 01 00       jmpl    *77848
+// DISASM-NEXT:    11066:       68 38 00 00 00          pushl   $56
+// DISASM-NEXT:    1106b:       e9 d0 ff ff ff          jmp     -48 <.plt+0x20>
+
+.text
+.type foo STT_GNU_IFUNC
+.globl foo
+foo:
+ ret
+
+.type bar STT_GNU_IFUNC
+.globl bar
+bar:
+ ret
+
+.globl _start
+_start:
+ call foo
+ call bar
+ call bar2
+ call zed2