From e08118c401d5434b7b3a57039263f4fa9b1f7d1a Mon Sep 17 00:00:00 2001
From: Mark Adler <madler@alumni.caltech.edu>
Date: Mon, 3 Oct 2016 22:33:26 -0700
Subject: 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.
---
crc32.c | 12 ++++++++++++
1 file changed, 12 insertions(+)
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] ^ \
--
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