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1 ================================
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2 Fuzzing LLVM libraries and tools
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3 ================================
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4
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5 .. contents::
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6 :local:
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7 :depth: 2
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8
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9 Introduction
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10 ============
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11
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12 The LLVM tree includes a number of fuzzers for various components. These are
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13 built on top of :doc:`LibFuzzer <LibFuzzer>`.
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14
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15
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16 Available Fuzzers
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17 =================
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18
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19 clang-fuzzer
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20 ------------
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21
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22 A |generic fuzzer| that tries to compile textual input as C++ code. Some of the
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23 bugs this fuzzer has reported are `on bugzilla`__ and `on OSS Fuzz's
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24 tracker`__.
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25
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26 __ https://llvm.org/pr23057
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27 __ https://bugs.chromium.org/p/oss-fuzz/issues/list?q=proj-llvm+clang-fuzzer
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28
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29 clang-proto-fuzzer
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30 ------------------
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31
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32 A |protobuf fuzzer| that compiles valid C++ programs generated from a protobuf
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33 class that describes a subset of the C++ language.
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34
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35 This fuzzer accepts clang command line options after `ignore_remaining_args=1`.
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36 For example, the following command will fuzz clang with a higher optimization
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37 level:
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38
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39 .. code-block:: shell
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40
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41 % bin/clang-proto-fuzzer <corpus-dir> -ignore_remaining_args=1 -O3
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42
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43 clang-format-fuzzer
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44 -------------------
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45
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46 A |generic fuzzer| that runs clang-format_ on C++ text fragments. Some of the
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47 bugs this fuzzer has reported are `on bugzilla`__
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48 and `on OSS Fuzz's tracker`__.
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49
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50 .. _clang-format: https://clang.llvm.org/docs/ClangFormat.html
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51 __ https://llvm.org/pr23052
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52 __ https://bugs.chromium.org/p/oss-fuzz/issues/list?q=proj-llvm+clang-format-fuzzer
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53
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54 llvm-as-fuzzer
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55 --------------
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56
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57 A |generic fuzzer| that tries to parse text as :doc:`LLVM assembly <LangRef>`.
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58 Some of the bugs this fuzzer has reported are `on bugzilla`__.
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59
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60 __ https://llvm.org/pr24639
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61
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62 llvm-dwarfdump-fuzzer
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63 ---------------------
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64
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65 A |generic fuzzer| that interprets inputs as object files and runs
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66 :doc:`llvm-dwarfdump <CommandGuide/llvm-dwarfdump>` on them. Some of the bugs
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67 this fuzzer has reported are `on OSS Fuzz's tracker`__
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68
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69 __ https://bugs.chromium.org/p/oss-fuzz/issues/list?q=proj-llvm+llvm-dwarfdump-fuzzer
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70
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71 llvm-demangle-fuzzer
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72 ---------------------
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73
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74 A |generic fuzzer| for the Itanium demangler used in various LLVM tools. We've
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75 fuzzed __cxa_demangle to death, why not fuzz LLVM's implementation of the same
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76 function!
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77
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78 llvm-isel-fuzzer
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79 ----------------
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80
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81 A |LLVM IR fuzzer| aimed at finding bugs in instruction selection.
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82
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83 This fuzzer accepts flags after `ignore_remaining_args=1`. The flags match
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84 those of :doc:`llc <CommandGuide/llc>` and the triple is required. For example,
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85 the following command would fuzz AArch64 with :doc:`GlobalISel`:
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86
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87 .. code-block:: shell
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88
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89 % bin/llvm-isel-fuzzer <corpus-dir> -ignore_remaining_args=1 -mtriple aarch64 -global-isel -O0
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90
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91 Some flags can also be specified in the binary name itself in order to support
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92 OSS Fuzz, which has trouble with required arguments. To do this, you can copy
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93 or move ``llvm-isel-fuzzer`` to ``llvm-isel-fuzzer--x-y-z``, separating options
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94 from the binary name using "--". The valid options are architecture names
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95 (``aarch64``, ``x86_64``), optimization levels (``O0``, ``O2``), or specific
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96 keywords, like ``gisel`` for enabling global instruction selection. In this
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97 mode, the same example could be run like so:
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98
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99 .. code-block:: shell
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100
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101 % bin/llvm-isel-fuzzer--aarch64-O0-gisel <corpus-dir>
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102
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103 llvm-mc-assemble-fuzzer
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104 -----------------------
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105
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106 A |generic fuzzer| that fuzzes the MC layer's assemblers by treating inputs as
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107 target specific assembly.
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108
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109 Note that this fuzzer has an unusual command line interface which is not fully
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110 compatible with all of libFuzzer's features. Fuzzer arguments must be passed
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111 after ``--fuzzer-args``, and any ``llc`` flags must use two dashes. For
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112 example, to fuzz the AArch64 assembler you might use the following command:
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113
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114 .. code-block:: console
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115
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116 llvm-mc-fuzzer --triple=aarch64-linux-gnu --fuzzer-args -max_len=4
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117
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118 This scheme will likely change in the future.
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119
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120 llvm-mc-disassemble-fuzzer
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121 --------------------------
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122
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123 A |generic fuzzer| that fuzzes the MC layer's disassemblers by treating inputs
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124 as assembled binary data.
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125
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126 Note that this fuzzer has an unusual command line interface which is not fully
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127 compatible with all of libFuzzer's features. See the notes above about
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128 ``llvm-mc-assemble-fuzzer`` for details.
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129
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130
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131 .. |generic fuzzer| replace:: :ref:`generic fuzzer <fuzzing-llvm-generic>`
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132 .. |protobuf fuzzer|
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133 replace:: :ref:`libprotobuf-mutator based fuzzer <fuzzing-llvm-protobuf>`
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134 .. |LLVM IR fuzzer|
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135 replace:: :ref:`structured LLVM IR fuzzer <fuzzing-llvm-ir>`
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136
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137
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138 Mutators and Input Generators
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139 =============================
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140
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141 The inputs for a fuzz target are generated via random mutations of a
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142 :ref:`corpus <libfuzzer-corpus>`. There are a few options for the kinds of
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143 mutations that a fuzzer in LLVM might want.
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144
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145 .. _fuzzing-llvm-generic:
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146
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147 Generic Random Fuzzing
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148 ----------------------
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149
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150 The most basic form of input mutation is to use the built in mutators of
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151 LibFuzzer. These simply treat the input corpus as a bag of bits and make random
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152 mutations. This type of fuzzer is good for stressing the surface layers of a
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153 program, and is good at testing things like lexers, parsers, or binary
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154 protocols.
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155
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156 Some of the in-tree fuzzers that use this type of mutator are `clang-fuzzer`_,
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157 `clang-format-fuzzer`_, `llvm-as-fuzzer`_, `llvm-dwarfdump-fuzzer`_,
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158 `llvm-mc-assemble-fuzzer`_, and `llvm-mc-disassemble-fuzzer`_.
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159
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160 .. _fuzzing-llvm-protobuf:
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161
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162 Structured Fuzzing using ``libprotobuf-mutator``
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163 ------------------------------------------------
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164
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165 We can use libprotobuf-mutator_ in order to perform structured fuzzing and
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166 stress deeper layers of programs. This works by defining a protobuf class that
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167 translates arbitrary data into structurally interesting input. Specifically, we
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168 use this to work with a subset of the C++ language and perform mutations that
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169 produce valid C++ programs in order to exercise parts of clang that are more
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170 interesting than parser error handling.
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171
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172 To build this kind of fuzzer you need `protobuf`_ and its dependencies
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173 installed, and you need to specify some extra flags when configuring the build
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174 with :doc:`CMake <CMake>`. For example, `clang-proto-fuzzer`_ can be enabled by
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175 adding ``-DCLANG_ENABLE_PROTO_FUZZER=ON`` to the flags described in
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176 :ref:`building-fuzzers`.
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177
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178 The only in-tree fuzzer that uses ``libprotobuf-mutator`` today is
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179 `clang-proto-fuzzer`_.
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180
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181 .. _libprotobuf-mutator: https://github.com/google/libprotobuf-mutator
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182 .. _protobuf: https://github.com/google/protobuf
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183
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184 .. _fuzzing-llvm-ir:
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185
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186 Structured Fuzzing of LLVM IR
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187 -----------------------------
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188
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189 We also use a more direct form of structured fuzzing for fuzzers that take
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190 :doc:`LLVM IR <LangRef>` as input. This is achieved through the ``FuzzMutate``
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191 library, which was `discussed at EuroLLVM 2017`_.
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192
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193 The ``FuzzMutate`` library is used to structurally fuzz backends in
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194 `llvm-isel-fuzzer`_.
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195
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196 .. _discussed at EuroLLVM 2017: https://www.youtube.com/watch?v=UBbQ_s6hNgg
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197
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198
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199 Building and Running
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200 ====================
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201
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202 .. _building-fuzzers:
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203
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204 Configuring LLVM to Build Fuzzers
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205 ---------------------------------
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206
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207 Fuzzers will be built and linked to libFuzzer by default as long as you build
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208 LLVM with sanitizer coverage enabled. You would typically also enable at least
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209 one sanitizer to find bugs faster. The most common way to build the fuzzers is
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210 by adding the following two flags to your CMake invocation:
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211 ``-DLLVM_USE_SANITIZER=Address -DLLVM_USE_SANITIZE_COVERAGE=On``.
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212
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213 .. note:: If you have ``compiler-rt`` checked out in an LLVM tree when building
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214 with sanitizers, you'll want to specify ``-DLLVM_BUILD_RUNTIME=Off``
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215 to avoid building the sanitizers themselves with sanitizers enabled.
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216
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217 Continuously Running and Finding Bugs
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218 -------------------------------------
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219
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220 There used to be a public buildbot running LLVM fuzzers continuously, and while
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221 this did find issues, it didn't have a very good way to report problems in an
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222 actionable way. Because of this, we're moving towards using `OSS Fuzz`_ more
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223 instead.
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224
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225 You can browse the `LLVM project issue list`_ for the bugs found by
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226 `LLVM on OSS Fuzz`_. These are also mailed to the `llvm-bugs mailing
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227 list`_.
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228
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229 .. _OSS Fuzz: https://github.com/google/oss-fuzz
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230 .. _LLVM project issue list:
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231 https://bugs.chromium.org/p/oss-fuzz/issues/list?q=Proj-llvm
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232 .. _LLVM on OSS Fuzz:
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233 https://github.com/google/oss-fuzz/blob/master/projects/llvm
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234 .. _llvm-bugs mailing list:
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235 http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
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236
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237
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238 Utilities for Writing Fuzzers
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239 =============================
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240
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241 There are some utilities available for writing fuzzers in LLVM.
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242
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243 Some helpers for handling the command line interface are available in
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244 ``include/llvm/FuzzMutate/FuzzerCLI.h``, including functions to parse command
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245 line options in a consistent way and to implement standalone main functions so
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246 your fuzzer can be built and tested when not built against libFuzzer.
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247
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248 There is also some handling of the CMake config for fuzzers, where you should
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249 use the ``add_llvm_fuzzer`` to set up fuzzer targets. This function works
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250 similarly to functions such as ``add_llvm_tool``, but they take care of linking
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251 to LibFuzzer when appropriate and can be passed the ``DUMMY_MAIN`` argument to
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252 enable standalone testing.
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