On Fri, Jan 15, 2021 at 07:51:54AM +1100, Dave Chinner wrote:

quoted

@@ -813,7 +818,7 @@ xfs_setattr_nonsize(
 	 * 	     Posix ACL code seems to care about this issue either.
 	 */
 	if (mask & ATTR_MODE) {
-		error = posix_acl_chmod(&init_user_ns, inode, inode->i_mode);
+		error = posix_acl_chmod(mnt_userns, inode, inode->i_mode);
 		if (error)
 			return error;
 	}

@@ -868,7 +873,7 @@ xfs_setattr_size(
 		 * Use the regular setattr path to update the timestamps.
 		 */
 		iattr->ia_valid &= ~ATTR_SIZE;
-		return xfs_setattr_nonsize(ip, iattr);
+		return xfs_setattr_nonsize(&init_user_ns, ip, iattr);

Shouldn't that be passing mnt_userns?

As Christian already explained we an't hit this with anything related
to uids/gids, the only thing that will be updated are the timestamps,
as also mentioned in the comment that only makes it partially into the
diff context.

quoted

 	trace_xfs_setattr(ip);
 
-	error = xfs_vn_change_ok(dentry, iattr);
+	error = xfs_vn_change_ok(mnt_userns, dentry, iattr);
 	if (error)
 		return error;
 	return xfs_setattr_size(ip, iattr);

And this passing mnt_userns down into xfs_setattr_size()?  Seems
like a bit of a landmine...

That being said we could just pass down the argument, even if it doesn't
make much sense for the size update.