vfs: Don't create inodes with a uid or gid unknown to the vfs

 *  1. We can't do it if child already exists (open has special treatment for

 *     this case, but since we are inlined it's OK)

 *  2. We can't do it if dir is read-only (done in permission())

 *  3. We should have write and exec permissions on dir

 *  4. We can't do it if dir is immutable (done in permission())

 *  3. We can't do it if the fs can't represent the fsuid or fsgid.

 *  4. We should have write and exec permissions on dir

 *  5. We can't do it if dir is immutable (done in permission())

 */

static inline int may_create(struct inode *dir, struct dentry *child)

{

	struct user_namespace *s_user_ns;

	audit_inode_child(dir, child, AUDIT_TYPE_CHILD_CREATE);

	if (child->d_inode)

		return -EEXIST;

	if (IS_DEADDIR(dir))

		return -ENOENT;

	s_user_ns = dir->i_sb->s_user_ns;

	if (!kuid_has_mapping(s_user_ns, current_fsuid()) ||

	    !kgid_has_mapping(s_user_ns, current_fsgid()))

		return -EOVERFLOW;

	return inode_permission(dir, MAY_WRITE | MAY_EXEC);

}