1 //===- EhFrame.cpp -------------------------------------------------------===//
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // .eh_frame section contains information on how to unwind the stack when
11 // an exception is thrown. The section consists of sequence of CIE and FDE
12 // records. The linker needs to merge CIEs and associate FDEs to CIEs.
13 // That means the linker has to understand the format of the section.
15 // This file contains a few utility functions to read .eh_frame contents.
17 //===----------------------------------------------------------------------===//
21 #include "InputSection.h"
22 #include "Relocations.h"
24 #include "lld/Common/ErrorHandler.h"
25 #include "lld/Common/Strings.h"
26 #include "llvm/BinaryFormat/Dwarf.h"
27 #include "llvm/Object/ELF.h"
30 using namespace llvm::ELF;
31 using namespace llvm::dwarf;
32 using namespace llvm::object;
35 using namespace lld::elf;
40 EhReader(InputSectionBase *S, ArrayRef<uint8_t> D) : IS(S), D(D) {}
41 size_t readEhRecordSize();
42 uint8_t getFdeEncoding();
45 template <class P> void failOn(const P *Loc, const Twine &Msg) {
46 fatal("corrupted .eh_frame: " + Msg + "\n>>> defined in " +
47 IS->getObjMsg((const uint8_t *)Loc - IS->Data.data()));
51 void skipBytes(size_t Count);
52 StringRef readString();
61 size_t elf::readEhRecordSize(InputSectionBase *S, size_t Off) {
62 return EhReader(S, S->Data.slice(Off)).readEhRecordSize();
65 // .eh_frame section is a sequence of records. Each record starts with
66 // a 4 byte length field. This function reads the length.
67 size_t EhReader::readEhRecordSize() {
69 failOn(D.data(), "CIE/FDE too small");
71 // First 4 bytes of CIE/FDE is the size of the record.
72 // If it is 0xFFFFFFFF, the next 8 bytes contain the size instead,
73 // but we do not support that format yet.
74 uint64_t V = read32(D.data());
76 failOn(D.data(), "CIE/FDE too large");
77 uint64_t Size = V + 4;
79 failOn(D.data(), "CIE/FDE ends past the end of the section");
83 // Read a byte and advance D by one byte.
84 uint8_t EhReader::readByte() {
86 failOn(D.data(), "unexpected end of CIE");
87 uint8_t B = D.front();
92 void EhReader::skipBytes(size_t Count) {
94 failOn(D.data(), "CIE is too small");
98 // Read a null-terminated string.
99 StringRef EhReader::readString() {
100 const uint8_t *End = std::find(D.begin(), D.end(), '\0');
102 failOn(D.data(), "corrupted CIE (failed to read string)");
103 StringRef S = toStringRef(D.slice(0, End - D.begin()));
104 D = D.slice(S.size() + 1);
108 // Skip an integer encoded in the LEB128 format.
109 // Actual number is not of interest because only the runtime needs it.
110 // But we need to be at least able to skip it so that we can read
111 // the field that follows a LEB128 number.
112 void EhReader::skipLeb128() {
113 const uint8_t *ErrPos = D.data();
115 uint8_t Val = D.front();
117 if ((Val & 0x80) == 0)
120 failOn(ErrPos, "corrupted CIE (failed to read LEB128)");
123 static size_t getAugPSize(unsigned Enc) {
124 switch (Enc & 0x0f) {
125 case DW_EH_PE_absptr:
126 case DW_EH_PE_signed:
127 return Config->Wordsize;
128 case DW_EH_PE_udata2:
129 case DW_EH_PE_sdata2:
131 case DW_EH_PE_udata4:
132 case DW_EH_PE_sdata4:
134 case DW_EH_PE_udata8:
135 case DW_EH_PE_sdata8:
141 void EhReader::skipAugP() {
142 uint8_t Enc = readByte();
143 if ((Enc & 0xf0) == DW_EH_PE_aligned)
144 failOn(D.data() - 1, "DW_EH_PE_aligned encoding is not supported");
145 size_t Size = getAugPSize(Enc);
147 failOn(D.data() - 1, "unknown FDE encoding");
148 if (Size >= D.size())
149 failOn(D.data() - 1, "corrupted CIE");
153 uint8_t elf::getFdeEncoding(EhSectionPiece *P) {
154 return EhReader(P->Sec, P->data()).getFdeEncoding();
157 uint8_t EhReader::getFdeEncoding() {
159 int Version = readByte();
160 if (Version != 1 && Version != 3)
162 "FDE version 1 or 3 expected, but got " + Twine(Version));
164 StringRef Aug = readString();
166 // Skip code and data alignment factors.
170 // Skip the return address register. In CIE version 1 this is a single
171 // byte. In CIE version 3 this is an unsigned LEB128.
177 // We only care about an 'R' value, but other records may precede an 'R'
178 // record. Unfortunately records are not in TLV (type-length-value) format,
179 // so we need to teach the linker how to skip records for each type.
195 failOn(Aug.data(), "unknown .eh_frame augmentation string: " + Aug);
197 return DW_EH_PE_absptr;