xref: /freebsd/contrib/llvm-project/lld/ELF/EhFrame.cpp (revision dc318a4ffabcbfa23bb56a33403aad36e6de30af)
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 
28 using namespace llvm;
29 using namespace llvm::ELF;
30 using namespace llvm::dwarf;
31 using namespace llvm::object;
32 using namespace lld;
33 using namespace lld::elf;
34 
35 namespace {
36 class EhReader {
37 public:
38   EhReader(InputSectionBase *s, ArrayRef<uint8_t> d) : isec(s), d(d) {}
39   size_t readEhRecordSize();
40   uint8_t getFdeEncoding();
41 
42 private:
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->data().data()));
46   }
47 
48   uint8_t readByte();
49   void skipBytes(size_t count);
50   StringRef readString();
51   void skipLeb128();
52   void skipAugP();
53 
54   InputSectionBase *isec;
55   ArrayRef<uint8_t> d;
56 };
57 }
58 
59 size_t elf::readEhRecordSize(InputSectionBase *s, size_t off) {
60   return EhReader(s, s->data().slice(off)).readEhRecordSize();
61 }
62 
63 // .eh_frame section is a sequence of records. Each record starts with
64 // a 4 byte length field. This function reads the length.
65 size_t EhReader::readEhRecordSize() {
66   if (d.size() < 4)
67     failOn(d.data(), "CIE/FDE too small");
68 
69   // First 4 bytes of CIE/FDE is the size of the record.
70   // If it is 0xFFFFFFFF, the next 8 bytes contain the size instead,
71   // but we do not support that format yet.
72   uint64_t v = read32(d.data());
73   if (v == UINT32_MAX)
74     failOn(d.data(), "CIE/FDE too large");
75   uint64_t size = v + 4;
76   if (size > d.size())
77     failOn(d.data(), "CIE/FDE ends past the end of the section");
78   return size;
79 }
80 
81 // Read a byte and advance D by one byte.
82 uint8_t EhReader::readByte() {
83   if (d.empty())
84     failOn(d.data(), "unexpected end of CIE");
85   uint8_t b = d.front();
86   d = d.slice(1);
87   return b;
88 }
89 
90 void EhReader::skipBytes(size_t count) {
91   if (d.size() < count)
92     failOn(d.data(), "CIE is too small");
93   d = d.slice(count);
94 }
95 
96 // Read a null-terminated string.
97 StringRef EhReader::readString() {
98   const uint8_t *end = llvm::find(d, '\0');
99   if (end == d.end())
100     failOn(d.data(), "corrupted CIE (failed to read string)");
101   StringRef s = toStringRef(d.slice(0, end - d.begin()));
102   d = d.slice(s.size() + 1);
103   return s;
104 }
105 
106 // Skip an integer encoded in the LEB128 format.
107 // Actual number is not of interest because only the runtime needs it.
108 // But we need to be at least able to skip it so that we can read
109 // the field that follows a LEB128 number.
110 void EhReader::skipLeb128() {
111   const uint8_t *errPos = d.data();
112   while (!d.empty()) {
113     uint8_t val = d.front();
114     d = d.slice(1);
115     if ((val & 0x80) == 0)
116       return;
117   }
118   failOn(errPos, "corrupted CIE (failed to read LEB128)");
119 }
120 
121 static size_t getAugPSize(unsigned enc) {
122   switch (enc & 0x0f) {
123   case DW_EH_PE_absptr:
124   case DW_EH_PE_signed:
125     return config->wordsize;
126   case DW_EH_PE_udata2:
127   case DW_EH_PE_sdata2:
128     return 2;
129   case DW_EH_PE_udata4:
130   case DW_EH_PE_sdata4:
131     return 4;
132   case DW_EH_PE_udata8:
133   case DW_EH_PE_sdata8:
134     return 8;
135   }
136   return 0;
137 }
138 
139 void EhReader::skipAugP() {
140   uint8_t enc = readByte();
141   if ((enc & 0xf0) == DW_EH_PE_aligned)
142     failOn(d.data() - 1, "DW_EH_PE_aligned encoding is not supported");
143   size_t size = getAugPSize(enc);
144   if (size == 0)
145     failOn(d.data() - 1, "unknown FDE encoding");
146   if (size >= d.size())
147     failOn(d.data() - 1, "corrupted CIE");
148   d = d.slice(size);
149 }
150 
151 uint8_t elf::getFdeEncoding(EhSectionPiece *p) {
152   return EhReader(p->sec, p->data()).getFdeEncoding();
153 }
154 
155 uint8_t EhReader::getFdeEncoding() {
156   skipBytes(8);
157   int version = readByte();
158   if (version != 1 && version != 3)
159     failOn(d.data() - 1,
160            "FDE version 1 or 3 expected, but got " + Twine(version));
161 
162   StringRef aug = readString();
163 
164   // Skip code and data alignment factors.
165   skipLeb128();
166   skipLeb128();
167 
168   // Skip the return address register. In CIE version 1 this is a single
169   // byte. In CIE version 3 this is an unsigned LEB128.
170   if (version == 1)
171     readByte();
172   else
173     skipLeb128();
174 
175   // We only care about an 'R' value, but other records may precede an 'R'
176   // record. Unfortunately records are not in TLV (type-length-value) format,
177   // so we need to teach the linker how to skip records for each type.
178   for (char c : aug) {
179     if (c == 'R')
180       return readByte();
181     if (c == 'z') {
182       skipLeb128();
183       continue;
184     }
185     if (c == 'P') {
186       skipAugP();
187       continue;
188     }
189     if (c == 'L') {
190       readByte();
191       continue;
192     }
193     failOn(aug.data(), "unknown .eh_frame augmentation string: " + aug);
194   }
195   return DW_EH_PE_absptr;
196 }
197