1//===- EhFrame.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// .eh_frame section contains information on how to unwind the stack when
10// an exception is thrown. The section consists of sequence of CIE and FDE
11// records. The linker needs to merge CIEs and associate FDEs to CIEs.
12// That means the linker has to understand the format of the section.
13//
14// This file contains a few utility functions to read .eh_frame contents.
15//
16//===----------------------------------------------------------------------===//
17
18#include "EhFrame.h"
19#include "Config.h"
20#include "InputSection.h"
21#include "Relocations.h"
22#include "Target.h"
23#include "lld/Common/ErrorHandler.h"
24#include "lld/Common/Strings.h"
25#include "llvm/BinaryFormat/Dwarf.h"
26#include "llvm/Object/ELF.h"
27
28using namespace llvm;
29using namespace llvm::ELF;
30using namespace llvm::dwarf;
31using namespace llvm::object;
32using namespace lld;
33using namespace lld::elf;
34
35namespace {
36class EhReader {
37public:
38  EhReader(InputSectionBase *s, ArrayRef<uint8_t> d) : isec(s), d(d) {}
39  uint8_t getFdeEncoding();
40  bool hasLSDA();
41
42private:
43  template <class P> void failOn(const P *loc, const Twine &msg) {
44    fatal("corrupted .eh_frame: " + msg + "\n>>> defined in " +
45          isec->getObjMsg((const uint8_t *)loc - isec->content().data()));
46  }
47
48  uint8_t readByte();
49  void skipBytes(size_t count);
50  StringRef readString();
51  void skipLeb128();
52  void skipAugP();
53  StringRef getAugmentation();
54
55  InputSectionBase *isec;
56  ArrayRef<uint8_t> d;
57};
58}
59
60// Read a byte and advance D by one byte.
61uint8_t EhReader::readByte() {
62  if (d.empty())
63    failOn(d.data(), "unexpected end of CIE");
64  uint8_t b = d.front();
65  d = d.slice(1);
66  return b;
67}
68
69void EhReader::skipBytes(size_t count) {
70  if (d.size() < count)
71    failOn(d.data(), "CIE is too small");
72  d = d.slice(count);
73}
74
75// Read a null-terminated string.
76StringRef EhReader::readString() {
77  const uint8_t *end = llvm::find(d, '\0');
78  if (end == d.end())
79    failOn(d.data(), "corrupted CIE (failed to read string)");
80  StringRef s = toStringRef(d.slice(0, end - d.begin()));
81  d = d.slice(s.size() + 1);
82  return s;
83}
84
85// Skip an integer encoded in the LEB128 format.
86// Actual number is not of interest because only the runtime needs it.
87// But we need to be at least able to skip it so that we can read
88// the field that follows a LEB128 number.
89void EhReader::skipLeb128() {
90  const uint8_t *errPos = d.data();
91  while (!d.empty()) {
92    uint8_t val = d.front();
93    d = d.slice(1);
94    if ((val & 0x80) == 0)
95      return;
96  }
97  failOn(errPos, "corrupted CIE (failed to read LEB128)");
98}
99
100static size_t getAugPSize(unsigned enc) {
101  switch (enc & 0x0f) {
102  case DW_EH_PE_absptr:
103  case DW_EH_PE_signed:
104    return config->wordsize;
105  case DW_EH_PE_udata2:
106  case DW_EH_PE_sdata2:
107    return 2;
108  case DW_EH_PE_udata4:
109  case DW_EH_PE_sdata4:
110    return 4;
111  case DW_EH_PE_udata8:
112  case DW_EH_PE_sdata8:
113    return 8;
114  }
115  return 0;
116}
117
118void EhReader::skipAugP() {
119  uint8_t enc = readByte();
120  if ((enc & 0xf0) == DW_EH_PE_aligned)
121    failOn(d.data() - 1, "DW_EH_PE_aligned encoding is not supported");
122  size_t size = getAugPSize(enc);
123  if (size == 0)
124    failOn(d.data() - 1, "unknown FDE encoding");
125  if (size >= d.size())
126    failOn(d.data() - 1, "corrupted CIE");
127  d = d.slice(size);
128}
129
130uint8_t elf::getFdeEncoding(EhSectionPiece *p) {
131  return EhReader(p->sec, p->data()).getFdeEncoding();
132}
133
134bool elf::hasLSDA(const EhSectionPiece &p) {
135  return EhReader(p.sec, p.data()).hasLSDA();
136}
137
138StringRef EhReader::getAugmentation() {
139  skipBytes(8);
140  int version = readByte();
141  if (version != 1 && version != 3)
142    failOn(d.data() - 1,
143           "FDE version 1 or 3 expected, but got " + Twine(version));
144
145  StringRef aug = readString();
146
147  // Skip code and data alignment factors.
148  skipLeb128();
149  skipLeb128();
150
151  // Skip the return address register. In CIE version 1 this is a single
152  // byte. In CIE version 3 this is an unsigned LEB128.
153  if (version == 1)
154    readByte();
155  else
156    skipLeb128();
157  return aug;
158}
159
160uint8_t EhReader::getFdeEncoding() {
161  // We only care about an 'R' value, but other records may precede an 'R'
162  // record. Unfortunately records are not in TLV (type-length-value) format,
163  // so we need to teach the linker how to skip records for each type.
164  StringRef aug = getAugmentation();
165  for (char c : aug) {
166    if (c == 'R')
167      return readByte();
168    if (c == 'z')
169      skipLeb128();
170    else if (c == 'L')
171      readByte();
172    else if (c == 'P')
173      skipAugP();
174    else if (c != 'B' && c != 'S' && c != 'G')
175      failOn(aug.data(), "unknown .eh_frame augmentation string: " + aug);
176  }
177  return DW_EH_PE_absptr;
178}
179
180bool EhReader::hasLSDA() {
181  StringRef aug = getAugmentation();
182  for (char c : aug) {
183    if (c == 'L')
184      return true;
185    if (c == 'z')
186      skipLeb128();
187    else if (c == 'P')
188      skipAugP();
189    else if (c == 'R')
190      readByte();
191    else if (c != 'B' && c != 'S' && c != 'G')
192      failOn(aug.data(), "unknown .eh_frame augmentation string: " + aug);
193  }
194  return false;
195}
196