Re: [PATCH 2.6.7-mm1] MBR centralization

[FabF <fabian.frederick@skynet.be>]

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On Thu, 2004-06-24 at 09:38, Anton Altaparmakov wrote:
> On Thu, 24 Jun 2004, Anton Altaparmakov wrote:
> > On Thu, 24 Jun 2004, FabF wrote:
> >> Here's a new version:
> >> 	-macro simplification (We're calling from struct & buffer so static 
> >> fx
> >> conversion seems troublesome)
> >> 	-(p)
> >> 	-MBR patch only
> >
> > Your patch is wrong:
> >
> > diff -Naur orig/include/linux/genhd.h edited/include/linux/genhd.h
> > --- orig/include/linux/genhd.h  2004-06-16 07:18:59.000000000 +0200
> > +++ edited/include/linux/genhd.h        2004-06-24 08:02:45.151489490 +0200
> > @@ -17,6 +17,9 @@
> > #include <linux/string.h>
> > #include <linux/fs.h>
> >
> > +/*Master boot record magic numbers*/
> > +#define MSDOS_MBR(p) le16_to_cpu((u16)*(p)) == 0xaa55
> >
> > This macro is completely broken for all p with a type size other than 16 
> > bits.  E.g. if you have "char *p" then *(p) gives you a single byte which you 
> > then cast to u16 so you will never see the 0xaa55.
> >
> > If you want to do it this way you need to cast p to (u16*) first like so:
> >
> > #define MSDOS_MBR(p)	(le16_to_cpup((u16*)(p)) == 0xaa55)
> >
> > Note using _cpup() means you don't have to do the dereferencing yourself 
> > which looks much cleaner IMO.  You do still need the cast as it is missing 
> > from various macros in the kernel (I consider this a bug but this is 
> > orthogonal to this patch).
> 
> While I am at it, the above macro is even further optimized by moving the 
> endianness conversion to the constant so it is applied at compile time 
> rather than run time like so:
> 
> #define MSDOS_MBR(p)	((*(u16*)(p)) == __constant_cpu_to_le16(0xaa55)
Thanks Anton :) Applied.It worked before but in semi-evaluation.

Regards,
FabF

> 
> Best regards,
> 
>  	Anton

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diff -Naur orig/fs/partitions/efi.c edited/fs/partitions/efi.c
--- orig/fs/partitions/efi.c	2004-06-16 07:19:23.000000000 +0200
+++ edited/fs/partitions/efi.c	2004-06-24 01:43:45.000000000 +0200
@@ -145,7 +145,7 @@
 	int i, found = 0, signature = 0;
 	if (!mbr)
 		return 0;
-	signature = (le16_to_cpu(mbr->signature) == MSDOS_MBR_SIGNATURE);
+	signature = MSDOS_MBR(&mbr->signature);
 	for (i = 0; signature && i < 4; i++) {
 		if (mbr->partition_record[i].sys_ind ==
                     EFI_PMBR_OSTYPE_EFI_GPT) {
diff -Naur orig/fs/partitions/efi.h edited/fs/partitions/efi.h
--- orig/fs/partitions/efi.h	2004-06-16 07:19:43.000000000 +0200
+++ edited/fs/partitions/efi.h	2004-06-24 01:20:46.000000000 +0200
@@ -34,7 +34,6 @@
 #include <linux/string.h>
 #include <linux/efi.h>
 
-#define MSDOS_MBR_SIGNATURE 0xaa55
 #define EFI_PMBR_OSTYPE_EFI 0xEF
 #define EFI_PMBR_OSTYPE_EFI_GPT 0xEE
 
diff -Naur orig/fs/partitions/msdos.c edited/fs/partitions/msdos.c
--- orig/fs/partitions/msdos.c	2004-06-16 07:19:37.000000000 +0200
+++ edited/fs/partitions/msdos.c	2004-06-24 08:00:05.000000000 +0200
@@ -50,15 +50,6 @@
 		SYS_IND(p) == LINUX_EXTENDED_PARTITION);
 }
 
-#define MSDOS_LABEL_MAGIC1	0x55
-#define MSDOS_LABEL_MAGIC2	0xAA
-
-static inline int
-msdos_magic_present(unsigned char *p)
-{
-	return (p[0] == MSDOS_LABEL_MAGIC1 && p[1] == MSDOS_LABEL_MAGIC2);
-}
-
 /*
  * Create devices for each logical partition in an extended partition.
  * The logical partitions form a linked list, with each entry being
@@ -87,6 +78,7 @@
 	this_size = first_size;
 
 	while (1) {
+		/*FIXME : Some definition for 100*/
 		if (++loopct > 100)
 			return;
 		if (state->next == state->limit)
@@ -95,7 +87,7 @@
 		if (!data)
 			return;
 
-		if (!msdos_magic_present(data + 510))
+		if (!MSDOS_MBR (data+510))
 			goto done; 
 
 		p = (struct partition *) (data + 0x1be);
@@ -112,7 +104,7 @@
 		/* 
 		 * First process the data partition(s)
 		 */
-		for (i=0; i<4; i++, p++) {
+		for (i = 0; i < 4; i++, p++) {
 			u32 offs, size, next;
 			if (!NR_SECTS(p) || is_extended_partition(p))
 				continue;
@@ -146,7 +138,7 @@
 		 * It should be a link to the next logical partition.
 		 */
 		p -= 4;
-		for (i=0; i<4; i++, p++)
+		for (i = 0; i < 4; i++, p++)
 			if (NR_SECTS(p) && is_extended_partition(p))
 				break;
 		if (i == 4)
@@ -342,7 +334,7 @@
 	/* The first sector of a Minix partition can have either
 	 * a secondary MBR describing its subpartitions, or
 	 * the normal boot sector. */
-	if (msdos_magic_present (data + 510) &&
+	if (MSDOS_MBR(data + 510) &&
 	    SYS_IND(p) == MINIX_PARTITION) { /* subpartition table present */
 
 		printk(" %s%d: <minix:", state->name, origin);
@@ -385,7 +377,7 @@
 	data = read_dev_sector(bdev, 0, &sect);
 	if (!data)
 		return -1;
-	if (!msdos_magic_present(data + 510)) {
+	if (!MSDOS_MBR(data + 510)) {
 		put_dev_sector(sect);
 		return 0;
 	}
@@ -403,7 +395,7 @@
 			return 0;
 		}
 	}
-
+	/*EFI : Extensible Firmware Initiative partitions ?*/
 #ifdef CONFIG_EFI_PARTITION
 	p = (struct partition *) (data + 0x1be);
 	for (slot = 1 ; slot <= 4 ; slot++, p++) {
@@ -433,6 +425,7 @@
 			   extended partition, but leave room for LILO */
 			put_partition(state, slot, start, size == 1 ? 1 : 2);
 			printk(" <");
+			/*parsing extended for logical partitions*/
 			parse_extended(state, bdev, start, size);
 			printk(" >");
 			continue;
diff -Naur orig/include/linux/genhd.h edited/include/linux/genhd.h
--- orig/include/linux/genhd.h	2004-06-16 07:18:59.000000000 +0200
+++ edited/include/linux/genhd.h	2004-06-24 17:11:58.447759035 +0200
@@ -17,6 +17,9 @@
 #include <linux/string.h>
 #include <linux/fs.h>
 
+/*Master boot record magic numbers*/
+#define MSDOS_MBR(p)    ((*(u16*)(p)) == __constant_cpu_to_le16(0xaa55))
+
 enum {
 /* These three have identical behaviour; use the second one if DOS FDISK gets
    confused about extended/logical partitions starting past cylinder 1023. */