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