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/slab.h>
  29 #include <linux/init.h>
  30 #include <asm/atomic.h>
  31 #include "crc32defs.h"
  32 #if CRC_LE_BITS == 8
  33 #define tole(x) __constant_cpu_to_le32(x)
  34 #define tobe(x) __constant_cpu_to_be32(x)
  35 #else
  36 #define tole(x) (x)
  37 #define tobe(x) (x)
  38 #endif
  39 #include "crc32table.h"
  40
  41 MODULE_AUTHOR("Matt Domsch <Matt_Domsch@dell.com>");
  42 MODULE_DESCRIPTION("Ethernet CRC32 calculations");
  43 MODULE_LICENSE("GPL");
  44
  45 /**
  46  * crc32_le() - Calculate bitwise little-endian Ethernet AUTODIN II CRC32
  47  * @crc: seed value for computation.  ~0 for Ethernet, sometimes 0 for
  48  *      other uses, or the previous crc32 value if computing incrementally.
  49  * @p: pointer to buffer over which CRC is run
  50  * @len: length of buffer @p
  51  */
  52 u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len);
  53
  54 #if CRC_LE_BITS == 1
  55 /*
  56  * In fact, the table-based code will work in this case, but it can be
  57  * simplified by inlining the table in ?: form.
  58  */
  59
  60 u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len)
  61 {
  62         int i;
  63         while (len--) {
  64                 crc ^= *p++;
  65                 for (i = 0; i < 8; i++)
  66                         crc = (crc >> 1) ^ ((crc & 1) ? CRCPOLY_LE : 0);
  67         }
  68         return crc;
  69 }
  70 #else                           /* Table-based approach */
  71
  72 u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len)
  73 {
  74 # if CRC_LE_BITS == 8
  75         const u32      *b =(u32 *)p;
  76         const u32      *tab = crc32table_le;
  77
  78 # ifdef __LITTLE_ENDIAN
  79 #  define DO_CRC(x) crc = tab[ (crc ^ (x)) & 255 ] ^ (crc>>8)
  80 # else
  81 #  define DO_CRC(x) crc = tab[ ((crc >> 24) ^ (x)) & 255] ^ (crc<<8)
  82 # endif
  83
  84         crc = __cpu_to_le32(crc);
  85         /* Align it */
  86         if(unlikely(((long)b)&3 && len)){
  87                 do {
  88                         u8 *p = (u8 *)b;
  89                         DO_CRC(*p++);
  90                         b = (void *)p;
  91                 } while ((--len) && ((long)b)&3 );
  92         }
  93         if(likely(len >= 4)){
  94                 /* load data 32 bits wide, xor data 32 bits wide. */
  95                 size_t save_len = len & 3;
  96                 len = len >> 2;
  97                 --b; /* use pre increment below(*++b) for speed */
  98                 do {
  99                         crc ^= *++b;
 100                         DO_CRC(0);
 101                         DO_CRC(0);
 102                         DO_CRC(0);
 103                         DO_CRC(0);
 104                 } while (--len);
 105                 b++; /* point to next byte(s) */
 106                 len = save_len;
 107         }
 108         /* And the last few bytes */
 109         if(len){
 110                 do {
 111                         u8 *p = (u8 *)b;
 112                         DO_CRC(*p++);
 113                         b = (void *)p;
 114                 } while (--len);
 115         }
 116
 117         return __le32_to_cpu(crc);
 118 #undef ENDIAN_SHIFT
 119 #undef DO_CRC
 120
 121 # elif CRC_LE_BITS == 4
 122         while (len--) {
 123                 crc ^= *p++;
 124                 crc = (crc >> 4) ^ crc32table_le[crc & 15];
 125                 crc = (crc >> 4) ^ crc32table_le[crc & 15];
 126         }
 127         return crc;
 128 # elif CRC_LE_BITS == 2
 129         while (len--) {
 130                 crc ^= *p++;
 131                 crc = (crc >> 2) ^ crc32table_le[crc & 3];
 132                 crc = (crc >> 2) ^ crc32table_le[crc & 3];
 133                 crc = (crc >> 2) ^ crc32table_le[crc & 3];
 134                 crc = (crc >> 2) ^ crc32table_le[crc & 3];
 135         }
 136         return crc;
 137 # endif
 138 }
 139 #endif
 140
 141 /**
 142  * crc32_be() - Calculate bitwise big-endian Ethernet AUTODIN II CRC32
 143  * @crc: seed value for computation.  ~0 for Ethernet, sometimes 0 for
 144  *      other uses, or the previous crc32 value if computing incrementally.
 145  * @p: pointer to buffer over which CRC is run
 146  * @len: length of buffer @p
 147  */
 148 u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len);
 149
 150 #if CRC_BE_BITS == 1
 151 /*
 152  * In fact, the table-based code will work in this case, but it can be
 153  * simplified by inlining the table in ?: form.
 154  */
 155
 156 u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len)
 157 {
 158         int i;
 159         while (len--) {
 160                 crc ^= *p++ << 24;
 161                 for (i = 0; i < 8; i++)
 162                         crc =
 163                             (crc << 1) ^ ((crc & 0x80000000) ? CRCPOLY_BE :
 164                                           0);
 165         }
 166         return crc;
 167 }
 168
 169 #else                           /* Table-based approach */
 170 u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len)
 171 {
 172 # if CRC_BE_BITS == 8
 173         const u32      *b =(u32 *)p;
 174         const u32      *tab = crc32table_be;
 175
 176 # ifdef __LITTLE_ENDIAN
 177 #  define DO_CRC(x) crc = tab[ (crc ^ (x)) & 255 ] ^ (crc>>8)
 178 # else
 179 #  define DO_CRC(x) crc = tab[ ((crc >> 24) ^ (x)) & 255] ^ (crc<<8)
 180 # endif
 181
 182         crc = __cpu_to_be32(crc);
 183         /* Align it */
 184         if(unlikely(((long)b)&3 && len)){
 185                 do {
 186                         u8 *p = (u8 *)b;
 187                         DO_CRC(*p++);
 188                         b = (u32 *)p;
 189                 } while ((--len) && ((long)b)&3 );
 190         }
 191         if(likely(len >= 4)){
 192                 /* load data 32 bits wide, xor data 32 bits wide. */
 193                 size_t save_len = len & 3;
 194                 len = len >> 2;
 195                 --b; /* use pre increment below(*++b) for speed */
 196                 do {
 197                         crc ^= *++b;
 198                         DO_CRC(0);
 199                         DO_CRC(0);
 200                         DO_CRC(0);
 201                         DO_CRC(0);
 202                 } while (--len);
 203                 b++; /* point to next byte(s) */
 204                 len = save_len;
 205         }
 206         /* And the last few bytes */
 207         if(len){
 208                 do {
 209                         u8 *p = (u8 *)b;
 210                         DO_CRC(*p++);
 211                         b = (void *)p;
 212                 } while (--len);
 213         }
 214         return __be32_to_cpu(crc);
 215 #undef ENDIAN_SHIFT
 216 #undef DO_CRC
 217
 218 # elif CRC_BE_BITS == 4
 219         while (len--) {
 220                 crc ^= *p++ << 24;
 221                 crc = (crc << 4) ^ crc32table_be[crc >> 28];
 222                 crc = (crc << 4) ^ crc32table_be[crc >> 28];
 223         }
 224         return crc;
 225 # elif CRC_BE_BITS == 2
 226         while (len--) {
 227                 crc ^= *p++ << 24;
 228                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
 229                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
 230                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
 231                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
 232         }
 233         return crc;
 234 # endif
 235 }
 236 #endif
 237
 238 EXPORT_SYMBOL(crc32_le);
 239 EXPORT_SYMBOL(crc32_be);
 240
 241 /*
 242  * A brief CRC tutorial.
 243  *
 244  * A CRC is a long-division remainder.  You add the CRC to the message,
 245  * and the whole thing (message+CRC) is a multiple of the given
 246  * CRC polynomial.  To check the CRC, you can either check that the
 247  * CRC matches the recomputed value, *or* you can check that the
 248  * remainder computed on the message+CRC is 0.  This latter approach
 249  * is used by a lot of hardware implementations, and is why so many
 250  * protocols put the end-of-frame flag after the CRC.
 251  *
 252  * It's actually the same long division you learned in school, except that
 253  * - We're working in binary, so the digits are only 0 and 1, and
 254  * - When dividing polynomials, there are no carries.  Rather than add and
 255  *   subtract, we just xor.  Thus, we tend to get a bit sloppy about
 256  *   the difference between adding and subtracting.
 257  *
 258  * A 32-bit CRC polynomial is actually 33 bits long.  But since it's
 259  * 33 bits long, bit 32 is always going to be set, so usually the CRC
 260  * is written in hex with the most significant bit omitted.  (If you're
 261  * familiar with the IEEE 754 floating-point format, it's the same idea.)
 262  *
 263  * Note that a CRC is computed over a string of *bits*, so you have
 264  * to decide on the endianness of the bits within each byte.  To get
 265  * the best error-detecting properties, this should correspond to the
 266  * order they're actually sent.  For example, standard RS-232 serial is
 267  * little-endian; the most significant bit (sometimes used for parity)
 268  * is sent last.  And when appending a CRC word to a message, you should
 269  * do it in the right order, matching the endianness.
 270  *
 271  * Just like with ordinary division, the remainder is always smaller than
 272  * the divisor (the CRC polynomial) you're dividing by.  Each step of the
 273  * division, you take one more digit (bit) of the dividend and append it
 274  * to the current remainder.  Then you figure out the appropriate multiple
 275  * of the divisor to subtract to being the remainder back into range.
 276  * In binary, it's easy - it has to be either 0 or 1, and to make the
 277  * XOR cancel, it's just a copy of bit 32 of the remainder.
 278  *
 279  * When computing a CRC, we don't care about the quotient, so we can
 280  * throw the quotient bit away, but subtract the appropriate multiple of
 281  * the polynomial from the remainder and we're back to where we started,
 282  * ready to process the next bit.
 283  *
 284  * A big-endian CRC written this way would be coded like:
 285  * for (i = 0; i < input_bits; i++) {
 286  *      multiple = remainder & 0x80000000 ? CRCPOLY : 0;
 287  *      remainder = (remainder << 1 | next_input_bit()) ^ multiple;
 288  * }
 289  * Notice how, to get at bit 32 of the shifted remainder, we look
 290  * at bit 31 of the remainder *before* shifting it.
 291  *
 292  * But also notice how the next_input_bit() bits we're shifting into
 293  * the remainder don't actually affect any decision-making until
 294  * 32 bits later.  Thus, the first 32 cycles of this are pretty boring.
 295  * Also, to add the CRC to a message, we need a 32-bit-long hole for it at
 296  * the end, so we have to add 32 extra cycles shifting in zeros at the
 297  * end of every message,
 298  *
 299  * So the standard trick is to rearrage merging in the next_input_bit()
 300  * until the moment it's needed.  Then the first 32 cycles can be precomputed,
 301  * and merging in the final 32 zero bits to make room for the CRC can be
 302  * skipped entirely.
 303  * This changes the code to:
 304  * for (i = 0; i < input_bits; i++) {
 305  *      remainder ^= next_input_bit() << 31;
 306  *      multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
 307  *      remainder = (remainder << 1) ^ multiple;
 308  * }
 309  * With this optimization, the little-endian code is simpler:
 310  * for (i = 0; i < input_bits; i++) {
 311  *      remainder ^= next_input_bit();
 312  *      multiple = (remainder & 1) ? CRCPOLY : 0;
 313  *      remainder = (remainder >> 1) ^ multiple;
 314  * }
 315  *
 316  * Note that the other details of endianness have been hidden in CRCPOLY
 317  * (which must be bit-reversed) and next_input_bit().
 318  *
 319  * However, as long as next_input_bit is returning the bits in a sensible
 320  * order, we can actually do the merging 8 or more bits at a time rather
 321  * than one bit at a time:
 322  * for (i = 0; i < input_bytes; i++) {
 323  *      remainder ^= next_input_byte() << 24;
 324  *      for (j = 0; j < 8; j++) {
 325  *              multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
 326  *              remainder = (remainder << 1) ^ multiple;
 327  *      }
 328  * }
 329  * Or in little-endian:
 330  * for (i = 0; i < input_bytes; i++) {
 331  *      remainder ^= next_input_byte();
 332  *      for (j = 0; j < 8; j++) {
 333  *              multiple = (remainder & 1) ? CRCPOLY : 0;
 334  *              remainder = (remainder << 1) ^ multiple;
 335  *      }
 336  * }
 337  * If the input is a multiple of 32 bits, you can even XOR in a 32-bit
 338  * word at a time and increase the inner loop count to 32.
 339  *
 340  * You can also mix and match the two loop styles, for example doing the
 341  * bulk of a message byte-at-a-time and adding bit-at-a-time processing
 342  * for any fractional bytes at the end.
 343  *
 344  * The only remaining optimization is to the byte-at-a-time table method.
 345  * Here, rather than just shifting one bit of the remainder to decide
 346  * in the correct multiple to subtract, we can shift a byte at a time.
 347  * This produces a 40-bit (rather than a 33-bit) intermediate remainder,
 348  * but again the multiple of the polynomial to subtract depends only on
 349  * the high bits, the high 8 bits in this case.
 350  *
 351  * The multiple we need in that case is the low 32 bits of a 40-bit
 352  * value whose high 8 bits are given, and which is a multiple of the
 353  * generator polynomial.  This is simply the CRC-32 of the given
 354  * one-byte message.
 355  *
 356  * Two more details: normally, appending zero bits to a message which
 357  * is already a multiple of a polynomial produces a larger multiple of that
 358  * polynomial.  To enable a CRC to detect this condition, it's common to
 359  * invert the CRC before appending it.  This makes the remainder of the
 360  * message+crc come out not as zero, but some fixed non-zero value.
 361  *
 362  * The same problem applies to zero bits prepended to the message, and
 363  * a similar solution is used.  Instead of starting with a remainder of
 364  * 0, an initial remainder of all ones is used.  As long as you start
 365  * the same way on decoding, it doesn't make a difference.
 366  */
 367
 368 #ifdef UNITTEST
 369
 370 #include <stdlib.h>
 371 #include <stdio.h>
 372
 373 #if 0                           /*Not used at present */
 374 static void
 375 buf_dump(char const *prefix, unsigned char const *buf, size_t len)
 376 {
 377         fputs(prefix, stdout);
 378         while (len--)
 379                 printf(" %02x", *buf++);
 380         putchar('\n');
 381
 382 }
 383 #endif
 384
 385 static void bytereverse(unsigned char *buf, size_t len)
 386 {
 387         while (len--) {
 388                 unsigned char x = bitrev8(*buf);
 389                 *buf++ = x;
 390         }
 391 }
 392
 393 static void random_garbage(unsigned char *buf, size_t len)
 394 {
 395         while (len--)
 396                 *buf++ = (unsigned char) random();
 397 }
 398
 399 #if 0                           /* Not used at present */
 400 static void store_le(u32 x, unsigned char *buf)
 401 {
 402         buf[0] = (unsigned char) x;
 403         buf[1] = (unsigned char) (x >> 8);
 404         buf[2] = (unsigned char) (x >> 16);
 405         buf[3] = (unsigned char) (x >> 24);
 406 }
 407 #endif
 408
 409 static void store_be(u32 x, unsigned char *buf)
 410 {
 411         buf[0] = (unsigned char) (x >> 24);
 412         buf[1] = (unsigned char) (x >> 16);
 413         buf[2] = (unsigned char) (x >> 8);
 414         buf[3] = (unsigned char) x;
 415 }
 416
 417 /*
 418  * This checks that CRC(buf + CRC(buf)) = 0, and that
 419  * CRC commutes with bit-reversal.  This has the side effect
 420  * of bytewise bit-reversing the input buffer, and returns
 421  * the CRC of the reversed buffer.
 422  */
 423 static u32 test_step(u32 init, unsigned char *buf, size_t len)
 424 {
 425         u32 crc1, crc2;
 426         size_t i;
 427
 428         crc1 = crc32_be(init, buf, len);
 429         store_be(crc1, buf + len);
 430         crc2 = crc32_be(init, buf, len + 4);
 431         if (crc2)
 432                 printf("\nCRC cancellation fail: 0x%08x should be 0\n",
 433                        crc2);
 434
 435         for (i = 0; i <= len + 4; i++) {
 436                 crc2 = crc32_be(init, buf, i);
 437                 crc2 = crc32_be(crc2, buf + i, len + 4 - i);
 438                 if (crc2)
 439                         printf("\nCRC split fail: 0x%08x\n", crc2);
 440         }
 441
 442         /* Now swap it around for the other test */
 443
 444         bytereverse(buf, len + 4);
 445         init = bitrev32(init);
 446         crc2 = bitrev32(crc1);
 447         if (crc1 != bitrev32(crc2))
 448                 printf("\nBit reversal fail: 0x%08x -> 0x%08x -> 0x%08x\n",
 449                        crc1, crc2, bitrev32(crc2));
 450         crc1 = crc32_le(init, buf, len);
 451         if (crc1 != crc2)
 452                 printf("\nCRC endianness fail: 0x%08x != 0x%08x\n", crc1,
 453                        crc2);
 454         crc2 = crc32_le(init, buf, len + 4);
 455         if (crc2)
 456                 printf("\nCRC cancellation fail: 0x%08x should be 0\n",
 457                        crc2);
 458
 459         for (i = 0; i <= len + 4; i++) {
 460                 crc2 = crc32_le(init, buf, i);
 461                 crc2 = crc32_le(crc2, buf + i, len + 4 - i);
 462                 if (crc2)
 463                         printf("\nCRC split fail: 0x%08x\n", crc2);
 464         }
 465
 466         return crc1;
 467 }
 468
 469 #define SIZE 64
 470 #define INIT1 0
 471 #define INIT2 0
 472
 473 int main(void)
 474 {
 475         unsigned char buf1[SIZE + 4];
 476         unsigned char buf2[SIZE + 4];
 477         unsigned char buf3[SIZE + 4];
 478         int i, j;
 479         u32 crc1, crc2, crc3;
 480
 481         for (i = 0; i <= SIZE; i++) {
 482                 printf("\rTesting length %d...", i);
 483                 fflush(stdout);
 484                 random_garbage(buf1, i);
 485                 random_garbage(buf2, i);
 486                 for (j = 0; j < i; j++)
 487                         buf3[j] = buf1[j] ^ buf2[j];
 488
 489                 crc1 = test_step(INIT1, buf1, i);
 490                 crc2 = test_step(INIT2, buf2, i);
 491                 /* Now check that CRC(buf1 ^ buf2) = CRC(buf1) ^ CRC(buf2) */
 492                 crc3 = test_step(INIT1 ^ INIT2, buf3, i);
 493                 if (crc3 != (crc1 ^ crc2))
 494                         printf("CRC XOR fail: 0x%08x != 0x%08x ^ 0x%08x\n",
 495                                crc3, crc1, crc2);
 496         }
 497         printf("\nAll test complete.  No failures expected.\n");
 498         return 0;
 499 }
 500
 501 #endif                          /* UNITTEST */