diff options
Diffstat (limited to 'zlib/crc32.c')
| -rw-r--r-- | zlib/crc32.c | 76 |
1 files changed, 49 insertions, 27 deletions
diff --git a/zlib/crc32.c b/zlib/crc32.c index 689b2883b43..b39c7e1253e 100644 --- a/zlib/crc32.c +++ b/zlib/crc32.c @@ -11,6 +11,14 @@ /* @(#) $Id$ */ +/* + Note on the use of DYNAMIC_CRC_TABLE: there is no mutex or semaphore + protection on the static variables used to control the first-use generation + of the crc tables. Therefore, if you #define DYNAMIC_CRC_TABLE, you should + first call get_crc_table() to initialize the tables before allowing more than + one thread to use crc32(). + */ + #ifdef MAKECRCH # include <stdio.h> # ifndef DYNAMIC_CRC_TABLE @@ -58,7 +66,7 @@ #ifdef DYNAMIC_CRC_TABLE -local int crc_table_empty = 1; +local volatile int crc_table_empty = 1; local unsigned long FAR crc_table[TBLS][256]; local void make_crc_table OF((void)); #ifdef MAKECRCH @@ -95,38 +103,51 @@ local void make_crc_table() { unsigned long c; int n, k; - unsigned long poly; /* polynomial exclusive-or pattern */ + unsigned long poly; /* polynomial exclusive-or pattern */ /* terms of polynomial defining this crc (except x^32): */ + static volatile int first = 1; /* flag to limit concurrent making */ static const unsigned char p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26}; - /* make exclusive-or pattern from polynomial (0xedb88320UL) */ - poly = 0UL; - for (n = 0; n < sizeof(p)/sizeof(unsigned char); n++) - poly |= 1UL << (31 - p[n]); - - /* generate a crc for every 8-bit value */ - for (n = 0; n < 256; n++) { - c = (unsigned long)n; - for (k = 0; k < 8; k++) - c = c & 1 ? poly ^ (c >> 1) : c >> 1; - crc_table[0][n] = c; - } + /* See if another task is already doing this (not thread-safe, but better + than nothing -- significantly reduces duration of vulnerability in + case the advice about DYNAMIC_CRC_TABLE is ignored) */ + if (first) { + first = 0; + + /* make exclusive-or pattern from polynomial (0xedb88320UL) */ + poly = 0UL; + for (n = 0; n < sizeof(p)/sizeof(unsigned char); n++) + poly |= 1UL << (31 - p[n]); + + /* generate a crc for every 8-bit value */ + for (n = 0; n < 256; n++) { + c = (unsigned long)n; + for (k = 0; k < 8; k++) + c = c & 1 ? poly ^ (c >> 1) : c >> 1; + crc_table[0][n] = c; + } #ifdef BYFOUR - /* generate crc for each value followed by one, two, and three zeros, and - then the byte reversal of those as well as the first table */ - for (n = 0; n < 256; n++) { - c = crc_table[0][n]; - crc_table[4][n] = REV(c); - for (k = 1; k < 4; k++) { - c = crc_table[0][c & 0xff] ^ (c >> 8); - crc_table[k][n] = c; - crc_table[k + 4][n] = REV(c); + /* generate crc for each value followed by one, two, and three zeros, + and then the byte reversal of those as well as the first table */ + for (n = 0; n < 256; n++) { + c = crc_table[0][n]; + crc_table[4][n] = REV(c); + for (k = 1; k < 4; k++) { + c = crc_table[0][c & 0xff] ^ (c >> 8); + crc_table[k][n] = c; + crc_table[k + 4][n] = REV(c); + } } - } #endif /* BYFOUR */ - crc_table_empty = 0; + crc_table_empty = 0; + } + else { /* not first */ + /* wait for the other guy to finish (not efficient, but rare) */ + while (crc_table_empty) + ; + } #ifdef MAKECRCH /* write out CRC tables to crc32.h */ @@ -180,9 +201,10 @@ local void write_table(out, table) const unsigned long FAR * ZEXPORT get_crc_table() { #ifdef DYNAMIC_CRC_TABLE - if (crc_table_empty) make_crc_table(); + if (crc_table_empty) + make_crc_table(); #endif /* DYNAMIC_CRC_TABLE */ - return (const unsigned long FAR *)crc_table; + return (const unsigned long FAR *)crc_table; } /* ========================================================================= */ |