Revert "(namespace) vfs: Don't modify inodes with a uid or gid unknown to the vfs"

BugLink: https://bugs.launchpad.net/bugs/1644165

This reverts commit ac7f3f73.

The kernel fix for bug #1634964 breaks LXD userspace, in particular the
following commits:

ac7f3f73 (namespace) vfs: Don't modify inodes with a uid or gid unknown to the vfs
ca52383a (namespace) vfs: Don't create inodes with a uid or gid unknown to the vfs

LXD 2.0.6 will include changes to support these kernel changes, but it isn't
available yet on xenial, so for now we just revert these commits.
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/attr.c

@@ -312,14 +312,6 @@ int notify_change(struct dentry * dentry, struct iattr * attr, struct inode **de
 	    !kgid_has_mapping(inode->i_sb->s_user_ns, attr->ia_gid))
 		return -EOVERFLOW;
 
-	/* Don't allow modifications of files with invalid uids or
-	 * gids unless those uids & gids are being made valid.
-	 */
-	if (!(ia_valid & ATTR_UID) && !uid_valid(inode->i_uid))
-		return -EOVERFLOW;
-	if (!(ia_valid & ATTR_GID) && !gid_valid(inode->i_gid))
-		return -EOVERFLOW;
-
 	error = security_inode_setattr(dentry, attr);
 	if (error)
 		return error;

fs/inode.c

@@ -1610,13 +1610,6 @@ bool atime_needs_update(const struct path *path, struct inode *inode)
 
 	if (inode->i_flags & S_NOATIME)
 		return false;
-
-	/* Atime updates will likely cause i_uid and i_gid to be written
-	 * back improprely if their true value is unknown to the vfs.
-	 */
-	if (HAS_UNMAPPED_ID(inode))
-		return false;
-
 	if (IS_NOATIME(inode))
 		return false;
 	if ((inode->i_sb->s_flags & MS_NODIRATIME) && S_ISDIR(inode->i_mode))

fs/namei.c

@@ -410,14 +410,6 @@ int __inode_permission(struct inode *inode, int mask)
 		 */
 		if (IS_IMMUTABLE(inode))
 			return -EACCES;
-
-		/*
-		 * Updating mtime will likely cause i_uid and i_gid to be
-		 * written back improperly if their true value is unknown
-		 * to the vfs.
-		 */
-		if (HAS_UNMAPPED_ID(inode))
-			return -EACCES;
 	}
 
 	retval = do_inode_permission(inode, mask);
@@ -2561,11 +2553,10 @@ EXPORT_SYMBOL(__check_sticky);
  *	c. have CAP_FOWNER capability
  *  6. If the victim is append-only or immutable we can't do antyhing with
  *     links pointing to it.
- *  7. If the victim has an unknown uid or gid we can't change the inode.
- *  8. If we were asked to remove a directory and victim isn't one - ENOTDIR.
- *  9. If we were asked to remove a non-directory and victim isn't one - EISDIR.
- * 10. We can't remove a root or mountpoint.
- * 11. We don't allow removal of NFS sillyrenamed files; it's handled by
+ *  7. If we were asked to remove a directory and victim isn't one - ENOTDIR.
+ *  8. If we were asked to remove a non-directory and victim isn't one - EISDIR.
+ *  9. We can't remove a root or mountpoint.
+ * 10. We don't allow removal of NFS sillyrenamed files; it's handled by
  *     nfs_async_unlink().
  */
 static int may_delete(struct inode *dir, struct dentry *victim, bool isdir)
@@ -2587,7 +2578,7 @@ static int may_delete(struct inode *dir, struct dentry *victim, bool isdir)
 		return -EPERM;
 
 	if (check_sticky(dir, inode) || IS_APPEND(inode) ||
-	    IS_IMMUTABLE(inode) || IS_SWAPFILE(inode) || HAS_UNMAPPED_ID(inode))
+	    IS_IMMUTABLE(inode) || IS_SWAPFILE(inode))
 		return -EPERM;
 	if (isdir) {
 		if (!d_is_dir(victim))
@@ -4028,13 +4019,6 @@ int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_de
 	 */
 	if (IS_APPEND(inode) || IS_IMMUTABLE(inode))
 		return -EPERM;
-	/*
-	 * Updating the link count will likely cause i_uid and i_gid to
-	 * be writen back improperly if their true value is unknown to
-	 * the vfs.
-	 */
-	if (HAS_UNMAPPED_ID(inode))
-		return -EPERM;
 	if (!dir->i_op->link)
 		return -EPERM;
 	if (S_ISDIR(inode->i_mode))

fs/xattr.c

@@ -38,13 +38,6 @@ xattr_permission(struct inode *inode, const char *name, int mask)
 	if (mask & MAY_WRITE) {
 		if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
 			return -EPERM;
-		/*
-		 * Updating an xattr will likely cause i_uid and i_gid
-		 * to be writen back improperly if their true value is
-		 * unknown to the vfs.
-		 */
-		if (HAS_UNMAPPED_ID(inode))
-			return -EPERM;
 	}
 
 	/*

include/linux/fs.h

@@ -1832,11 +1832,6 @@ struct super_operations {
 #define IS_WHITEOUT(inode)	(S_ISCHR(inode->i_mode) && \
 				 (inode)->i_rdev == WHITEOUT_DEV)
 
-static inline bool HAS_UNMAPPED_ID(struct inode *inode)
-{
-	return !uid_valid(inode->i_uid) || !gid_valid(inode->i_gid);
-}
-
 /*
  * Inode state bits.  Protected by inode->i_lock
  *