The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/lib/crc32.c

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    1 /* 
    2  * Oct 15, 2000 Matt Domsch <Matt_Domsch@dell.com>
    3  * Nicer crc32 functions/docs submitted by linux@horizon.com.  Thanks!
    4  *
    5  * Oct 12, 2000 Matt Domsch <Matt_Domsch@dell.com>
    6  * Same crc32 function was used in 5 other places in the kernel.
    7  * I made one version, and deleted the others.
    8  * There are various incantations of crc32().  Some use a seed of 0 or ~0.
    9  * Some xor at the end with ~0.  The generic crc32() function takes
   10  * seed as an argument, and doesn't xor at the end.  Then individual
   11  * users can do whatever they need.
   12  *   drivers/net/smc9194.c uses seed ~0, doesn't xor with ~0.
   13  *   fs/jffs2 uses seed 0, doesn't xor with ~0.
   14  *   fs/partitions/efi.c uses seed ~0, xor's with ~0.
   15  * 
   16  */
   17 
   18 #include <linux/crc32.h>
   19 #include <linux/kernel.h>
   20 #include <linux/module.h>
   21 #include <linux/config.h>
   22 #include <linux/types.h>
   23 #include <linux/slab.h>
   24 #include <linux/init.h>
   25 #include <asm/atomic.h>
   26 #include "crc32defs.h"
   27 #if CRC_LE_BITS == 8
   28 #define tole(x) __constant_cpu_to_le32(x)
   29 #define tobe(x) __constant_cpu_to_be32(x)
   30 #else
   31 #define tole(x) (x)
   32 #define tobe(x) (x)
   33 #endif
   34 #include "crc32table.h"
   35 
   36 #if __GNUC__ >= 3       /* 2.x has "attribute", but only 3.0 has "pure */
   37 #define attribute(x) __attribute__(x)
   38 #else
   39 #define attribute(x)
   40 #endif
   41 
   42 /*
   43  * This code is in the public domain; copyright abandoned.
   44  * Liability for non-performance of this code is limited to the amount
   45  * you paid for it.  Since it is distributed for free, your refund will
   46  * be very very small.  If it breaks, you get to keep both pieces.
   47  */
   48 
   49 MODULE_AUTHOR("Matt Domsch <Matt_Domsch@dell.com>");
   50 MODULE_DESCRIPTION("Ethernet CRC32 calculations");
   51 MODULE_LICENSE("GPL and additional rights");
   52 
   53 #if CRC_LE_BITS == 1
   54 /*
   55  * In fact, the table-based code will work in this case, but it can be
   56  * simplified by inlining the table in ?: form.
   57  */
   58 
   59 /**
   60  * crc32_le() - Calculate bitwise little-endian Ethernet AUTODIN II CRC32
   61  * @crc - seed value for computation.  ~0 for Ethernet, sometimes 0 for
   62  *        other uses, or the previous crc32 value if computing incrementally.
   63  * @p   - pointer to buffer over which CRC is run
   64  * @len - length of buffer @p
   65  * 
   66  */
   67 u32 attribute((pure)) crc32_le(u32 crc, unsigned char const *p, size_t len)
   68 {
   69         int i;
   70         while (len--) {
   71                 crc ^= *p++;
   72                 for (i = 0; i < 8; i++)
   73                         crc = (crc >> 1) ^ ((crc & 1) ? CRCPOLY_LE : 0);
   74         }
   75         return crc;
   76 }
   77 #else                           /* Table-based approach */
   78 
   79 /**
   80  * crc32_le() - Calculate bitwise little-endian Ethernet AUTODIN II CRC32
   81  * @crc - seed value for computation.  ~0 for Ethernet, sometimes 0 for
   82  *        other uses, or the previous crc32 value if computing incrementally.
   83  * @p   - pointer to buffer over which CRC is run
   84  * @len - length of buffer @p
   85  * 
   86  */
   87 u32 attribute((pure)) crc32_le(u32 crc, unsigned char const *p, size_t len)
   88 {
   89 # if CRC_LE_BITS == 8
   90         const u32      *b =(u32 *)p;
   91         const u32      *tab = crc32table_le;
   92 
   93 # ifdef __LITTLE_ENDIAN
   94 #  define DO_CRC(x) crc = tab[ (crc ^ (x)) & 255 ] ^ (crc>>8)
   95 # else
   96 #  define DO_CRC(x) crc = tab[ ((crc >> 24) ^ (x)) & 255] ^ (crc<<8)
   97 # endif
   98 
   99         crc = __cpu_to_le32(crc);
  100         /* Align it */
  101         if(unlikely(((long)b)&3 && len)){
  102                 do {
  103                         DO_CRC(*((u8 *)b)++);
  104                 } while ((--len) && ((long)b)&3 );
  105         }
  106         if(likely(len >= 4)){
  107                 /* load data 32 bits wide, xor data 32 bits wide. */
  108                 size_t save_len = len & 3;
  109                 len = len >> 2;
  110                 --b; /* use pre increment below(*++b) for speed */
  111                 do {
  112                         crc ^= *++b;
  113                         DO_CRC(0);
  114                         DO_CRC(0);
  115                         DO_CRC(0);
  116                         DO_CRC(0);
  117                 } while (--len);
  118                 b++; /* point to next byte(s) */
  119                 len = save_len;
  120         }
  121         /* And the last few bytes */
  122         if(len){
  123                 do {
  124                         DO_CRC(*((u8 *)b)++);
  125                 } while (--len);
  126         }
  127 
  128         return __le32_to_cpu(crc);
  129 #undef ENDIAN_SHIFT
  130 #undef DO_CRC
  131 
  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 #if CRC_BE_BITS == 1
  153 /*
  154  * In fact, the table-based code will work in this case, but it can be
  155  * simplified by inlining the table in ?: form.
  156  */
  157 
  158 /**
  159  * crc32_be() - Calculate bitwise big-endian Ethernet AUTODIN II CRC32
  160  * @crc - seed value for computation.  ~0 for Ethernet, sometimes 0 for
  161  *        other uses, or the previous crc32 value if computing incrementally.
  162  * @p   - pointer to buffer over which CRC is run
  163  * @len - length of buffer @p
  164  * 
  165  */
  166 u32 attribute((pure)) crc32_be(u32 crc, unsigned char const *p, size_t len)
  167 {
  168         int i;
  169         while (len--) {
  170                 crc ^= *p++ << 24;
  171                 for (i = 0; i < 8; i++)
  172                         crc =
  173                             (crc << 1) ^ ((crc & 0x80000000) ? CRCPOLY_BE :
  174                                           0);
  175         }
  176         return crc;
  177 }
  178 
  179 #else                           /* Table-based approach */
  180 /**
  181  * crc32_be() - Calculate bitwise big-endian Ethernet AUTODIN II CRC32
  182  * @crc - seed value for computation.  ~0 for Ethernet, sometimes 0 for
  183  *        other uses, or the previous crc32 value if computing incrementally.
  184  * @p   - pointer to buffer over which CRC is run
  185  * @len - length of buffer @p
  186  * 
  187  */
  188 u32 attribute((pure)) crc32_be(u32 crc, unsigned char const *p, size_t len)
  189 {
  190 # if CRC_BE_BITS == 8
  191         const u32      *b =(u32 *)p;
  192         const u32      *tab = crc32table_be;
  193 
  194 # ifdef __LITTLE_ENDIAN
  195 #  define DO_CRC(x) crc = tab[ (crc ^ (x)) & 255 ] ^ (crc>>8)
  196 # else
  197 #  define DO_CRC(x) crc = tab[ ((crc >> 24) ^ (x)) & 255] ^ (crc<<8)
  198 # endif
  199 
  200         crc = __cpu_to_be32(crc);
  201         /* Align it */
  202         if(unlikely(((long)b)&3 && len)){
  203                 do {
  204                         DO_CRC(*((u8 *)b)++);
  205                 } while ((--len) && ((long)b)&3 );
  206         }
  207         if(likely(len >= 4)){
  208                 /* load data 32 bits wide, xor data 32 bits wide. */
  209                 size_t save_len = len & 3;
  210                 len = len >> 2;
  211                 --b; /* use pre increment below(*++b) for speed */
  212                 do {
  213                         crc ^= *++b;
  214                         DO_CRC(0);
  215                         DO_CRC(0);
  216                         DO_CRC(0);
  217                         DO_CRC(0);
  218                 } while (--len);
  219                 b++; /* point to next byte(s) */
  220                 len = save_len;
  221         }
  222         /* And the last few bytes */
  223         if(len){
  224                 do {
  225                         DO_CRC(*((u8 *)b)++);
  226                 } while (--len);
  227         }
  228         return __be32_to_cpu(crc);
  229 #undef ENDIAN_SHIFT
  230 #undef DO_CRC
  231 
  232 # elif CRC_BE_BITS == 4
  233         while (len--) {
  234                 crc ^= *p++ << 24;
  235                 crc = (crc << 4) ^ crc32table_be[crc >> 28];
  236                 crc = (crc << 4) ^ crc32table_be[crc >> 28];
  237         }
  238         return crc;
  239 # elif CRC_BE_BITS == 2
  240         while (len--) {
  241                 crc ^= *p++ << 24;
  242                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
  243                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
  244                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
  245                 crc = (crc << 2) ^ crc32table_be[crc >> 30];
  246         }
  247         return crc;
  248 # endif
  249 }
  250 #endif
  251 
  252 u32 bitreverse(u32 x)
  253 {
  254         x = (x >> 16) | (x << 16);
  255         x = (x >> 8 & 0x00ff00ff) | (x << 8 & 0xff00ff00);
  256         x = (x >> 4 & 0x0f0f0f0f) | (x << 4 & 0xf0f0f0f0);
  257         x = (x >> 2 & 0x33333333) | (x << 2 & 0xcccccccc);
  258         x = (x >> 1 & 0x55555555) | (x << 1 & 0xaaaaaaaa);
  259         return x;
  260 }
  261 
  262 #ifndef CONFIG_CRC32 
  263         /* To ensure that this file is pulled in from lib/lib.a if it's
  264            configured in but nothing in-kernel uses it, we export its
  265            symbols from kernel/ksyms.c in the CONFIG_CRC32=y case.
  266            Otherwise (either modular or pulled in by the makefile magic)
  267            we export them from here. */
  268 EXPORT_SYMBOL(crc32_le);
  269 EXPORT_SYMBOL(crc32_be);
  270 EXPORT_SYMBOL(bitreverse);
  271 #endif
  272 
  273 /*
  274  * A brief CRC tutorial.
  275  *
  276  * A CRC is a long-division remainder.  You add the CRC to the message,
  277  * and the whole thing (message+CRC) is a multiple of the given
  278  * CRC polynomial.  To check the CRC, you can either check that the
  279  * CRC matches the recomputed value, *or* you can check that the
  280  * remainder computed on the message+CRC is 0.  This latter approach
  281  * is used by a lot of hardware implementations, and is why so many
  282  * protocols put the end-of-frame flag after the CRC.
  283  *
  284  * It's actually the same long division you learned in school, except that
  285  * - We're working in binary, so the digits are only 0 and 1, and
  286  * - When dividing polynomials, there are no carries.  Rather than add and
  287  *   subtract, we just xor.  Thus, we tend to get a bit sloppy about
  288  *   the difference between adding and subtracting.
  289  *
  290  * A 32-bit CRC polynomial is actually 33 bits long.  But since it's
  291  * 33 bits long, bit 32 is always going to be set, so usually the CRC
  292  * is written in hex with the most significant bit omitted.  (If you're
  293  * familiar with the IEEE 754 floating-point format, it's the same idea.)
  294  *
  295  * Note that a CRC is computed over a string of *bits*, so you have
  296  * to decide on the endianness of the bits within each byte.  To get
  297  * the best error-detecting properties, this should correspond to the
  298  * order they're actually sent.  For example, standard RS-232 serial is
  299  * little-endian; the most significant bit (sometimes used for parity)
  300  * is sent last.  And when appending a CRC word to a message, you should
  301  * do it in the right order, matching the endianness.
  302  *
  303  * Just like with ordinary division, the remainder is always smaller than
  304  * the divisor (the CRC polynomial) you're dividing by.  Each step of the
  305  * division, you take one more digit (bit) of the dividend and append it
  306  * to the current remainder.  Then you figure out the appropriate multiple
  307  * of the divisor to subtract to being the remainder back into range.
  308  * In binary, it's easy - it has to be either 0 or 1, and to make the
  309  * XOR cancel, it's just a copy of bit 32 of the remainder.
  310  *
  311  * When computing a CRC, we don't care about the quotient, so we can
  312  * throw the quotient bit away, but subtract the appropriate multiple of
  313  * the polynomial from the remainder and we're back to where we started,
  314  * ready to process the next bit.
  315  *
  316  * A big-endian CRC written this way would be coded like:
  317  * for (i = 0; i < input_bits; i++) {
  318  *      multiple = remainder & 0x80000000 ? CRCPOLY : 0;
  319  *      remainder = (remainder << 1 | next_input_bit()) ^ multiple;
  320  * }
  321  * Notice how, to get at bit 32 of the shifted remainder, we look
  322  * at bit 31 of the remainder *before* shifting it.
  323  *
  324  * But also notice how the next_input_bit() bits we're shifting into
  325  * the remainder don't actually affect any decision-making until
  326  * 32 bits later.  Thus, the first 32 cycles of this are pretty boring.
  327  * Also, to add the CRC to a message, we need a 32-bit-long hole for it at
  328  * the end, so we have to add 32 extra cycles shifting in zeros at the
  329  * end of every message,
  330  *
  331  * So the standard trick is to rearrage merging in the next_input_bit()
  332  * until the moment it's needed.  Then the first 32 cycles can be precomputed,
  333  * and merging in the final 32 zero bits to make room for the CRC can be
  334  * skipped entirely.
  335  * This changes the code to:
  336  * for (i = 0; i < input_bits; i++) {
  337  *      remainder ^= next_input_bit() << 31;
  338  *      multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
  339  *      remainder = (remainder << 1) ^ multiple;
  340  * }
  341  * With this optimization, the little-endian code is simpler:
  342  * for (i = 0; i < input_bits; i++) {
  343  *      remainder ^= next_input_bit();
  344  *      multiple = (remainder & 1) ? CRCPOLY : 0;
  345  *      remainder = (remainder >> 1) ^ multiple;
  346  * }
  347  *
  348  * Note that the other details of endianness have been hidden in CRCPOLY
  349  * (which must be bit-reversed) and next_input_bit().
  350  *
  351  * However, as long as next_input_bit is returning the bits in a sensible
  352  * order, we can actually do the merging 8 or more bits at a time rather
  353  * than one bit at a time:
  354  * for (i = 0; i < input_bytes; i++) {
  355  *      remainder ^= next_input_byte() << 24;
  356  *      for (j = 0; j < 8; j++) {
  357  *              multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
  358  *              remainder = (remainder << 1) ^ multiple;
  359  *      }
  360  * }
  361  * Or in little-endian:
  362  * for (i = 0; i < input_bytes; i++) {
  363  *      remainder ^= next_input_byte();
  364  *      for (j = 0; j < 8; j++) {
  365  *              multiple = (remainder & 1) ? CRCPOLY : 0;
  366  *              remainder = (remainder << 1) ^ multiple;
  367  *      }
  368  * }
  369  * If the input is a multiple of 32 bits, you can even XOR in a 32-bit
  370  * word at a time and increase the inner loop count to 32.
  371  *
  372  * You can also mix and match the two loop styles, for example doing the
  373  * bulk of a message byte-at-a-time and adding bit-at-a-time processing
  374  * for any fractional bytes at the end.
  375  *
  376  * The only remaining optimization is to the byte-at-a-time table method.
  377  * Here, rather than just shifting one bit of the remainder to decide
  378  * in the correct multiple to subtract, we can shift a byte at a time.
  379  * This produces a 40-bit (rather than a 33-bit) intermediate remainder,
  380  * but again the multiple of the polynomial to subtract depends only on
  381  * the high bits, the high 8 bits in this case.  
  382  *
  383  * The multile we need in that case is the low 32 bits of a 40-bit
  384  * value whose high 8 bits are given, and which is a multiple of the
  385  * generator polynomial.  This is simply the CRC-32 of the given
  386  * one-byte message.
  387  *
  388  * Two more details: normally, appending zero bits to a message which
  389  * is already a multiple of a polynomial produces a larger multiple of that
  390  * polynomial.  To enable a CRC to detect this condition, it's common to
  391  * invert the CRC before appending it.  This makes the remainder of the
  392  * message+crc come out not as zero, but some fixed non-zero value.
  393  *
  394  * The same problem applies to zero bits prepended to the message, and
  395  * a similar solution is used.  Instead of starting with a remainder of
  396  * 0, an initial remainder of all ones is used.  As long as you start
  397  * the same way on decoding, it doesn't make a difference.
  398  */
  399 
  400 #if UNITTEST
  401 
  402 #include <stdlib.h>
  403 #include <stdio.h>
  404 
  405 #if 0                           /*Not used at present */
  406 static void
  407 buf_dump(char const *prefix, unsigned char const *buf, size_t len)
  408 {
  409         fputs(prefix, stdout);
  410         while (len--)
  411                 printf(" %02x", *buf++);
  412         putchar('\n');
  413 
  414 }
  415 #endif
  416 
  417 static void bytereverse(unsigned char *buf, size_t len)
  418 {
  419         while (len--) {
  420                 unsigned char x = *buf;
  421                 x = (x >> 4) | (x << 4);
  422                 x = (x >> 2 & 0x33) | (x << 2 & 0xcc);
  423                 x = (x >> 1 & 0x55) | (x << 1 & 0xaa);
  424                 *buf++ = x;
  425         }
  426 }
  427 
  428 static void random_garbage(unsigned char *buf, size_t len)
  429 {
  430         while (len--)
  431                 *buf++ = (unsigned char) random();
  432 }
  433 
  434 #if 0                           /* Not used at present */
  435 static void store_le(u32 x, unsigned char *buf)
  436 {
  437         buf[0] = (unsigned char) x;
  438         buf[1] = (unsigned char) (x >> 8);
  439         buf[2] = (unsigned char) (x >> 16);
  440         buf[3] = (unsigned char) (x >> 24);
  441 }
  442 #endif
  443 
  444 static void store_be(u32 x, unsigned char *buf)
  445 {
  446         buf[0] = (unsigned char) (x >> 24);
  447         buf[1] = (unsigned char) (x >> 16);
  448         buf[2] = (unsigned char) (x >> 8);
  449         buf[3] = (unsigned char) x;
  450 }
  451 
  452 /*
  453  * This checks that CRC(buf + CRC(buf)) = 0, and that
  454  * CRC commutes with bit-reversal.  This has the side effect
  455  * of bytewise bit-reversing the input buffer, and returns
  456  * the CRC of the reversed buffer.
  457  */
  458 static u32 test_step(u32 init, unsigned char *buf, size_t len)
  459 {
  460         u32 crc1, crc2;
  461         size_t i;
  462 
  463         crc1 = crc32_be(init, buf, len);
  464         store_be(crc1, buf + len);
  465         crc2 = crc32_be(init, buf, len + 4);
  466         if (crc2)
  467                 printf("\nCRC cancellation fail: 0x%08x should be 0\n",
  468                        crc2);
  469 
  470         for (i = 0; i <= len + 4; i++) {
  471                 crc2 = crc32_be(init, buf, i);
  472                 crc2 = crc32_be(crc2, buf + i, len + 4 - i);
  473                 if (crc2)
  474                         printf("\nCRC split fail: 0x%08x\n", crc2);
  475         }
  476 
  477         /* Now swap it around for the other test */
  478 
  479         bytereverse(buf, len + 4);
  480         init = bitreverse(init);
  481         crc2 = bitreverse(crc1);
  482         if (crc1 != bitreverse(crc2))
  483                 printf("\nBit reversal fail: 0x%08x -> %0x08x -> 0x%08x\n",
  484                        crc1, crc2, bitreverse(crc2));
  485         crc1 = crc32_le(init, buf, len);
  486         if (crc1 != crc2)
  487                 printf("\nCRC endianness fail: 0x%08x != 0x%08x\n", crc1,
  488                        crc2);
  489         crc2 = crc32_le(init, buf, len + 4);
  490         if (crc2)
  491                 printf("\nCRC cancellation fail: 0x%08x should be 0\n",
  492                        crc2);
  493 
  494         for (i = 0; i <= len + 4; i++) {
  495                 crc2 = crc32_le(init, buf, i);
  496                 crc2 = crc32_le(crc2, buf + i, len + 4 - i);
  497                 if (crc2)
  498                         printf("\nCRC split fail: 0x%08x\n", crc2);
  499         }
  500 
  501         return crc1;
  502 }
  503 
  504 #define SIZE 64
  505 #define INIT1 0
  506 #define INIT2 0
  507 
  508 int main(void)
  509 {
  510         unsigned char buf1[SIZE + 4];
  511         unsigned char buf2[SIZE + 4];
  512         unsigned char buf3[SIZE + 4];
  513         int i, j;
  514         u32 crc1, crc2, crc3;
  515 
  516         for (i = 0; i <= SIZE; i++) {
  517                 printf("\rTesting length %d...", i);
  518                 fflush(stdout);
  519                 random_garbage(buf1, i);
  520                 random_garbage(buf2, i);
  521                 for (j = 0; j < i; j++)
  522                         buf3[j] = buf1[j] ^ buf2[j];
  523 
  524                 crc1 = test_step(INIT1, buf1, i);
  525                 crc2 = test_step(INIT2, buf2, i);
  526                 /* Now check that CRC(buf1 ^ buf2) = CRC(buf1) ^ CRC(buf2) */
  527                 crc3 = test_step(INIT1 ^ INIT2, buf3, i);
  528                 if (crc3 != (crc1 ^ crc2))
  529                         printf("CRC XOR fail: 0x%08x != 0x%08x ^ 0x%08x\n",
  530                                crc3, crc1, crc2);
  531         }
  532         printf("\nAll test complete.  No failures expected.\n");
  533         return 0;
  534 }
  535 
  536 #endif                          /* UNITTEST */

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