lib/crc32.c

   1 /*
   2  * Oct 15, 2000 Matt Domsch <Matt_Domsch@dell.com>
   3  * Nicer crc32 functions/docs submitted by linux@horizon.com.  Thanks!
   4  * Code was from the public domain, copyright abandoned.  Code was
   5  * subsequently included in the kernel, thus was re-licensed under the
   6  * GNU GPL v2.
   7  *
   8  * Oct 12, 2000 Matt Domsch <Matt_Domsch@dell.com>
   9  * Same crc32 function was used in 5 other places in the kernel.
  10  * I made one version, and deleted the others.
  11  * There are various incantations of crc32().  Some use a seed of 0 or ~0.
  12  * Some xor at the end with ~0.  The generic crc32() function takes
  13  * seed as an argument, and doesn't xor at the end.  Then individual
  14  * users can do whatever they need.
  15  *   drivers/net/smc9194.c uses seed ~0, doesn't xor with ~0.
  16  *   fs/jffs2 uses seed 0, doesn't xor with ~0.
  17  *   fs/partitions/efi.c uses seed ~0, xor's with ~0.
  18  *
  19  * This source code is licensed under the GNU General Public License,
  20  * Version 2.  See the file COPYING for more details.
  21  */
  22
  23 #include <linux/crc32.h>
  24 #include <linux/kernel.h>
  25 #include <linux/module.h>
  26 #include <linux/compiler.h>
  27 #include <linux/types.h>
  28 #include <linux/init.h>
  29 #include <linux/atomic.h>
  30 #include "crc32defs.h"
  31 #if CRC_LE_BITS == 8
  32 # define tole(x) __constant_cpu_to_le32(x)
  33 #else
  34 # define tole(x) (x)
  35 #endif
  36
  37 #if CRC_BE_BITS == 8
  38 # define tobe(x) __constant_cpu_to_be32(x)
  39 #else
  40 # define tobe(x) (x)
  41 #endif
  42 #include "crc32table.h"
  43
  44 MODULE_AUTHOR("Matt Domsch <Matt_Domsch@dell.com>");
  45 MODULE_DESCRIPTION("Ethernet CRC32 calculations");
  46 MODULE_LICENSE("GPL");
  47
  48 #if CRC_LE_BITS == 8 || CRC_BE_BITS == 8
  49
  50 static inline u32
  51 crc32_body(u32 crc, unsigned char const *buf, size_t len, const u32 (*tab)[256])
  52 {
  53 # ifdef __LITTLE_ENDIAN
  54 #  define DO_CRC(x) crc = t0[(crc ^ (x)) & 255] ^ (crc >> 8)
  55 #  define DO_CRC4 crc = t3[(crc) & 255] ^ \
  56                 t2[(crc >> 8) & 255] ^ \
  57                 t1[(crc >> 16) & 255] ^ \
  58                 t0[(crc >> 24) & 255]
  59 # else
  60 #  define DO_CRC(x) crc = t0[((crc >> 24) ^ (x)) & 255] ^ (crc << 8)
  61 #  define DO_CRC4 crc = t0[(crc) & 255] ^ \
  62                 t1[(crc >> 8) & 255] ^  \
  63                 t2[(crc >> 16) & 255] ^ \
  64                 t3[(crc >> 24) & 255]
  65 # endif
  66         const u32 *b;
  67         size_t    rem_len;
  68         const u32 *t0=tab[0], *t1=tab[1], *t2=tab[2], *t3=tab[3];
  69
  70         /* Align it */
  71         if (unlikely((long)buf & 3 && len)) {
  72                 do {
  73                         DO_CRC(*buf++);
  74                 } while ((--len) && ((long)buf)&3);
  75         }
  76         rem_len = len & 3;
  77         /* load data 32 bits wide, xor data 32 bits wide. */
  78         len = len >> 2;
  79         b = (const u32 *)buf;
  80         for (--b; len; --len) {
  81                 crc ^= *++b; /* use pre increment for speed */
  82                 DO_CRC4;
  83         }
  84         len = rem_len;
  85         /* And the last few bytes */
  86         if (len) {
  87                 u8 *p = (u8 *)(b + 1) - 1;
  88                 do {
  89                         DO_CRC(*++p); /* use pre increment for speed */
  90                 } while (--len);
  91         }
  92         return crc;
  93 #undef DO_CRC
  94 #undef DO_CRC4
  95 }
  96 #endif
  97 /**
  98  * crc32_le() - Calculate bitwise little-endian Ethernet AUTODIN II CRC32
  99  * @crc: seed value for computation.  ~0 for Ethernet, sometimes 0 for
 100  *      other uses, or the previous crc32 value if computing incrementally.
 101  * @p: pointer to buffer over which CRC is run
 102  * @len: length of buffer @p
 103  */
 104 u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len);
 105
 106 #if CRC_LE_BITS == 1
 107 /*
 108  * In fact, the table-based code will work in this case, but it can be
 109  * simplified by inlining the table in ?: form.
 110  */
 111
 112 u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len)
 113 {
 114         int i;
 115         while (len--) {
 116                 crc ^= *p++;
 117                 for (i = 0; i < 8; i++)
 118                         crc = (crc >> 1) ^ ((crc & 1) ? CRCPOLY_LE : 0);
 119         }
 120         return crc;
 121 }
 122 #else                           /* Table-based approach */
 123
 124 u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len)
 125 {
 126 # if CRC_LE_BITS == 8
 127         const u32      (*tab)[] = crc32table_le;
 128
 129         crc = __cpu_to_le32(crc);
 130         crc = crc32_body(crc, p, len, tab);
 131         return __le32_to_cpu(crc);
 132 # elif CRC_LE_BITS == 4
 133         while (len--) {
 134                 crc ^= *p++;
 135                 crc = (crc >> 4) ^ crc32table_le[crc & 15];
 136                 crc = (crc >> 4) ^ crc32table_le[crc & 15];
 137         }
 138         return crc;
 139 # elif CRC_LE_BITS == 2
 140         while (len--) {
 141                 crc ^= *p++;
 142                 crc = (crc >> 2) ^ crc32table_le[crc & 3];
 143                 crc = (crc >> 2) ^ crc32table_le[crc & 3];
 144                 crc = (crc >> 2) ^ crc32table_le[crc & 3];
 145                 crc = (crc >> 2) ^ crc32table_le[crc & 3];
 146         }
 147         return crc;
 148 # endif
 149 }
 150 #endif
 151
 152 /**
 153  * crc32_be() - Calculate bitwise big-endian Ethernet AUTODIN II CRC32
 154  * @crc: seed value for computation.  ~0 for Ethernet, sometimes 0 for
 155  *      other uses, or the previous crc32 value if computing incrementally.
 156  * @p: pointer to buffer over which CRC is run
 157  * @len: length of buffer @p
 158  */
 159 u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len);
 160
 161 #if CRC_BE_BITS == 1
 162 /*
 163  * In fact, the table-based code will work in this case, but it can be
 164  * simplified by inlining the table in ?: form.
 165  */
 166
 167 u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len)
 168 {
 169         int i;
 170         while (len--) {
 171                 crc ^= *p++ << 24;
 172                 for (i = 0; i < 8; i++)
 173                         crc =
 174                             (crc << 1) ^ ((crc & 0x80000000) ? CRCPOLY_BE :
 175                                           0);
 176         }
 177         return crc;
 178 }
 179
 180 #else                           /* Table-based approach */
 181 u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len)
 182 {
 183 # if CRC_BE_BITS == 8
 184         const u32      (*tab)[] = crc32table_be;
 185
 186         crc = __cpu_to_be32(crc);
 187         crc = crc32_body(crc, p, len, tab);
 188         return __be32_to_cpu(crc);
 189 # elif CRC_BE_BITS == 4
 190         while (len--) {
 191                 crc ^= *p++ << 24;
 192                 crc = (crc << 4) ^ crc32table_be[crc >> 28];
 193                 crc = (crc << 4) ^ crc32table_be[crc >> 28];
 194         }
 195         return crc;
 196 # elif CRC_BE_BITS == 2
 197         while (len--) {
 198                 crc ^= *p++ << 24;
 199                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
 200                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
 201                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
 202                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
 203         }
 204         return crc;
 205 # endif
 206 }
 207 #endif
 208
 209 EXPORT_SYMBOL(crc32_le);
 210 EXPORT_SYMBOL(crc32_be);
 211
 212 /*
 213  * A brief CRC tutorial.
 214  *
 215  * A CRC is a long-division remainder.  You add the CRC to the message,
 216  * and the whole thing (message+CRC) is a multiple of the given
 217  * CRC polynomial.  To check the CRC, you can either check that the
 218  * CRC matches the recomputed value, *or* you can check that the
 219  * remainder computed on the message+CRC is 0.  This latter approach
 220  * is used by a lot of hardware implementations, and is why so many
 221  * protocols put the end-of-frame flag after the CRC.
 222  *
 223  * It's actually the same long division you learned in school, except that
 224  * - We're working in binary, so the digits are only 0 and 1, and
 225  * - When dividing polynomials, there are no carries.  Rather than add and
 226  *   subtract, we just xor.  Thus, we tend to get a bit sloppy about
 227  *   the difference between adding and subtracting.
 228  *
 229  * A 32-bit CRC polynomial is actually 33 bits long.  But since it's
 230  * 33 bits long, bit 32 is always going to be set, so usually the CRC
 231  * is written in hex with the most significant bit omitted.  (If you're
 232  * familiar with the IEEE 754 floating-point format, it's the same idea.)
 233  *
 234  * Note that a CRC is computed over a string of *bits*, so you have
 235  * to decide on the endianness of the bits within each byte.  To get
 236  * the best error-detecting properties, this should correspond to the
 237  * order they're actually sent.  For example, standard RS-232 serial is
 238  * little-endian; the most significant bit (sometimes used for parity)
 239  * is sent last.  And when appending a CRC word to a message, you should
 240  * do it in the right order, matching the endianness.
 241  *
 242  * Just like with ordinary division, the remainder is always smaller than
 243  * the divisor (the CRC polynomial) you're dividing by.  Each step of the
 244  * division, you take one more digit (bit) of the dividend and append it
 245  * to the current remainder.  Then you figure out the appropriate multiple
 246  * of the divisor to subtract to being the remainder back into range.
 247  * In binary, it's easy - it has to be either 0 or 1, and to make the
 248  * XOR cancel, it's just a copy of bit 32 of the remainder.
 249  *
 250  * When computing a CRC, we don't care about the quotient, so we can
 251  * throw the quotient bit away, but subtract the appropriate multiple of
 252  * the polynomial from the remainder and we're back to where we started,
 253  * ready to process the next bit.
 254  *
 255  * A big-endian CRC written this way would be coded like:
 256  * for (i = 0; i < input_bits; i++) {
 257  *      multiple = remainder & 0x80000000 ? CRCPOLY : 0;
 258  *      remainder = (remainder << 1 | next_input_bit()) ^ multiple;
 259  * }
 260  * Notice how, to get at bit 32 of the shifted remainder, we look
 261  * at bit 31 of the remainder *before* shifting it.
 262  *
 263  * But also notice how the next_input_bit() bits we're shifting into
 264  * the remainder don't actually affect any decision-making until
 265  * 32 bits later.  Thus, the first 32 cycles of this are pretty boring.
 266  * Also, to add the CRC to a message, we need a 32-bit-long hole for it at
 267  * the end, so we have to add 32 extra cycles shifting in zeros at the
 268  * end of every message,
 269  *
 270  * So the standard trick is to rearrage merging in the next_input_bit()
 271  * until the moment it's needed.  Then the first 32 cycles can be precomputed,
 272  * and merging in the final 32 zero bits to make room for the CRC can be
 273  * skipped entirely.
 274  * This changes the code to:
 275  * for (i = 0; i < input_bits; i++) {
 276  *      remainder ^= next_input_bit() << 31;
 277  *      multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
 278  *      remainder = (remainder << 1) ^ multiple;
 279  * }
 280  * With this optimization, the little-endian code is simpler:
 281  * for (i = 0; i < input_bits; i++) {
 282  *      remainder ^= next_input_bit();
 283  *      multiple = (remainder & 1) ? CRCPOLY : 0;
 284  *      remainder = (remainder >> 1) ^ multiple;
 285  * }
 286  *
 287  * Note that the other details of endianness have been hidden in CRCPOLY
 288  * (which must be bit-reversed) and next_input_bit().
 289  *
 290  * However, as long as next_input_bit is returning the bits in a sensible
 291  * order, we can actually do the merging 8 or more bits at a time rather
 292  * than one bit at a time:
 293  * for (i = 0; i < input_bytes; i++) {
 294  *      remainder ^= next_input_byte() << 24;
 295  *      for (j = 0; j < 8; j++) {
 296  *              multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
 297  *              remainder = (remainder << 1) ^ multiple;
 298  *      }
 299  * }
 300  * Or in little-endian:
 301  * for (i = 0; i < input_bytes; i++) {
 302  *      remainder ^= next_input_byte();
 303  *      for (j = 0; j < 8; j++) {
 304  *              multiple = (remainder & 1) ? CRCPOLY : 0;
 305  *              remainder = (remainder << 1) ^ multiple;
 306  *      }
 307  * }
 308  * If the input is a multiple of 32 bits, you can even XOR in a 32-bit
 309  * word at a time and increase the inner loop count to 32.
 310  *
 311  * You can also mix and match the two loop styles, for example doing the
 312  * bulk of a message byte-at-a-time and adding bit-at-a-time processing
 313  * for any fractional bytes at the end.
 314  *
 315  * The only remaining optimization is to the byte-at-a-time table method.
 316  * Here, rather than just shifting one bit of the remainder to decide
 317  * in the correct multiple to subtract, we can shift a byte at a time.
 318  * This produces a 40-bit (rather than a 33-bit) intermediate remainder,
 319  * but again the multiple of the polynomial to subtract depends only on
 320  * the high bits, the high 8 bits in this case.
 321  *
 322  * The multiple we need in that case is the low 32 bits of a 40-bit
 323  * value whose high 8 bits are given, and which is a multiple of the
 324  * generator polynomial.  This is simply the CRC-32 of the given
 325  * one-byte message.
 326  *
 327  * Two more details: normally, appending zero bits to a message which
 328  * is already a multiple of a polynomial produces a larger multiple of that
 329  * polynomial.  To enable a CRC to detect this condition, it's common to
 330  * invert the CRC before appending it.  This makes the remainder of the
 331  * message+crc come out not as zero, but some fixed non-zero value.
 332  *
 333  * The same problem applies to zero bits prepended to the message, and
 334  * a similar solution is used.  Instead of starting with a remainder of
 335  * 0, an initial remainder of all ones is used.  As long as you start
 336  * the same way on decoding, it doesn't make a difference.
 337  */
 338
 339 #ifdef UNITTEST
 340
 341 #include <stdlib.h>
 342 #include <stdio.h>
 343
 344 #if 0                           /*Not used at present */
 345 static void
 346 buf_dump(char const *prefix, unsigned char const *buf, size_t len)
 347 {
 348         fputs(prefix, stdout);
 349         while (len--)
 350                 printf(" %02x", *buf++);
 351         putchar('\n');
 352
 353 }
 354 #endif
 355
 356 static void bytereverse(unsigned char *buf, size_t len)
 357 {
 358         while (len--) {
 359                 unsigned char x = bitrev8(*buf);
 360                 *buf++ = x;
 361         }
 362 }
 363
 364 static void random_garbage(unsigned char *buf, size_t len)
 365 {
 366         while (len--)
 367                 *buf++ = (unsigned char) random();
 368 }
 369
 370 #if 0                           /* Not used at present */
 371 static void store_le(u32 x, unsigned char *buf)
 372 {
 373         buf[0] = (unsigned char) x;
 374         buf[1] = (unsigned char) (x >> 8);
 375         buf[2] = (unsigned char) (x >> 16);
 376         buf[3] = (unsigned char) (x >> 24);
 377 }
 378 #endif
 379
 380 static void store_be(u32 x, unsigned char *buf)
 381 {
 382         buf[0] = (unsigned char) (x >> 24);
 383         buf[1] = (unsigned char) (x >> 16);
 384         buf[2] = (unsigned char) (x >> 8);
 385         buf[3] = (unsigned char) x;
 386 }
 387
 388 /*
 389  * This checks that CRC(buf + CRC(buf)) = 0, and that
 390  * CRC commutes with bit-reversal.  This has the side effect
 391  * of bytewise bit-reversing the input buffer, and returns
 392  * the CRC of the reversed buffer.
 393  */
 394 static u32 test_step(u32 init, unsigned char *buf, size_t len)
 395 {
 396         u32 crc1, crc2;
 397         size_t i;
 398
 399         crc1 = crc32_be(init, buf, len);
 400         store_be(crc1, buf + len);
 401         crc2 = crc32_be(init, buf, len + 4);
 402         if (crc2)
 403                 printf("\nCRC cancellation fail: 0x%08x should be 0\n",
 404                        crc2);
 405
 406         for (i = 0; i <= len + 4; i++) {
 407                 crc2 = crc32_be(init, buf, i);
 408                 crc2 = crc32_be(crc2, buf + i, len + 4 - i);
 409                 if (crc2)
 410                         printf("\nCRC split fail: 0x%08x\n", crc2);
 411         }
 412
 413         /* Now swap it around for the other test */
 414
 415         bytereverse(buf, len + 4);
 416         init = bitrev32(init);
 417         crc2 = bitrev32(crc1);
 418         if (crc1 != bitrev32(crc2))
 419                 printf("\nBit reversal fail: 0x%08x -> 0x%08x -> 0x%08x\n",
 420                        crc1, crc2, bitrev32(crc2));
 421         crc1 = crc32_le(init, buf, len);
 422         if (crc1 != crc2)
 423                 printf("\nCRC endianness fail: 0x%08x != 0x%08x\n", crc1,
 424                        crc2);
 425         crc2 = crc32_le(init, buf, len + 4);
 426         if (crc2)
 427                 printf("\nCRC cancellation fail: 0x%08x should be 0\n",
 428                        crc2);
 429
 430         for (i = 0; i <= len + 4; i++) {
 431                 crc2 = crc32_le(init, buf, i);
 432                 crc2 = crc32_le(crc2, buf + i, len + 4 - i);
 433                 if (crc2)
 434                         printf("\nCRC split fail: 0x%08x\n", crc2);
 435         }
 436
 437         return crc1;
 438 }
 439
 440 #define SIZE 64
 441 #define INIT1 0
 442 #define INIT2 0
 443
 444 int main(void)
 445 {
 446         unsigned char buf1[SIZE + 4];
 447         unsigned char buf2[SIZE + 4];
 448         unsigned char buf3[SIZE + 4];
 449         int i, j;
 450         u32 crc1, crc2, crc3;
 451
 452         for (i = 0; i <= SIZE; i++) {
 453                 printf("\rTesting length %d...", i);
 454                 fflush(stdout);
 455                 random_garbage(buf1, i);
 456                 random_garbage(buf2, i);
 457                 for (j = 0; j < i; j++)
 458                         buf3[j] = buf1[j] ^ buf2[j];
 459
 460                 crc1 = test_step(INIT1, buf1, i);
 461                 crc2 = test_step(INIT2, buf2, i);
 462                 /* Now check that CRC(buf1 ^ buf2) = CRC(buf1) ^ CRC(buf2) */
 463                 crc3 = test_step(INIT1 ^ INIT2, buf3, i);
 464                 if (crc3 != (crc1 ^ crc2))
 465                         printf("CRC XOR fail: 0x%08x != 0x%08x ^ 0x%08x\n",
 466                                crc3, crc1, crc2);
 467         }
 468         printf("\nAll test complete.  No failures expected.\n");
 469         return 0;
 470 }
 471
 472 #endif                          /* UNITTEST */