Note the violation of the strict aliasing rule in crc32.c.

See the comment for more details. This is in response to an issue
raised as a result of a security audit of the zlib code by Trail
of Bits and TrustInSoft, in support of the Mozilla Foundation.

1 files changed, 12 insertions, 0 deletions

diff --git a/crc32.c b/crc32.c
index 05733f4..d49f39c 100644
--- a/crc32.c
+++ b/crc32.c
@@ -237,6 +237,18 @@ unsigned long ZEXPORT crc32(crc, buf, len)
 
 #ifdef BYFOUR
 
+/*
+   This BYFOUR code accesses the passed unsigned char * buffer with a 32-bit
+   integer pointer type. This violates the strict aliasing rule, where a
+   compiler can assume, for optimization purposes, that two pointers to
+   fundamentally different types won't ever point to the same memory. This can
+   manifest as a problem only if one of the pointers is written to. This code
+   only reads from those pointers. So long as this code remains isolated in
+   this compilation unit, there won't be a problem. For this reason, this code
+   should not be copied and pasted into a compilation unit in which other code
+   writes to the buffer that is passed to these routines.
+ */
+
 /* ========================================================================= */
 #define DOLIT4 c ^= *buf4++; \
         c = crc_table[3][c & 0xff] ^ crc_table[2][(c >> 8) & 0xff] ^ \