[PATCH 1/2] block-sha1: avoid pointer conversion that violates alignment constraints
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 <mcree@orcon.net.nz>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
---
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(-)
Show changes to block-sha1/sha1.c +3 −3
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
Jonathan Nieder