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