vfs: add whiteout support

Whiteout isn't actually a new file type, but is represented as a char
device (Linus's idea) with 0/0 device number.

This has several advantages compared to introducing a new whiteout file
type:

- no userspace API changes (e.g. trivial to make backups of upper layer
filesystem, without losing whiteouts)

- no fs image format changes (you can boot an old kernel/fsck without
whiteout support and things won't break)

- implementation is trivial

Signed-off-by: Miklos Szeredi <[e-mail]>

fs/namei.c

···
 	return sys_renameat2(AT_FDCWD, oldname, AT_FDCWD, newname, 0);
 }
 
+int vfs_whiteout(struct inode *dir, struct dentry *dentry)
+{
+	int error = may_create(dir, dentry);
+	if (error)
+		return error;
+
+	if (!dir->i_op->mknod)
+		return -EPERM;
+
+	return dir->i_op->mknod(dir, dentry,
+				S_IFCHR | WHITEOUT_MODE, WHITEOUT_DEV);
+}
+EXPORT_SYMBOL(vfs_whiteout);
+
 int readlink_copy(char __user *buffer, int buflen, const char *link)
 {
 	int len = PTR_ERR(link);

include/linux/fs.h

···
 #define ATTR_TIMES_SET	(1 << 16)
 
 /*
+ * Whiteout is represented by a char device.  The following constants define the
+ * mode and device number to use.
+ */
+#define WHITEOUT_MODE 0
+#define WHITEOUT_DEV 0
+
+/*
  * This is the Inode Attributes structure, used for notify_change().  It
  * uses the above definitions as flags, to know which values have changed.
  * Also, in this manner, a Filesystem can look at only the values it cares
···
 extern int vfs_rmdir(struct inode *, struct dentry *);
 extern int vfs_unlink(struct inode *, struct dentry *, struct inode **);
 extern int vfs_rename(struct inode *, struct dentry *, struct inode *, struct dentry *, struct inode **, unsigned int);
+extern int vfs_whiteout(struct inode *, struct dentry *);
 
 /*
  * VFS dentry helper functions.
···
 #define IS_IMA(inode)		((inode)->i_flags & S_IMA)
 #define IS_AUTOMOUNT(inode)	((inode)->i_flags & S_AUTOMOUNT)
 #define IS_NOSEC(inode)		((inode)->i_flags & S_NOSEC)
+
+#define IS_WHITEOUT(inode)	(S_ISCHR(inode->i_mode) && \
+				 (inode)->i_rdev == WHITEOUT_DEV)
 
 /*
  * Inode state bits.  Protected by inode->i_lock