new helper: ext2_image_size()

... implemented that way since the next commit will leave it
almost alone in ext2_fs.h - most of the file (including
struct ext2_super_block) is going to move to fs/ext2/ext2.h.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/blackfin/kernel/setup.c

@@ -550,6 +550,7 @@ static __init void memory_setup(void)
 {
 #ifdef CONFIG_MTD_UCLINUX
 	unsigned long mtd_phys = 0;
+	unsigned long n;
 #endif
 	unsigned long max_mem;
 
@@ -593,9 +594,9 @@ static __init void memory_setup(void)
 	mtd_size = PAGE_ALIGN(*((unsigned long *)(mtd_phys + 8)));
 
 # if defined(CONFIG_EXT2_FS) || defined(CONFIG_EXT3_FS)
-	if (*((unsigned short *)(mtd_phys + 0x438)) == EXT2_SUPER_MAGIC)
-		mtd_size =
-		    PAGE_ALIGN(*((unsigned long *)(mtd_phys + 0x404)) << 10);
+	n = ext2_image_size((void *)(mtd_phys + 0x400));
+	if (n)
+		mtd_size = PAGE_ALIGN(n * 1024);
 # endif
 
 # if defined(CONFIG_CRAMFS)

include/linux/ext2_fs.h

@@ -536,4 +536,26 @@ enum {
 					 ~EXT2_DIR_ROUND)
 #define EXT2_MAX_REC_LEN		((1<<16)-1)
 
+#define EXT2_SB_MAGIC_OFFSET	0x38
+#define EXT2_SB_BLOCKS_OFFSET	0x04
+#define EXT2_SB_BSIZE_OFFSET	0x18
+
+static inline u64 ext2_image_size(void *ext2_sb)
+{
+	__u8 *p = ext2_sb;
+	if (*(__le16 *)(p + EXT2_SB_MAGIC_OFFSET) != cpu_to_le16(EXT2_SUPER_MAGIC))
+		return 0;
+	return (u64)le32_to_cpup((__le32 *)(p + EXT2_SB_BLOCKS_OFFSET)) <<
+		le32_to_cpup((__le32 *)(p + EXT2_SB_BSIZE_OFFSET));
+}
+
+static inline void verify_offsets(void)
+{
+#define A(x,y) BUILD_BUG_ON(x != offsetof(struct ext2_super_block, y));
+	A(EXT2_SB_MAGIC_OFFSET, s_magic);
+	A(EXT2_SB_BLOCKS_OFFSET, s_blocks_count);
+	A(EXT2_SB_BSIZE_OFFSET, s_log_block_size);
+#undef A
+}
+
 #endif	/* _LINUX_EXT2_FS_H */

init/do_mounts_rd.c

@@ -54,20 +54,19 @@ identify_ramdisk_image(int fd, int start_block, decompress_fn *decompressor)
 {
 	const int size = 512;
 	struct minix_super_block *minixsb;
-	struct ext2_super_block *ext2sb;
 	struct romfs_super_block *romfsb;
 	struct cramfs_super *cramfsb;
 	struct squashfs_super_block *squashfsb;
 	int nblocks = -1;
 	unsigned char *buf;
 	const char *compress_name;
+	unsigned long n;
 
 	buf = kmalloc(size, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;
 
 	minixsb = (struct minix_super_block *) buf;
-	ext2sb = (struct ext2_super_block *) buf;
 	romfsb = (struct romfs_super_block *) buf;
 	cramfsb = (struct cramfs_super *) buf;
 	squashfsb = (struct squashfs_super_block *) buf;
@@ -150,12 +149,12 @@ identify_ramdisk_image(int fd, int start_block, decompress_fn *decompressor)
 	}
 
 	/* Try ext2 */
-	if (ext2sb->s_magic == cpu_to_le16(EXT2_SUPER_MAGIC)) {
+	n = ext2_image_size(buf);
+	if (n) {
 		printk(KERN_NOTICE
 		       "RAMDISK: ext2 filesystem found at block %d\n",
 		       start_block);
-		nblocks = le32_to_cpu(ext2sb->s_blocks_count) <<
-			le32_to_cpu(ext2sb->s_log_block_size);
+		nblocks = n;
 		goto done;
 	}