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-rw-r--r--lib/libz/crc32.c76
1 files changed, 49 insertions, 27 deletions
diff --git a/lib/libz/crc32.c b/lib/libz/crc32.c
index 689b2883b43c..b39c7e1253e1 100644
--- a/lib/libz/crc32.c
+++ b/lib/libz/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;
}
/* ========================================================================= */