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Fix from Chris Demetriou to modify the way in which arm32 filecore 

checksums are calculated paying attention to the fact that the way
the checksum works a sectors filled with a single byte value will
always checksum correctly.
This commit is contained in:
mark 1998-06-07 20:33:22 +00:00
parent 74b990a7b4
commit e7424be3c9
1 changed files with 56 additions and 26 deletions
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82
sbin/disklabel/disklabel.c
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@ -1,4 +1,4 @@
/* $NetBSD: disklabel.c,v 1.54 1998/03/26 01:37:49 cgd Exp $ */
/* $NetBSD: disklabel.c,v 1.55 1998/06/07 20:33:22 mark Exp $ */
/*
* Copyright (c) 1987, 1993
@ -47,7 +47,7 @@ __COPYRIGHT("@(#) Copyright (c) 1987, 1993\n\
static char sccsid[] = "@(#)disklabel.c 8.4 (Berkeley) 5/4/95";
/* from static char sccsid[] = "@(#)disklabel.c 1.2 (Symmetric) 11/28/85"; */
#else
__RCSID("$NetBSD: disklabel.c,v 1.54 1998/03/26 01:37:49 cgd Exp $");
__RCSID("$NetBSD: disklabel.c,v 1.55 1998/06/07 20:33:22 mark Exp $");
#endif
#endif /* not lint */
@ -144,7 +144,7 @@ static char mbrsig[2] = {0x55, 0xaa};
#ifdef __arm32__
static u_int filecore_partition_offset;
static u_int get_filecore_partition __P((int));
static u_int filecore_checksum __P((u_char *));
static int filecore_checksum __P((u_char *));
#endif /* __arm32__ */
#if defined(__i386__) || (defined(__arm32__) && defined(notyet))
static void confirm __P((const char *));
@ -646,42 +646,72 @@ readmbr(f)
#ifdef __arm32__
/*
* static u_int filecore_checksum(u_char *bootblock)
* static int filecore_checksum(u_char *bootblock)
*
* Calculates the filecore boot block checksum. This is used to validate
* a filecore boot block on the disc. If a boot block is validated then
* a filecore boot block on the disk. If a boot block is validated then
* it is used to locate the partition table. If the boot block is not
* validated, it is assumed that the whole disc is NetBSD.
* validated, it is assumed that the whole disk is NetBSD.
*
* The basic algorithm is:
*
* for (each byte in block, excluding checksum) {
* sum += byte;
* if (sum > 255)
* sum -= 255;
* }
*
* That's equivalent to summing all of the bytes in the block
* (excluding the checksum byte, of course), then calculating the
* checksum as "cksum = sum - ((sum - 1) / 255) * 255)". That
* expression may or may not yield a faster checksum function,
* but it's easier to reason about.
*
* Note that if you have a block filled with bytes of a single
* value "X" (regardless of that value!) and calculate the cksum
* of the block (excluding the checksum byte), you will _always_
* end up with a checksum of X. (Do the math; that can be derived
* from the checksum calculation function!) That means that
* blocks which contain bytes which all have the same value will
* always checksum properly. That's a _very_ unlikely occurence
* (probably impossible, actually) for a valid filecore boot block,
* so we treat such blocks as invalid.
*/
/*
* This can be coded better using add with carry but as it is used rarely
* there is not much point writing it in assembly.
*/
static u_int
static int
filecore_checksum(bootblock)
u_char *bootblock;
{
u_char byte0, accum_diff;
u_int sum;
u_int loop;
int i;
sum = 0;
accum_diff = 0;
byte0 = bootblock[0];
for (loop = 0; loop < 512; ++loop)
sum += bootblock[loop];
if (sum == 0) return(0xffff);
sum = 0;
for (loop = 0; loop < 511; ++loop) {
sum += bootblock[loop];
if (sum > 255)
sum -= 255;
/*
* Sum the contents of the block, keeping track of whether
* or not all bytes are the same. If 'accum_diff' ends up
* being zero, all of the bytes are, in fact, the same.
*/
for (i = 0; i < 511; ++i) {
sum += bootblock[i];
accum_diff |= bootblock[i] ^ byte0;
}
return(sum);
/*
* Check to see if the checksum byte is the same as the
* rest of the bytes, too. (Note that if all of the bytes
* are the same except the checksum, a checksum compare
* won't succeed, but that's not our problem.)
*/
accum_diff |= bootblock[i] ^ byte0;
/* All bytes in block are the same; call it invalid. */
if (accum_diff == 0)
return (-1);
return (sum - ((sum - 1) / 255) * 255);
}
/*