commit d1d577490c15a0c6862473d7576352a9f18ef811

Author: Mark Adler <madler@alumni.caltech.edu>

Date: Wed Sep 28 20:20:25 2016 -0700

Avoid pre-decrement of pointer in big-endian CRC calculation.

There was a small optimization for PowerPCs to pre-increment a

pointer when accessing a word, instead of post-incrementing. This

required prefacing the loop with a decrement of the pointer,

possibly pointing before the object passed. This is not compliant

with the C standard, for which decrementing a pointer before its

allocated memory is undefined. When tested on a modern PowerPC

with a modern compiler, the optimization no longer has any effect.

Due to all that, and per the recommendation of a security audit of

the zlib code by Trail of Bits and TrustInSoft, in support of the

Mozilla Foundation, this "optimization" was removed, in order to

avoid the possibility of undefined behavior.

diff --git a/crc32.c b/crc32.c

index 979a719..05733f4 100644

--- a/crc32.c

+++ b/crc32.c

@@ -278,7 +278,7 @@ local unsigned long crc32_little(crc, buf, len)

}

/* ========================================================================= */

-#define DOBIG4 c ^= *++buf4; \

+#define DOBIG4 c ^= *buf4++; \

c = crc_table[4][c & 0xff] ^ crc_table[5][(c >> 8) & 0xff] ^ \

crc_table[6][(c >> 16) & 0xff] ^ crc_table[7][c >> 24]

#define DOBIG32 DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4

@@ -300,7 +300,6 @@ local unsigned long crc32_big(crc, buf, len)

}

buf4 = (const z_crc_t FAR *)(const void FAR *)buf;

- buf4--;

while (len >= 32) {

DOBIG32;

len -= 32;

@@ -309,7 +308,6 @@ local unsigned long crc32_big(crc, buf, len)

DOBIG4;

len -= 4;

}

- buf4++;

buf = (const unsigned char FAR *)buf4;

if (len) do {