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>

1 files changed, 22 insertions, 0 deletions

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 */