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>

3 files changed, 30 insertions, 8 deletions

diff --git a/arch/blackfin/kernel/setup.c b/arch/blackfin/kernel/setup.c
index 2aa019368504..2ad747e909fb 100644
--- a/arch/blackfin/kernel/setup.c
+++ b/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)
diff --git a/include/linux/ext2_fs.h b/include/linux/ext2_fs.h
index f28dba518a4a..5f65ec75b48a 100644
--- a/include/linux/ext2_fs.h
+++ b/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 */
diff --git a/init/do_mounts_rd.c b/init/do_mounts_rd.c
index 01f1306aa26e..6212586df29a 100644
--- a/init/do_mounts_rd.c
+++ b/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;
         }