1 // -*- mode: c++ -*- 2 3 // Copyright (c) 2010, Google Inc. 4 // All rights reserved. 5 // 6 // Redistribution and use in source and binary forms, with or without 7 // modification, are permitted provided that the following conditions are 8 // met: 9 // 10 // * Redistributions of source code must retain the above copyright 11 // notice, this list of conditions and the following disclaimer. 12 // * Redistributions in binary form must reproduce the above 13 // copyright notice, this list of conditions and the following disclaimer 14 // in the documentation and/or other materials provided with the 15 // distribution. 16 // * Neither the name of Google Inc. nor the names of its 17 // contributors may be used to endorse or promote products derived from 18 // this software without specific prior written permission. 19 // 20 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 21 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 22 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 23 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 24 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 25 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 26 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 27 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 28 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 29 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 30 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 31 32 // Original author: Jim Blandy <jimb@mozilla.com> <jimb@red-bean.com> 33 34 // byte_cursor.h: Classes for parsing values from a buffer of bytes. 35 // The ByteCursor class provides a convenient interface for reading 36 // fixed-size integers of arbitrary endianness, being thorough about 37 // checking for buffer overruns. 38 39 #ifndef COMMON_BYTE_CURSOR_H_ 40 #define COMMON_BYTE_CURSOR_H_ 41 42 #include <assert.h> 43 #include <stdint.h> 44 #include <stdlib.h> 45 #include <string.h> 46 #include <string> 47 48 #include "common/using_std_string.h" 49 50 namespace google_breakpad { 51 52 // A buffer holding a series of bytes. 53 struct ByteBuffer { ByteBufferByteBuffer54 ByteBuffer() : start(0), end(0) { } ByteBufferByteBuffer55 ByteBuffer(const uint8_t *set_start, size_t set_size) 56 : start(set_start), end(set_start + set_size) { } ~ByteBufferByteBuffer57 ~ByteBuffer() { }; 58 59 // Equality operators. Useful in unit tests, and when we're using 60 // ByteBuffers to refer to regions of a larger buffer. 61 bool operator==(const ByteBuffer &that) const { 62 return start == that.start && end == that.end; 63 } 64 bool operator!=(const ByteBuffer &that) const { 65 return start != that.start || end != that.end; 66 } 67 68 // Not C++ style guide compliant, but this definitely belongs here. SizeByteBuffer69 size_t Size() const { 70 assert(start <= end); 71 return end - start; 72 } 73 74 const uint8_t *start, *end; 75 }; 76 77 // A cursor pointing into a ByteBuffer that can parse numbers of various 78 // widths and representations, strings, and data blocks, advancing through 79 // the buffer as it goes. All ByteCursor operations check that accesses 80 // haven't gone beyond the end of the enclosing ByteBuffer. 81 class ByteCursor { 82 public: 83 // Create a cursor reading bytes from the start of BUFFER. By default, the 84 // cursor reads multi-byte values in little-endian form. 85 ByteCursor(const ByteBuffer *buffer, bool big_endian = false) buffer_(buffer)86 : buffer_(buffer), here_(buffer->start), 87 big_endian_(big_endian), complete_(true) { } 88 89 // Accessor and setter for this cursor's endianness flag. big_endian()90 bool big_endian() const { return big_endian_; } set_big_endian(bool big_endian)91 void set_big_endian(bool big_endian) { big_endian_ = big_endian; } 92 93 // Accessor and setter for this cursor's current position. The setter 94 // returns a reference to this cursor. here()95 const uint8_t *here() const { return here_; } set_here(const uint8_t * here)96 ByteCursor &set_here(const uint8_t *here) { 97 assert(buffer_->start <= here && here <= buffer_->end); 98 here_ = here; 99 return *this; 100 } 101 102 // Return the number of bytes available to read at the cursor. Available()103 size_t Available() const { return size_t(buffer_->end - here_); } 104 105 // Return true if this cursor is at the end of its buffer. AtEnd()106 bool AtEnd() const { return Available() == 0; } 107 108 // When used as a boolean value this cursor converts to true if all 109 // prior reads have been completed, or false if we ran off the end 110 // of the buffer. 111 operator bool() const { return complete_; } 112 113 // Read a SIZE-byte integer at this cursor, signed if IS_SIGNED is true, 114 // unsigned otherwise, using the cursor's established endianness, and set 115 // *RESULT to the number. If we read off the end of our buffer, clear 116 // this cursor's complete_ flag, and store a dummy value in *RESULT. 117 // Return a reference to this cursor. 118 template<typename T> Read(size_t size,bool is_signed,T * result)119 ByteCursor &Read(size_t size, bool is_signed, T *result) { 120 if (CheckAvailable(size)) { 121 T v = 0; 122 if (big_endian_) { 123 for (size_t i = 0; i < size; i++) 124 v = (v << 8) + here_[i]; 125 } else { 126 // This loop condition looks weird, but size_t is unsigned, so 127 // decrementing i after it is zero yields the largest size_t value. 128 for (size_t i = size - 1; i < size; i--) 129 v = (v << 8) + here_[i]; 130 } 131 if (is_signed && size < sizeof(T)) { 132 size_t sign_bit = (T)1 << (size * 8 - 1); 133 v = (v ^ sign_bit) - sign_bit; 134 } 135 here_ += size; 136 *result = v; 137 } else { 138 *result = (T) 0xdeadbeef; 139 } 140 return *this; 141 } 142 143 // Read an integer, using the cursor's established endianness and 144 // *RESULT's size and signedness, and set *RESULT to the number. If we 145 // read off the end of our buffer, clear this cursor's complete_ flag. 146 // Return a reference to this cursor. 147 template<typename T> 148 ByteCursor &operator>>(T &result) { 149 bool T_is_signed = (T)-1 < 0; 150 return Read(sizeof(T), T_is_signed, &result); 151 } 152 153 // Copy the SIZE bytes at the cursor to BUFFER, and advance this 154 // cursor to the end of them. If we read off the end of our buffer, 155 // clear this cursor's complete_ flag, and set *POINTER to NULL. 156 // Return a reference to this cursor. Read(uint8_t * buffer,size_t size)157 ByteCursor &Read(uint8_t *buffer, size_t size) { 158 if (CheckAvailable(size)) { 159 memcpy(buffer, here_, size); 160 here_ += size; 161 } 162 return *this; 163 } 164 165 // Set STR to a copy of the '\0'-terminated string at the cursor. If the 166 // byte buffer does not contain a terminating zero, clear this cursor's 167 // complete_ flag, and set STR to the empty string. Return a reference to 168 // this cursor. CString(string * str)169 ByteCursor &CString(string *str) { 170 const uint8_t *end 171 = static_cast<const uint8_t *>(memchr(here_, '\0', Available())); 172 if (end) { 173 str->assign(reinterpret_cast<const char *>(here_), end - here_); 174 here_ = end + 1; 175 } else { 176 str->clear(); 177 here_ = buffer_->end; 178 complete_ = false; 179 } 180 return *this; 181 } 182 183 // Like CString(STR), but extract the string from a fixed-width buffer 184 // LIMIT bytes long, which may or may not contain a terminating '\0' 185 // byte. Specifically: 186 // 187 // - If there are not LIMIT bytes available at the cursor, clear the 188 // cursor's complete_ flag and set STR to the empty string. 189 // 190 // - Otherwise, if the LIMIT bytes at the cursor contain any '\0' 191 // characters, set *STR to a copy of the bytes before the first '\0', 192 // and advance the cursor by LIMIT bytes. 193 // 194 // - Otherwise, set *STR to a copy of those LIMIT bytes, and advance the 195 // cursor by LIMIT bytes. CString(string * str,size_t limit)196 ByteCursor &CString(string *str, size_t limit) { 197 if (CheckAvailable(limit)) { 198 const uint8_t *end 199 = static_cast<const uint8_t *>(memchr(here_, '\0', limit)); 200 if (end) 201 str->assign(reinterpret_cast<const char *>(here_), end - here_); 202 else 203 str->assign(reinterpret_cast<const char *>(here_), limit); 204 here_ += limit; 205 } else { 206 str->clear(); 207 } 208 return *this; 209 } 210 211 // Set *POINTER to point to the SIZE bytes at the cursor, and advance 212 // this cursor to the end of them. If SIZE is omitted, don't move the 213 // cursor. If we read off the end of our buffer, clear this cursor's 214 // complete_ flag, and set *POINTER to NULL. Return a reference to this 215 // cursor. 216 ByteCursor &PointTo(const uint8_t **pointer, size_t size = 0) { 217 if (CheckAvailable(size)) { 218 *pointer = here_; 219 here_ += size; 220 } else { 221 *pointer = NULL; 222 } 223 return *this; 224 } 225 226 // Skip SIZE bytes at the cursor. If doing so would advance us off 227 // the end of our buffer, clear this cursor's complete_ flag, and 228 // set *POINTER to NULL. Return a reference to this cursor. Skip(size_t size)229 ByteCursor &Skip(size_t size) { 230 if (CheckAvailable(size)) 231 here_ += size; 232 return *this; 233 } 234 235 private: 236 // If there are at least SIZE bytes available to read from the buffer, 237 // return true. Otherwise, set here_ to the end of the buffer, set 238 // complete_ to false, and return false. CheckAvailable(size_t size)239 bool CheckAvailable(size_t size) { 240 if (Available() >= size) { 241 return true; 242 } else { 243 here_ = buffer_->end; 244 complete_ = false; 245 return false; 246 } 247 } 248 249 // The buffer we're reading bytes from. 250 const ByteBuffer *buffer_; 251 252 // The next byte within buffer_ that we'll read. 253 const uint8_t *here_; 254 255 // True if we should read numbers in big-endian form; false if we 256 // should read in little-endian form. 257 bool big_endian_; 258 259 // True if we've been able to read all we've been asked to. 260 bool complete_; 261 }; 262 263 } // namespace google_breakpad 264 265 #endif // COMMON_BYTE_CURSOR_H_ 266