[ELF] - Do not fail on R_*_NONE relocations when parsing the debug info.

[grimar]

Summary

This is https://bugs.llvm.org//show_bug.cgi?id=38919.

Currently, LLD may report "unsupported relocation target while parsing debug info"
when parsing the debug information.

At the same time in the bug sample, it does that for zeroed R_X86_64_NONE relocation:

000000009cca  015e00000001 R_X86_64_64       0000000000000000  + 0
000000009cfa  000000000000 R_X86_64_NONE                        0
000000009d69  015f00000001 R_X86_64_64       0000000000000000  + 0

Which obviously has "invalid" target (not defined symbol) because it is zeroed out.
I believe the nature of R_*_NONE relocation assumes them should be ignored and
that is why they were invented. So this patch teaches LLD to stop reporting
debug information parsing errors for them.

[ruiu]

ELF/DWARF.cpp

80

Looks like calling getRelExpr is overkill as you don't really need a symbol and a relocation target location. You probably should define a more lightweight predicate or just Target->NoneRelType.

[grimar]

ELF/DWARF.cpp

80

I was not sure that introducing Target->NoneRelType is acceptable. But that is what I would really prefer here. I'll do that change.

[grimar]

• Addressed review comment.

[ruiu]

LGTM

ELF/DWARF.cpp

77–79

I don't think you need a comment, as a none relocation is a no-op in any context.

ELF/Arch/AArch64.cpp

 };
 } // namespace
 
 AArch64::AArch64() {
   CopyRel = R_AARCH64_COPY;
   RelativeRel = R_AARCH64_RELATIVE;
   IRelativeRel = R_AARCH64_IRELATIVE;
   GotRel = R_AARCH64_GLOB_DAT;
+  NoneRel = R_AARCH64_NONE;
   PltRel = R_AARCH64_JUMP_SLOT;
   TlsDescRel = R_AARCH64_TLSDESC;
   TlsGotRel = R_AARCH64_TLS_TPREL64;
   GotEntrySize = 8;
   GotPltEntrySize = 8;
   PltEntrySize = 16;
   PltHeaderSize = 32;
   DefaultMaxPageSize = 65536;

ELF/Arch/AMDGPU.cpp

public:
   RelExpr getRelExpr(RelType Type, const Symbol &S,
                      const uint8_t *Loc) const override;
 };
 } // namespace
 
 AMDGPU::AMDGPU() {
   RelativeRel = R_AMDGPU_RELATIVE64;
   GotRel = R_AMDGPU_ABS64;
+  NoneRel = R_AMDGPU_NONE;
   GotEntrySize = 8;
 }
 
 static uint32_t getEFlags(InputFile *File) {
   return cast<ObjFile<ELF64LE>>(File)->getObj().getHeader()->e_flags;
 }
 
 uint32_t AMDGPU::calcEFlags() const {

ELF/Arch/ARM.cpp

 };
 } // namespace
 
 ARM::ARM() {
   CopyRel = R_ARM_COPY;
   RelativeRel = R_ARM_RELATIVE;
   IRelativeRel = R_ARM_IRELATIVE;
   GotRel = R_ARM_GLOB_DAT;
+  NoneRel = R_ARM_NONE;
   PltRel = R_ARM_JUMP_SLOT;
   TlsGotRel = R_ARM_TLS_TPOFF32;
   TlsModuleIndexRel = R_ARM_TLS_DTPMOD32;
   TlsOffsetRel = R_ARM_TLS_DTPOFF32;
   GotBaseSymInGotPlt = false;
   GotEntrySize = 4;
   GotPltEntrySize = 4;
   PltEntrySize = 16;

ELF/Arch/AVR.cpp

 using namespace llvm::support::endian;
 using namespace llvm::ELF;
 using namespace lld;
 using namespace lld::elf;
 
 namespace {
 class AVR final : public TargetInfo {
 public:
+  AVR();
   RelExpr getRelExpr(RelType Type, const Symbol &S,
                      const uint8_t *Loc) const override;
   void relocateOne(uint8_t *Loc, RelType Type, uint64_t Val) const override;
 };
 } // namespace
 
+AVR::AVR() { NoneRel = R_AVR_NONE; }
+
 RelExpr AVR::getRelExpr(RelType Type, const Symbol &S,
                         const uint8_t *Loc) const {
   return R_ABS;
 }
 
 void AVR::relocateOne(uint8_t *Loc, RelType Type, uint64_t Val) const {
   switch (Type) {
   case R_AVR_CALL: {

ELF/Arch/Hexagon.cpp

RelExpr getRelExpr(RelType Type, const Symbol &S,
                      const uint8_t *Loc) const override;
   void relocateOne(uint8_t *Loc, RelType Type, uint64_t Val) const override;
 };
 } // namespace
 
 Hexagon::Hexagon() {
   // Hexagon Linux uses 64K pages by default.
   DefaultMaxPageSize = 0x10000;
+  NoneRel = R_HEX_NONE;
 }
 
 // Support V60 only at the moment.
 uint32_t Hexagon::calcEFlags() const { return 0x60; }
 
 static uint32_t applyMask(uint32_t Mask, uint32_t Data) {
   uint32_t Result = 0;
   size_t Off = 0;

ELF/Arch/Mips.cpp

template <class ELFT> MIPS<ELFT>::MIPS() {
   GotPltHeaderEntriesNum = 2;
   DefaultMaxPageSize = 65536;
   GotEntrySize = sizeof(typename ELFT::uint);
   GotPltEntrySize = sizeof(typename ELFT::uint);
   GotBaseSymInGotPlt = false;
   PltEntrySize = 16;
   PltHeaderSize = 32;
   CopyRel = R_MIPS_COPY;
+  NoneRel = R_MIPS_NONE;
   PltRel = R_MIPS_JUMP_SLOT;
   NeedsThunks = true;
   TrapInstr = 0xefefefef;
 
   if (ELFT::Is64Bits) {
     RelativeRel = (R_MIPS_64 << 8) | R_MIPS_REL32;
     TlsGotRel = R_MIPS_TLS_TPREL64;
     TlsModuleIndexRel = R_MIPS_TLS_DTPMOD64;

ELF/Arch/PPC.cpp

public:
   PPC();
   void relocateOne(uint8_t *Loc, RelType Type, uint64_t Val) const override;
   RelExpr getRelExpr(RelType Type, const Symbol &S,
                      const uint8_t *Loc) const override;
 };
 } // namespace
 
 PPC::PPC() {
+  NoneRel = R_PPC_NONE;
   GotBaseSymOff = 0x8000;
   GotBaseSymInGotPlt = false;
 }
 
 RelExpr PPC::getRelExpr(RelType Type, const Symbol &S,
                         const uint8_t *Loc) const {
   switch (Type) {
   case R_PPC_REL24:

ELF/Arch/PPC64.cpp

 }
 
 static uint32_t readInstrFromHalf16(const uint8_t *Loc) {
   return read32(Loc - (Config->EKind == ELF64BEKind ? 2 : 0));
 }
 
 PPC64::PPC64() {
   GotRel = R_PPC64_GLOB_DAT;
+  NoneRel = R_PPC64_NONE;
   PltRel = R_PPC64_JMP_SLOT;
   RelativeRel = R_PPC64_RELATIVE;
   IRelativeRel = R_PPC64_IRELATIVE;
   GotEntrySize = 8;
   PltEntrySize = 4;
   GotPltEntrySize = 8;
   GotBaseSymInGotPlt = false;
   GotBaseSymOff = 0x8000;

ELF/Arch/RISCV.cpp

 using namespace llvm::ELF;
 using namespace lld;
 using namespace lld::elf;
 
 namespace {
 
 class RISCV final : public TargetInfo {
 public:
+  RISCV();
   virtual uint32_t calcEFlags() const override;
   RelExpr getRelExpr(RelType Type, const Symbol &S,
                      const uint8_t *Loc) const override;
   void relocateOne(uint8_t *Loc, RelType Type, uint64_t Val) const override;
 };
 
 } // end anonymous namespace
 
+RISCV::RISCV() { NoneRel = R_RISCV_NONE; }
+
 static uint32_t getEFlags(InputFile *F) {
   if (Config->Is64)
     return cast<ObjFile<ELF64LE>>(F)->getObj().getHeader()->e_flags;
   else
     return cast<ObjFile<ELF32LE>>(F)->getObj().getHeader()->e_flags;
 }
 
 uint32_t RISCV::calcEFlags() const {

ELF/Arch/SPARCV9.cpp

void writePlt(uint8_t *Buf, uint64_t GotEntryAddr, uint64_t PltEntryAddr,
                 int32_t Index, unsigned RelOff) const override;
   void relocateOne(uint8_t *Loc, RelType Type, uint64_t Val) const override;
 };
 } // namespace
 
 SPARCV9::SPARCV9() {
   CopyRel = R_SPARC_COPY;
   GotRel = R_SPARC_GLOB_DAT;
+  NoneRel = R_SPARC_NONE;
   PltRel = R_SPARC_JMP_SLOT;
   RelativeRel = R_SPARC_RELATIVE;
   GotEntrySize = 8;
   PltEntrySize = 32;
   PltHeaderSize = 4 * PltEntrySize;
 
   PageSize = 8192;
   DefaultMaxPageSize = 0x100000;

ELF/Arch/X86.cpp

public:
   void relaxTlsIeToLe(uint8_t *Loc, RelType Type, uint64_t Val) const override;
   void relaxTlsLdToLe(uint8_t *Loc, RelType Type, uint64_t Val) const override;
 };
 } // namespace
 
 X86::X86() {
   CopyRel = R_386_COPY;
   GotRel = R_386_GLOB_DAT;
+  NoneRel = R_386_NONE;
   PltRel = R_386_JUMP_SLOT;
   IRelativeRel = R_386_IRELATIVE;
   RelativeRel = R_386_RELATIVE;
   TlsGotRel = R_386_TLS_TPOFF;
   TlsModuleIndexRel = R_386_TLS_DTPMOD32;
   TlsOffsetRel = R_386_TLS_DTPOFF32;
   GotEntrySize = 4;
   GotPltEntrySize = 4;

ELF/Arch/X86_64.cpp

private:
   void relaxGotNoPic(uint8_t *Loc, uint64_t Val, uint8_t Op,
                      uint8_t ModRm) const;
 };
 } // namespace
 
 template <class ELFT> X86_64<ELFT>::X86_64() {
   CopyRel = R_X86_64_COPY;
   GotRel = R_X86_64_GLOB_DAT;
+  NoneRel = R_X86_64_NONE;
   PltRel = R_X86_64_JUMP_SLOT;
   RelativeRel = R_X86_64_RELATIVE;
   IRelativeRel = R_X86_64_IRELATIVE;
   TlsGotRel = R_X86_64_TPOFF64;
   TlsModuleIndexRel = R_X86_64_DTPMOD64;
   TlsOffsetRel = R_X86_64_DTPOFF64;
   GotEntrySize = 8;
   GotPltEntrySize = 8;

ELF/DWARF.cpp

 // The section contains information to make gdb startup faster.
 // The format of the section is described at
 // https://sourceware.org/gdb/onlinedocs/gdb/Index-Section-Format.html.
 //
 //===----------------------------------------------------------------------===//
 
 #include "DWARF.h"
 #include "Symbols.h"
+#include "Target.h"
 #include "lld/Common/Memory.h"
 #include "llvm/DebugInfo/DWARF/DWARFDebugPubTable.h"
 #include "llvm/Object/ELFObjectFile.h"
 
 using namespace llvm;
 using namespace llvm::object;
 using namespace lld;
 using namespace lld::elf;
LLDDwarfObj<ELFT>::findAux(const InputSectionBase &Sec, uint64_t Pos,
   const ObjFile<ELFT> *File = Sec.getFile<ELFT>();
   uint32_t SymIndex = Rel.getSymbol(Config->IsMips64EL);
   const typename ELFT::Sym &Sym = File->getELFSyms()[SymIndex];
   uint32_t SecIndex = File->getSectionIndex(Sym);
 
   // Broken debug info can point to a non-Defined symbol.
   auto *DR = dyn_cast<Defined>(&File->getRelocTargetSym(Rel));
   if (!DR) {
-    error("unsupported relocation target while parsing debug info");
+    // Different tools may eliminate relocations and replace them with R_*_NONE
+    // relocations. That means "nothing to relocate". We should just ignore
+    // this case, as it is not an error.
+    RelType Type = Rel.getType(Config->IsMips64EL);
+    if (Type != Target->NoneRel)
+      error(toString(File) + ": relocation " + lld::toString(Type) + " at 0x" +
+            llvm::utohexstr(Rel.r_offset) + " has unsupported target");
     return None;
   }
   uint64_t Val = DR->Value + getAddend<ELFT>(Rel);
 
   // FIXME: We should be consistent about always adding the file
   // offset or not.
   if (DR->Section->Flags & ELF::SHF_ALLOC)
     Val += cast<InputSection>(DR->Section)->getOffsetInFile();

ELF/Target.h

public:
 
   // Offset of _GLOBAL_OFFSET_TABLE_ from base of .got or .got.plt section.
   uint64_t GotBaseSymOff = 0;
   // True if _GLOBAL_OFFSET_TABLE_ is relative to .got.plt, false if .got.
   bool GotBaseSymInGotPlt = true;
 
   RelType CopyRel;
   RelType GotRel;
+  RelType NoneRel;
   RelType PltRel;
   RelType RelativeRel;
   RelType IRelativeRel;
   RelType TlsDescRel;
   RelType TlsGotRel;
   RelType TlsModuleIndexRel;
   RelType TlsOffsetRel;
   unsigned GotEntrySize = 0;

test/ELF/debug-relocation-none.test

+# REQUIRES: x86
+# RUN: yaml2obj %s -o %t.o
+# RUN: not ld.lld %t.o -o /dev/null 2>&1 | FileCheck %s
+
+## Previously we would report an error saying the relocation in .debug_info
+## has an unsupported target.
+## Check we do not report debug information parsing errors when relocation
+## used is of type R_*_NONE, what actually means it should be ignored.
+
+# CHECK-NOT: error
+# CHECK: error: undefined symbol: bar
+# CHECK-NOT: error
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    Content:         '0000000000000000'
+  - Name:            .rela.text
+    Type:            SHT_RELA
+    AddressAlign:    8
+    Link:            .symtab
+    Info:            .text
+    Relocations:
+      - Offset:          0x0000000000000000
+        Symbol:          bar
+        Type:            R_X86_64_64
+  - Name:            .debug_line
+    Type:            SHT_PROGBITS
+    Content:         3300000002001C0000000101FB0E0D000101010100000001000001006162632E7300000000000009020000000000000000140208000101
+  - Name:            .rela.debug_line
+    AddressAlign:    8
+    Type:            SHT_RELA
+    Link:            .symtab
+    Info:            .debug_line
+    Relocations:
+      - Offset:          0x0000000000000029
+        Type:            R_X86_64_NONE
+  - Name:            .debug_info
+    Type:            SHT_PROGBITS
+    AddressAlign:    0x0000000000000001
+    Content:         0C000000040000000000080100000000
+  - Name:            .debug_abbrev
+    Type:            SHT_PROGBITS
+    AddressAlign:    0x0000000000000001
+    Content:         '0111001017000000'
+
+Symbols:
+  Global:
+    - Name:            _start
+      Section:         .text
+    - Name:            bar

test/ELF/undef-broken-debug.test

 # REQUIRES: x86
 # RUN: yaml2obj %s -o %t.o
 # RUN: not ld.lld %t.o -o /dev/null 2>&1 | FileCheck %s
 
 # The debug info has a broken relocation. Check that we don't crash
 # and still report the undefined symbol.
 
-# CHECK: error: unsupported relocation target while parsing debug info
+# CHECK: error: {{.*}}.o: relocation R_X86_64_64 at 0x29 has unsupported target
 # CHECK: error: undefined symbol: bar
 
 --- !ELF
 FileHeader:
   Class:           ELFCLASS64
   Data:            ELFDATA2LSB
   Type:            ET_REL
   Machine:         EM_X86_64