1 //===- DWARF.cpp ----------------------------------------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // The --gdb-index option instructs the linker to emit a .gdb_index section. 10 // The section contains information to make gdb startup faster. 11 // The format of the section is described at 12 // https://sourceware.org/gdb/onlinedocs/gdb/Index-Section-Format.html. 13 // 14 //===----------------------------------------------------------------------===// 15 16 #include "DWARF.h" 17 #include "InputSection.h" 18 #include "Symbols.h" 19 20 using namespace llvm; 21 using namespace llvm::object; 22 using namespace lld; 23 using namespace lld::elf; 24 25 template <class ELFT> LLDDwarfObj<ELFT>::LLDDwarfObj(ObjFile<ELFT> *obj) { 26 // Get the ELF sections to retrieve sh_flags. See the SHF_GROUP comment below. 27 ArrayRef<typename ELFT::Shdr> objSections = obj->template getELFShdrs<ELFT>(); 28 assert(objSections.size() == obj->getSections().size()); 29 for (auto [i, sec] : llvm::enumerate(obj->getSections())) { 30 if (!sec) 31 continue; 32 33 if (LLDDWARFSection *m = 34 StringSwitch<LLDDWARFSection *>(sec->name) 35 .Case(".debug_addr", &addrSection) 36 .Case(".debug_gnu_pubnames", &gnuPubnamesSection) 37 .Case(".debug_gnu_pubtypes", &gnuPubtypesSection) 38 .Case(".debug_line", &lineSection) 39 .Case(".debug_loclists", &loclistsSection) 40 .Case(".debug_names", &namesSection) 41 .Case(".debug_ranges", &rangesSection) 42 .Case(".debug_rnglists", &rnglistsSection) 43 .Case(".debug_str_offsets", &strOffsetsSection) 44 .Default(nullptr)) { 45 m->Data = toStringRef(sec->contentMaybeDecompress()); 46 m->sec = sec; 47 continue; 48 } 49 50 if (sec->name == ".debug_abbrev") 51 abbrevSection = toStringRef(sec->contentMaybeDecompress()); 52 else if (sec->name == ".debug_str") 53 strSection = toStringRef(sec->contentMaybeDecompress()); 54 else if (sec->name == ".debug_line_str") 55 lineStrSection = toStringRef(sec->contentMaybeDecompress()); 56 else if (sec->name == ".debug_info" && 57 !(objSections[i].sh_flags & ELF::SHF_GROUP)) { 58 // In DWARF v5, -fdebug-types-section places type units in .debug_info 59 // sections in COMDAT groups. They are not compile units and thus should 60 // be ignored for .gdb_index/diagnostics purposes. 61 // 62 // We use a simple heuristic: the compile unit does not have the SHF_GROUP 63 // flag. If we place compile units in COMDAT groups in the future, we may 64 // need to perform a lightweight parsing. We drop the SHF_GROUP flag when 65 // the InputSection was created, so we need to retrieve sh_flags from the 66 // associated ELF section header. 67 infoSection.Data = toStringRef(sec->contentMaybeDecompress()); 68 infoSection.sec = sec; 69 } 70 } 71 } 72 73 namespace { 74 template <class RelTy> struct LLDRelocationResolver { 75 // In the ELF ABIs, S sepresents the value of the symbol in the relocation 76 // entry. For Rela, the addend is stored as part of the relocation entry and 77 // is provided by the `findAux` method. 78 // In resolve() methods, the `type` and `offset` arguments would always be 0, 79 // because we don't set an owning object for the `RelocationRef` instance that 80 // we create in `findAux()`. 81 static uint64_t resolve(uint64_t /*type*/, uint64_t /*offset*/, uint64_t s, 82 uint64_t /*locData*/, int64_t addend) { 83 return s + addend; 84 } 85 }; 86 87 template <class ELFT> struct LLDRelocationResolver<Elf_Rel_Impl<ELFT, false>> { 88 // For Rel, the addend is extracted from the relocated location and is 89 // supplied by the caller. 90 static uint64_t resolve(uint64_t /*type*/, uint64_t /*offset*/, uint64_t s, 91 uint64_t locData, int64_t /*addend*/) { 92 return s + locData; 93 } 94 }; 95 } // namespace 96 97 // Find if there is a relocation at Pos in Sec. The code is a bit 98 // more complicated than usual because we need to pass a section index 99 // to llvm since it has no idea about InputSection. 100 template <class ELFT> 101 template <class RelTy> 102 std::optional<RelocAddrEntry> 103 LLDDwarfObj<ELFT>::findAux(const InputSectionBase &sec, uint64_t pos, 104 ArrayRef<RelTy> rels) const { 105 auto it = 106 partition_point(rels, [=](const RelTy &a) { return a.r_offset < pos; }); 107 if (it == rels.end() || it->r_offset != pos) 108 return std::nullopt; 109 const RelTy &rel = *it; 110 111 const ObjFile<ELFT> *file = sec.getFile<ELFT>(); 112 Ctx &ctx = sec.getCtx(); 113 uint32_t symIndex = rel.getSymbol(ctx.arg.isMips64EL); 114 const typename ELFT::Sym &sym = file->template getELFSyms<ELFT>()[symIndex]; 115 uint32_t secIndex = file->getSectionIndex(sym); 116 117 // An undefined symbol may be a symbol defined in a discarded section. We 118 // shall still resolve it. This is important for --gdb-index: the end address 119 // offset of an entry in .debug_ranges is relocated. If it is not resolved, 120 // its zero value will terminate the decoding of .debug_ranges prematurely. 121 Symbol &s = file->getRelocTargetSym(rel); 122 uint64_t val = 0; 123 if (auto *dr = dyn_cast<Defined>(&s)) 124 val = dr->value; 125 126 DataRefImpl d; 127 d.p = getAddend<ELFT>(rel); 128 return RelocAddrEntry{secIndex, RelocationRef(d, nullptr), 129 val, std::optional<object::RelocationRef>(), 130 0, LLDRelocationResolver<RelTy>::resolve}; 131 } 132 133 template <class ELFT> 134 std::optional<RelocAddrEntry> 135 LLDDwarfObj<ELFT>::find(const llvm::DWARFSection &s, uint64_t pos) const { 136 auto &sec = static_cast<const LLDDWARFSection &>(s); 137 const RelsOrRelas<ELFT> rels = 138 sec.sec->template relsOrRelas<ELFT>(/*supportsCrel=*/false); 139 if (rels.areRelocsRel()) 140 return findAux(*sec.sec, pos, rels.rels); 141 return findAux(*sec.sec, pos, rels.relas); 142 } 143 144 template class elf::LLDDwarfObj<ELF32LE>; 145 template class elf::LLDDwarfObj<ELF32BE>; 146 template class elf::LLDDwarfObj<ELF64LE>; 147 template class elf::LLDDwarfObj<ELF64BE>; 148