Subject: [PATCH 1/2] block-sha1: avoid pointer conversion that violates alignment constraints

From: Jonathan Nieder <[email-masked]>
Date: Sun, 22 Jul 2012 18:39:54 -0500

With 660231aa (block-sha1: support for architectures with memory
alignment restrictions, 2009-08-12), blk_SHA1_Update was modified to
access 32-bit chunks of memory one byte at a time on arches that
prefer that:

	#define get_be32(p)    ( \
		(*((unsigned char *)(p) + 0) << 24) | \
		(*((unsigned char *)(p) + 1) << 16) | \
		(*((unsigned char *)(p) + 2) <<  8) | \
		(*((unsigned char *)(p) + 3) <<  0) )

The code previously accessed these values by just using htonl(*p).

Unfortunately, Michael noticed on an Alpha machine that git was using
plain 32-bit reads anyway.  As soon as we convert a pointer to int *,
the compiler can assume that the object pointed to is correctly
aligned as an int (C99 section 6.3.2.3 "pointer conversions"
paragraph 7), and gcc takes full advantage by using a single 32-bit
load, resulting in a whole bunch of unaligned access traps.

So we need to obey the alignment constraints even when only dealing
with pointers instead of actual values.  Do so by changing the type
of 'data' to void *.  This patch renames 'data' to 'block' at the same
time to make sure all references are updated to reflect the new type.

Reported-tested-and-explained-by: Michael Cree <[email-masked]>
Signed-off-by: Jonathan Nieder <[email-masked]>
Acked-by: Linus Torvalds <[email-masked]>
---
Changes since v2:

 - fixed explanation of how the alignment assumption shows up in the
   assembler[1]
 - add Linus's ack

[1] http://thread.gmane.org/gmane.comp.version-control.git/201434/focus=201484

 block-sha1/sha1.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/block-sha1/sha1.c b/block-sha1/sha1.c
index d8934757..10fd94d1 100644
--- a/block-sha1/sha1.c
+++ b/block-sha1/sha1.c
@@ -100,7 +100,7 @@
  * Where do we get the source from? The first 16 iterations get it from
  * the input data, the next mix it from the 512-bit array.
  */
-#define SHA_SRC(t) get_be32(data + t)
+#define SHA_SRC(t) get_be32((unsigned char *) block + t*4)
 #define SHA_MIX(t) SHA_ROL(W(t+13) ^ W(t+8) ^ W(t+2) ^ W(t), 1)
 
 #define SHA_ROUND(t, input, fn, constant, A, B, C, D, E) do { \
@@ -114,7 +114,7 @@
 #define T_40_59(t, A, B, C, D, E) SHA_ROUND(t, SHA_MIX, ((B&C)+(D&(B^C))) , 0x8f1bbcdc, A, B, C, D, E )
 #define T_60_79(t, A, B, C, D, E) SHA_ROUND(t, SHA_MIX, (B^C^D) ,  0xca62c1d6, A, B, C, D, E )
 
-static void blk_SHA1_Block(blk_SHA_CTX *ctx, const unsigned int *data)
+static void blk_SHA1_Block(blk_SHA_CTX *ctx, const void *block)
 {
 	unsigned int A,B,C,D,E;
 	unsigned int array[16];
@@ -125,7 +125,7 @@ static void blk_SHA1_Block(blk_SHA_CTX *ctx, const unsigned int *data)
 	D = ctx->H[3];
 	E = ctx->H[4];
 
-	/* Round 1 - iterations 0-16 take their input from 'data' */
+	/* Round 1 - iterations 0-16 take their input from 'block' */
 	T_0_15( 0, A, B, C, D, E);
 	T_0_15( 1, E, A, B, C, D);
 	T_0_15( 2, D, E, A, B, C);
-- 
1.7.10.4