1 /*
2  *    Copyright (c) 2016, The OpenThread Authors.
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14  *       derived from this software without specific prior written permission.
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26  */
27 
28 /**
29  * @file
30  *   This file implements an HDLC-lite encoder and decoder.
31  */
32 
33 #include "hdlc.hpp"
34 
35 #include <stdlib.h>
36 
37 #include "common/code_utils.hpp"
38 
39 namespace ot {
40 namespace Hdlc {
41 
42 /**
43  * Updates an FCS.
44  *
45  * @param[in]  aFcs   The FCS to update.
46  * @param[in]  aByte  The input byte value.
47  *
48  * @returns The updated FCS.
49  */
50 static uint16_t UpdateFcs(uint16_t aFcs, uint8_t aByte);
51 
52 enum
53 {
54     kFlagXOn        = 0x11,
55     kFlagXOff       = 0x13,
56     kFlagSequence   = 0x7e, ///< HDLC Flag value
57     kEscapeSequence = 0x7d, ///< HDLC Escape value
58     kFlagSpecial    = 0xf8,
59 };
60 
61 /**
62  * FCS lookup table
63  */
64 enum
65 {
66     kInitFcs = 0xffff, ///< Initial FCS value.
67     kGoodFcs = 0xf0b8, ///< Good FCS value.
68     kFcsSize = 2,      ///< FCS size (number of bytes).
69 };
70 
UpdateFcs(uint16_t aFcs,uint8_t aByte)71 uint16_t UpdateFcs(uint16_t aFcs, uint8_t aByte)
72 {
73     static const uint16_t sFcsTable[256] = {
74         0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf, 0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5,
75         0xe97e, 0xf8f7, 0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e, 0x9cc9, 0x8d40, 0xbfdb, 0xae52,
76         0xdaed, 0xcb64, 0xf9ff, 0xe876, 0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd, 0xad4a, 0xbcc3,
77         0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5, 0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
78         0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974, 0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9,
79         0x2732, 0x36bb, 0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3, 0x5285, 0x430c, 0x7197, 0x601e,
80         0x14a1, 0x0528, 0x37b3, 0x263a, 0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72, 0x6306, 0x728f,
81         0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9, 0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
82         0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738, 0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862,
83         0x9af9, 0x8b70, 0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7, 0x0840, 0x19c9, 0x2b52, 0x3adb,
84         0x4e64, 0x5fed, 0x6d76, 0x7cff, 0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036, 0x18c1, 0x0948,
85         0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e, 0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
86         0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd, 0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226,
87         0xd0bd, 0xc134, 0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c, 0xc60c, 0xd785, 0xe51e, 0xf497,
88         0x8028, 0x91a1, 0xa33a, 0xb2b3, 0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb, 0xd68d, 0xc704,
89         0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232, 0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
90         0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1, 0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb,
91         0x0e70, 0x1ff9, 0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330, 0x7bc7, 0x6a4e, 0x58d5, 0x495c,
92         0x3de3, 0x2c6a, 0x1ef1, 0x0f78};
93     return (aFcs >> 8) ^ sFcsTable[(aFcs ^ aByte) & 0xff];
94 }
95 
HdlcByteNeedsEscape(uint8_t aByte)96 static bool HdlcByteNeedsEscape(uint8_t aByte)
97 {
98     bool rval;
99 
100     switch (aByte)
101     {
102     case kFlagXOn:
103     case kFlagXOff:
104     case kEscapeSequence:
105     case kFlagSequence:
106     case kFlagSpecial:
107         rval = true;
108         break;
109 
110     default:
111         rval = false;
112         break;
113     }
114 
115     return rval;
116 }
117 
Encoder(Spinel::FrameWritePointer & aWritePointer)118 Encoder::Encoder(Spinel::FrameWritePointer &aWritePointer)
119     : mWritePointer(aWritePointer)
120     , mFcs(0)
121 {
122 }
123 
BeginFrame(void)124 otError Encoder::BeginFrame(void)
125 {
126     mFcs = kInitFcs;
127 
128     return mWritePointer.WriteByte(kFlagSequence);
129 }
130 
Encode(uint8_t aByte)131 otError Encoder::Encode(uint8_t aByte)
132 {
133     otError error = OT_ERROR_NONE;
134 
135     if (HdlcByteNeedsEscape(aByte))
136     {
137         VerifyOrExit(mWritePointer.CanWrite(2), error = OT_ERROR_NO_BUFS);
138 
139         IgnoreReturnValue(mWritePointer.WriteByte(kEscapeSequence));
140         IgnoreReturnValue(mWritePointer.WriteByte(aByte ^ 0x20));
141     }
142     else
143     {
144         SuccessOrExit(error = mWritePointer.WriteByte(aByte));
145     }
146 
147     mFcs = UpdateFcs(mFcs, aByte);
148 
149 exit:
150     return error;
151 }
152 
Encode(const uint8_t * aData,uint16_t aLength)153 otError Encoder::Encode(const uint8_t *aData, uint16_t aLength)
154 {
155     otError                   error      = OT_ERROR_NONE;
156     uint16_t                  oldFcs     = mFcs;
157     Spinel::FrameWritePointer oldPointer = mWritePointer;
158 
159     while (aLength--)
160     {
161         SuccessOrExit(error = Encode(*aData++));
162     }
163 
164 exit:
165 
166     if (error != OT_ERROR_NONE)
167     {
168         mWritePointer = oldPointer;
169         mFcs          = oldFcs;
170     }
171 
172     return error;
173 }
174 
EndFrame(void)175 otError Encoder::EndFrame(void)
176 {
177     otError                   error      = OT_ERROR_NONE;
178     Spinel::FrameWritePointer oldPointer = mWritePointer;
179     uint16_t                  oldFcs     = mFcs;
180     uint16_t                  fcs        = mFcs;
181 
182     fcs ^= 0xffff;
183 
184     SuccessOrExit(error = Encode(fcs & 0xff));
185     SuccessOrExit(error = Encode(fcs >> 8));
186 
187     SuccessOrExit(error = mWritePointer.WriteByte(kFlagSequence));
188 
189 exit:
190 
191     if (error != OT_ERROR_NONE)
192     {
193         mWritePointer = oldPointer;
194         mFcs          = oldFcs;
195     }
196 
197     return error;
198 }
199 
Decoder(void)200 Decoder::Decoder(void)
201     : mState(kStateNoSync)
202     , mWritePointer(nullptr)
203     , mFrameHandler(nullptr)
204     , mContext(nullptr)
205     , mFcs(0)
206     , mDecodedLength(0)
207 {
208 }
209 
Init(Spinel::FrameWritePointer & aFrameWritePointer,FrameHandler aFrameHandler,void * aContext)210 void Decoder::Init(Spinel::FrameWritePointer &aFrameWritePointer, FrameHandler aFrameHandler, void *aContext)
211 {
212     mWritePointer = &aFrameWritePointer;
213     mFrameHandler = aFrameHandler;
214     mContext      = aContext;
215 }
216 
Reset(void)217 void Decoder::Reset(void)
218 {
219     mState         = kStateNoSync;
220     mFcs           = 0;
221     mDecodedLength = 0;
222 }
223 
Decode(const uint8_t * aData,uint16_t aLength)224 void Decoder::Decode(const uint8_t *aData, uint16_t aLength)
225 {
226     while (aLength--)
227     {
228         uint8_t byte = *aData++;
229 
230         switch (mState)
231         {
232         case kStateNoSync:
233             if (byte == kFlagSequence)
234             {
235                 mState         = kStateSync;
236                 mDecodedLength = 0;
237                 mFcs           = kInitFcs;
238             }
239 
240             break;
241 
242         case kStateSync:
243             switch (byte)
244             {
245             case kEscapeSequence:
246                 mState = kStateEscaped;
247                 break;
248 
249             case kFlagSequence:
250 
251                 if (mDecodedLength > 0)
252                 {
253                     otError error = OT_ERROR_PARSE;
254 
255                     if ((mDecodedLength >= kFcsSize)
256 #ifndef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
257                         && (mFcs == kGoodFcs)
258 #endif
259                     )
260                     {
261                         // Remove the FCS from the frame.
262                         mWritePointer->UndoLastWrites(kFcsSize);
263                         error = OT_ERROR_NONE;
264                     }
265 
266                     mFrameHandler(mContext, error);
267                 }
268 
269                 mDecodedLength = 0;
270                 mFcs           = kInitFcs;
271                 break;
272 
273             default:
274                 if (mWritePointer->CanWrite(sizeof(uint8_t)))
275                 {
276                     mFcs = UpdateFcs(mFcs, byte);
277                     IgnoreReturnValue(mWritePointer->WriteByte(byte));
278                     mDecodedLength++;
279                 }
280                 else
281                 {
282                     mFrameHandler(mContext, OT_ERROR_NO_BUFS);
283                     mState = kStateNoSync;
284                 }
285 
286                 break;
287             }
288 
289             break;
290 
291         case kStateEscaped:
292             if (mWritePointer->CanWrite(sizeof(uint8_t)))
293             {
294                 byte ^= 0x20;
295                 mFcs = UpdateFcs(mFcs, byte);
296                 IgnoreReturnValue(mWritePointer->WriteByte(byte));
297                 mDecodedLength++;
298                 mState = kStateSync;
299             }
300             else
301             {
302                 mFrameHandler(mContext, OT_ERROR_NO_BUFS);
303                 mState = kStateNoSync;
304             }
305 
306             break;
307         }
308     }
309 }
310 
311 } // namespace Hdlc
312 } // namespace ot
313