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. |
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18 | // this software without specific prior written permission. |
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22 | // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR |
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27 | // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
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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 { |
54 | ByteBuffer() : start(0), end(0) { } |
55 | ByteBuffer(const uint8_t* set_start, size_t set_size) |
56 | : start(set_start), end(set_start + set_size) { } |
57 | ~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. |
69 | size_t Size() const { |
70 | assert(start <= end); |
71 | return end - start; |
72 | } |
73 | |
74 | const uint8_t* start; |
75 | const uint8_t* end; |
76 | }; |
77 | |
78 | // A cursor pointing into a ByteBuffer that can parse numbers of various |
79 | // widths and representations, strings, and data blocks, advancing through |
80 | // the buffer as it goes. All ByteCursor operations check that accesses |
81 | // haven't gone beyond the end of the enclosing ByteBuffer. |
82 | class ByteCursor { |
83 | public: |
84 | // Create a cursor reading bytes from the start of BUFFER. By default, the |
85 | // cursor reads multi-byte values in little-endian form. |
86 | ByteCursor(const ByteBuffer* buffer, bool big_endian = false) |
87 | : buffer_(buffer), here_(buffer->start), |
88 | big_endian_(big_endian), complete_(true) { } |
89 | |
90 | // Accessor and setter for this cursor's endianness flag. |
91 | bool big_endian() const { return big_endian_; } |
92 | void set_big_endian(bool big_endian) { big_endian_ = big_endian; } |
93 | |
94 | // Accessor and setter for this cursor's current position. The setter |
95 | // returns a reference to this cursor. |
96 | const uint8_t* here() const { return here_; } |
97 | ByteCursor& set_here(const uint8_t* here) { |
98 | assert(buffer_->start <= here && here <= buffer_->end); |
99 | here_ = here; |
100 | return *this; |
101 | } |
102 | |
103 | // Return the number of bytes available to read at the cursor. |
104 | size_t Available() const { return size_t(buffer_->end - here_); } |
105 | |
106 | // Return true if this cursor is at the end of its buffer. |
107 | bool AtEnd() const { return Available() == 0; } |
108 | |
109 | // When used as a boolean value this cursor converts to true if all |
110 | // prior reads have been completed, or false if we ran off the end |
111 | // of the buffer. |
112 | operator bool() const { return complete_; } |
113 | |
114 | // Read a SIZE-byte integer at this cursor, signed if IS_SIGNED is true, |
115 | // unsigned otherwise, using the cursor's established endianness, and set |
116 | // *RESULT to the number. If we read off the end of our buffer, clear |
117 | // this cursor's complete_ flag, and store a dummy value in *RESULT. |
118 | // Return a reference to this cursor. |
119 | template<typename T> |
120 | ByteCursor& Read(size_t size, bool is_signed, T* result) { |
121 | if (CheckAvailable(size)) { |
122 | T v = 0; |
123 | if (big_endian_) { |
124 | for (size_t i = 0; i < size; i++) |
125 | v = (v << 8) + here_[i]; |
126 | } else { |
127 | // This loop condition looks weird, but size_t is unsigned, so |
128 | // decrementing i after it is zero yields the largest size_t value. |
129 | for (size_t i = size - 1; i < size; i--) |
130 | v = (v << 8) + here_[i]; |
131 | } |
132 | if (is_signed && size < sizeof(T)) { |
133 | size_t sign_bit = (T)1 << (size * 8 - 1); |
134 | v = (v ^ sign_bit) - sign_bit; |
135 | } |
136 | here_ += size; |
137 | *result = v; |
138 | } else { |
139 | *result = (T) 0xdeadbeef; |
140 | } |
141 | return *this; |
142 | } |
143 | |
144 | // Read an integer, using the cursor's established endianness and |
145 | // *RESULT's size and signedness, and set *RESULT to the number. If we |
146 | // read off the end of our buffer, clear this cursor's complete_ flag. |
147 | // Return a reference to this cursor. |
148 | template<typename T> |
149 | ByteCursor& operator>>(T& result) { |
150 | bool T_is_signed = (T)-1 < 0; |
151 | return Read(sizeof(T), T_is_signed, &result); |
152 | } |
153 | |
154 | // Copy the SIZE bytes at the cursor to BUFFER, and advance this |
155 | // cursor to the end of them. If we read off the end of our buffer, |
156 | // clear this cursor's complete_ flag, and set *POINTER to NULL. |
157 | // Return a reference to this cursor. |
158 | ByteCursor& Read(uint8_t* buffer, size_t size) { |
159 | if (CheckAvailable(size)) { |
160 | memcpy(buffer, here_, size); |
161 | here_ += size; |
162 | } |
163 | return *this; |
164 | } |
165 | |
166 | // Set STR to a copy of the '\0'-terminated string at the cursor. If the |
167 | // byte buffer does not contain a terminating zero, clear this cursor's |
168 | // complete_ flag, and set STR to the empty string. Return a reference to |
169 | // this cursor. |
170 | ByteCursor& CString(string* str) { |
171 | const uint8_t* end |
172 | = static_cast<const uint8_t*>(memchr(here_, '\0', Available())); |
173 | if (end) { |
174 | str->assign(reinterpret_cast<const char*>(here_), end - here_); |
175 | here_ = end + 1; |
176 | } else { |
177 | str->clear(); |
178 | here_ = buffer_->end; |
179 | complete_ = false; |
180 | } |
181 | return *this; |
182 | } |
183 | |
184 | // Like CString(STR), but extract the string from a fixed-width buffer |
185 | // LIMIT bytes long, which may or may not contain a terminating '\0' |
186 | // byte. Specifically: |
187 | // |
188 | // - If there are not LIMIT bytes available at the cursor, clear the |
189 | // cursor's complete_ flag and set STR to the empty string. |
190 | // |
191 | // - Otherwise, if the LIMIT bytes at the cursor contain any '\0' |
192 | // characters, set *STR to a copy of the bytes before the first '\0', |
193 | // and advance the cursor by LIMIT bytes. |
194 | // |
195 | // - Otherwise, set *STR to a copy of those LIMIT bytes, and advance the |
196 | // cursor by LIMIT bytes. |
197 | ByteCursor& CString(string* str, size_t limit) { |
198 | if (CheckAvailable(limit)) { |
199 | const uint8_t* end |
200 | = static_cast<const uint8_t*>(memchr(here_, '\0', limit)); |
201 | if (end) |
202 | str->assign(reinterpret_cast<const char*>(here_), end - here_); |
203 | else |
204 | str->assign(reinterpret_cast<const char*>(here_), limit); |
205 | here_ += limit; |
206 | } else { |
207 | str->clear(); |
208 | } |
209 | return *this; |
210 | } |
211 | |
212 | // Set *POINTER to point to the SIZE bytes at the cursor, and advance |
213 | // this cursor to the end of them. If SIZE is omitted, don't move the |
214 | // cursor. If we read off the end of our buffer, clear this cursor's |
215 | // complete_ flag, and set *POINTER to NULL. Return a reference to this |
216 | // cursor. |
217 | ByteCursor& PointTo(const uint8_t** pointer, size_t size = 0) { |
218 | if (CheckAvailable(size)) { |
219 | *pointer = here_; |
220 | here_ += size; |
221 | } else { |
222 | *pointer = NULL; |
223 | } |
224 | return *this; |
225 | } |
226 | |
227 | // Skip SIZE bytes at the cursor. If doing so would advance us off |
228 | // the end of our buffer, clear this cursor's complete_ flag, and |
229 | // set *POINTER to NULL. Return a reference to this cursor. |
230 | ByteCursor& Skip(size_t size) { |
231 | if (CheckAvailable(size)) |
232 | here_ += size; |
233 | return *this; |
234 | } |
235 | |
236 | private: |
237 | // If there are at least SIZE bytes available to read from the buffer, |
238 | // return true. Otherwise, set here_ to the end of the buffer, set |
239 | // complete_ to false, and return false. |
240 | bool CheckAvailable(size_t size) { |
241 | if (Available() >= size) { |
242 | return true; |
243 | } else { |
244 | here_ = buffer_->end; |
245 | complete_ = false; |
246 | return false; |
247 | } |
248 | } |
249 | |
250 | // The buffer we're reading bytes from. |
251 | const ByteBuffer* buffer_; |
252 | |
253 | // The next byte within buffer_ that we'll read. |
254 | const uint8_t* here_; |
255 | |
256 | // True if we should read numbers in big-endian form; false if we |
257 | // should read in little-endian form. |
258 | bool big_endian_; |
259 | |
260 | // True if we've been able to read all we've been asked to. |
261 | bool complete_; |
262 | }; |
263 | |
264 | } // namespace google_breakpad |
265 | |
266 | #endif // COMMON_BYTE_CURSOR_H_ |
267 | |