1// Copyright 2007 Google Inc.
2// Author: Lincoln Smith
3//
4// Licensed under the Apache License, Version 2.0 (the "License");
5// you may not use this file except in compliance with the License.
6// You may obtain a copy of the License at
7//
8//      http://www.apache.org/licenses/LICENSE-2.0
9//
10// Unless required by applicable law or agreed to in writing, software
11// distributed under the License is distributed on an "AS IS" BASIS,
12// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13// See the License for the specific language governing permissions and
14// limitations under the License.
15//
16// Implementation of the Address Cache and Address Encoding
17// algorithms described in sections 5.1 - 5.4 of RFC 3284 -
18// The VCDIFF Generic Differencing and Compression Data Format.
19// The RFC text can be found at http://www.faqs.org/rfcs/rfc3284.html
20//
21// Assumptions:
22//   * The VCDAddress type is large enough to hold any offset within
23//     the source and target windows.  The limit (for int32_t) is 2^31-1 bytes.
24//     The source (dictionary) should not approach this size limit;
25//     to compress a target file that is larger than
26//     INT_MAX - (dictionary size) bytes, the encoder must
27//     break it up into multiple target windows.
28
29#include <config.h>
30#include "addrcache.h"
31#include "logging.h"
32#include "varint_bigendian.h"
33#include "vcdiff_defs.h"  // RESULT_ERROR
34
35namespace open_vcdiff {
36
37// The constructor does not initialize near_addresses_ and same_addresses_.
38// Therefore, Init() must be called before any other method can be used.
39//
40// Arguments:
41//   near_cache_size: Size of the NEAR cache (number of 4-byte integers)
42//   same_cache_size: Size of the SAME cache (number of blocks of
43//                                            256 4-byte integers per block)
44//     Because the mode is expressed as a byte value,
45//     near_cache_size + same_cache_size should not exceed 254.
46//
47VCDiffAddressCache::VCDiffAddressCache(int near_cache_size,
48                                       int same_cache_size)
49    : near_cache_size_(near_cache_size),
50      same_cache_size_(same_cache_size),
51      next_slot_(0) { }
52
53VCDiffAddressCache::VCDiffAddressCache()
54    : near_cache_size_(kDefaultNearCacheSize),
55      same_cache_size_(kDefaultSameCacheSize),
56      next_slot_(0) { }
57
58// Sets up data structures needed to call other methods.  Operations that may
59// fail at runtime (for example, validating the provided near_cache_size_ and
60// same_cache_size_ parameters against their maximum allowed values) are
61// confined to this routine in order to guarantee that the class constructor
62// will never fail.  Other methods (except the destructor) cannot be invoked
63// until this method has been called successfully.  After the object has been
64// initialized and used, Init() can be called again to reset it to its initial
65// state.
66//
67// Return value: "true" if initialization succeeded, "false" if it failed.
68//     No other method except the destructor may be invoked if this function
69//     returns false.  The caller is responsible for checking the return value
70//     and providing an exit path in case of error.
71//
72bool VCDiffAddressCache::Init() {
73  // The mode is expressed as a byte value, so there is only room for 256 modes,
74  // including the two non-cached modes (SELF and HERE).  Do not allow a larger
75  // number of modes to be defined.  We do a separate sanity check for
76  // near_cache_size_ and same_cache_size_ because adding them together can
77  // cause an integer overflow if each is set to, say, INT_MAX.
78  if ((near_cache_size_ > (VCD_MAX_MODES - 2)) || (near_cache_size_ < 0)) {
79    LOG(ERROR) << "Near cache size " << near_cache_size_ << " is invalid"
80               << LOG_ENDL;
81    return false;
82  }
83  if ((same_cache_size_ > (VCD_MAX_MODES - 2)) || (same_cache_size_ < 0)) {
84    LOG(ERROR) << "Same cache size " << same_cache_size_ << " is invalid"
85               << LOG_ENDL;
86    return false;
87  }
88  if ((near_cache_size_ + same_cache_size_) > VCD_MAX_MODES - 2) {
89    LOG(ERROR) << "Using near cache size " << near_cache_size_
90               << " and same cache size " << same_cache_size_
91               << " would exceed maximum number of COPY modes ("
92               << VCD_MAX_MODES << ")" << LOG_ENDL;
93    return false;
94  }
95  if (near_cache_size_ > 0) {
96    near_addresses_.assign(near_cache_size_, 0);
97  }
98  if (same_cache_size_ > 0) {
99    same_addresses_.assign(same_cache_size_ * 256, 0);
100  }
101  next_slot_ = 0;  // in case Init() is called a second time to reinit
102  return true;
103}
104
105// This method will be called whenever an address is calculated for an
106// encoded or decoded COPY instruction, and will update the contents
107// of the SAME and NEAR caches.  It is vital that the use of
108// UpdateCache (called cache_update in the RFC examples) exactly match
109// the RFC standard, and that the same caching logic be used in the
110// decoder as in the encoder, in order for the decoded addresses to
111// match.
112//
113// Argument:
114//   address: This must be a valid address between 0 and
115//       (source window size + target window size).  It is assumed that
116//       these bounds have been checked before calling UpdateCache.
117//
118void VCDiffAddressCache::UpdateCache(VCDAddress address) {
119  if (near_cache_size_ > 0) {
120    near_addresses_[next_slot_] = address;
121    next_slot_ = (next_slot_ + 1) % near_cache_size_;
122  }
123  if (same_cache_size_ > 0) {
124    same_addresses_[address % (same_cache_size_ * 256)] = address;
125  }
126}
127
128// Determines the address mode that yields the most compact encoding
129// of the given address value, writes the encoded address into the
130// address stream, and returns the mode used.  The most compact encoding
131// is found by looking for the numerically lowest encoded address.
132// The Init() function must already have been called.
133//
134// Arguments:
135//   address: The address to be encoded.  Must be a non-negative integer
136//       between 0 and (here_address - 1).
137//   here_address: The current location in the target data (i.e., the
138//       position just after the last encoded value.)  Must be non-negative.
139//   encoded_addr: Points to an VCDAddress that will be replaced
140//       with the encoded representation of address.
141//       If WriteAddressAsVarintForMode returns true when passed
142//       the return value, then encoded_addr should be written
143//       into the delta file as a variable-length integer (Varint);
144//       otherwise, it should be written as a byte (unsigned char).
145//
146// Return value: A mode value between 0 and 255.  The mode will tell
147//       how to interpret the next value in the address stream.
148//       The values 0 and 1 correspond to SELF and HERE addressing.
149//
150// The function is guaranteed to succeed unless the conditions on the arguments
151// have not been met, in which case a LOG(DFATAL) message will be produced,
152// 0 will be returned, and *encoded_addr will be replaced with 0.
153//
154unsigned char VCDiffAddressCache::EncodeAddress(VCDAddress address,
155                                                VCDAddress here_address,
156                                                VCDAddress* encoded_addr) {
157  if (address < 0) {
158    LOG(DFATAL) << "EncodeAddress was passed a negative address: "
159                << address << LOG_ENDL;
160    *encoded_addr = 0;
161    return 0;
162  }
163  if (address >= here_address) {
164    LOG(DFATAL) << "EncodeAddress was called with address (" << address
165                << ") < here_address (" << here_address << ")" << LOG_ENDL;
166    *encoded_addr = 0;
167    return 0;
168  }
169  // Try using the SAME cache.  This method, if available, always
170  // results in the smallest encoding and takes priority over other modes.
171  if (same_cache_size() > 0) {
172    const VCDAddress same_cache_pos =
173        address % (same_cache_size() * 256);
174    if (SameAddress(same_cache_pos) == address) {
175      // This is the only mode for which an single byte will be written
176      // to the address stream instead of a variable-length integer.
177      UpdateCache(address);
178      *encoded_addr = same_cache_pos % 256;
179      return FirstSameMode() + (same_cache_pos / 256);  // SAME mode
180    }
181  }
182
183  // Try SELF mode
184  unsigned char best_mode = VCD_SELF_MODE;
185  VCDAddress best_encoded_address = address;
186
187  // Try HERE mode
188  {
189    const VCDAddress here_encoded_address = here_address - address;
190    if (here_encoded_address < best_encoded_address) {
191      best_mode = VCD_HERE_MODE;
192      best_encoded_address = here_encoded_address;
193    }
194  }
195
196  // Try using the NEAR cache
197  for (int i = 0; i < near_cache_size(); ++i) {
198    const VCDAddress near_encoded_address = address - NearAddress(i);
199    if ((near_encoded_address >= 0) &&
200        (near_encoded_address < best_encoded_address)) {
201      best_mode = FirstNearMode() + i;
202      best_encoded_address = near_encoded_address;
203    }
204  }
205
206  UpdateCache(address);
207  *encoded_addr = best_encoded_address;
208  return best_mode;
209}
210
211// Increments *byte_pointer and returns the byte it pointed to before the
212// increment.  The caller must check bounds to ensure that *byte_pointer
213// points to a valid address in memory.
214static unsigned char ParseByte(const char** byte_pointer) {
215  unsigned char byte_value = static_cast<unsigned char>(**byte_pointer);
216  ++(*byte_pointer);
217  return byte_value;
218}
219
220// Checks the given decoded address for validity.  Returns true if the
221// address is valid; otherwise, prints an error message to the log and
222// returns false.
223static bool IsDecodedAddressValid(VCDAddress decoded_address,
224                                  VCDAddress here_address) {
225  if (decoded_address < 0) {
226    LOG(ERROR) << "Decoded address " << decoded_address << " is invalid"
227               << LOG_ENDL;
228    return false;
229  } else if (decoded_address >= here_address) {
230    LOG(ERROR) << "Decoded address (" << decoded_address
231               << ") is beyond location in target file (" << here_address
232               << ")" << LOG_ENDL;
233    return false;
234  }
235  return true;
236}
237
238// Interprets the next value in the address_stream using the provided mode,
239// which may need to access the SAME or NEAR address cache.  Returns the
240// decoded address.
241// The Init() function must already have been called.
242//
243// Arguments:
244//   here_address: The current location in the source + target data (i.e., the
245//       location into which the COPY instruction will copy.)  By definition,
246//       all addresses between 0 and (here_address - 1) are valid, and
247//       any other address is invalid.
248//   mode: A byte value between 0 and (near_cache_size_ + same_cache_size_ + 1)
249//       which tells how to interpret the next value in the address stream.
250//       The values 0 and 1 correspond to SELF and HERE addressing.
251//       The validity of "mode" should already have been checked before
252//       calling this function.
253//   address_stream: Points to a pointer holding the position
254//       in the "Addresses section for COPYs" part of the input data.
255//       That section must already have been uncompressed
256//       using a secondary decompressor (if necessary.)
257//       This is an IN/OUT argument; the value of *address_stream will be
258//       incremented by the size of an integer, or (if the SAME cache
259//       was used) by the size of a byte (1).
260//   address_stream_end: Points to the position just after the end of
261//       the address stream buffer.  All addresses between *address_stream
262//       and address_stream_end should contain valid address data.
263//
264// Return value: If the input conditions were met, and the address section
265//     of the input data contains properly encoded addresses that match
266//     the instructions section, then an integer between 0 and here_address - 1
267//     will be returned, representing the address from which data should
268//     be copied from the source or target window into the output stream.
269//     If an invalid address value is found in address_stream, then
270//     RESULT_ERROR will be returned.  If the limit address_stream_end
271//     is reached before the address can be decoded, then
272//     RESULT_END_OF_DATA will be returned.  If more streamed data
273//     is expected, this means that the consumer should block and wait
274//     for more data before continuing to decode.  If no more data is expected,
275//     this return value signals an error condition.
276//
277VCDAddress VCDiffAddressCache::DecodeAddress(VCDAddress here_address,
278                                             unsigned char mode,
279                                             const char** address_stream,
280                                             const char* address_stream_end) {
281  if (here_address < 0) {
282    LOG(DFATAL) << "DecodeAddress was passed a negative value"
283                   " for here_address: " << here_address << LOG_ENDL;
284    return RESULT_ERROR;
285  }
286  const char* new_address_pos = *address_stream;
287  if (new_address_pos >= address_stream_end) {
288    return RESULT_END_OF_DATA;
289  }
290  VCDAddress decoded_address;
291  if (IsSameMode(mode)) {
292    // SAME mode expects a byte value as the encoded address
293    unsigned char encoded_address = ParseByte(&new_address_pos);
294    decoded_address = DecodeSameAddress(mode, encoded_address);
295  } else {
296    // All modes except SAME mode expect a VarintBE as the encoded address
297    int32_t encoded_address = VarintBE<int32_t>::Parse(address_stream_end,
298                                                       &new_address_pos);
299    switch (encoded_address) {
300      case RESULT_ERROR:
301        LOG(ERROR) << "Found invalid variable-length integer "
302                      "as encoded address value" << LOG_ENDL;
303        return RESULT_ERROR;
304      case RESULT_END_OF_DATA:
305        return RESULT_END_OF_DATA;
306      default:
307        break;
308    }
309    if (IsSelfMode(mode)) {
310      decoded_address = DecodeSelfAddress(encoded_address);
311    } else if (IsHereMode(mode)) {
312      decoded_address = DecodeHereAddress(encoded_address, here_address);
313    } else if (IsNearMode(mode)) {
314      decoded_address = DecodeNearAddress(mode, encoded_address);
315    } else {
316      LOG(DFATAL) << "Invalid mode value (" << static_cast<int>(mode)
317                  << ") passed to DecodeAddress; maximum mode value = "
318                  << static_cast<int>(LastMode()) << LOG_ENDL;
319      return RESULT_ERROR;
320    }
321  }
322  // Check for an out-of-bounds address (corrupt/malicious data)
323  if (!IsDecodedAddressValid(decoded_address, here_address)) {
324    return RESULT_ERROR;
325  }
326  *address_stream = new_address_pos;
327  UpdateCache(decoded_address);
328  return decoded_address;
329}
330
331}  // namespace open_vcdiff
332