From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [PATCH net 1/2] netlink: add NLA_U8_BUGGY attribute type Date: Tue, 05 Dec 2017 11:41:06 -0500 (EST) Message-ID: <20171205.114106.1013322969674769159.davem@davemloft.net> References: <20171202202332.10205-1-johannes@sipsolutions.net> <20171205.113145.172521292247335321.davem@davemloft.net> <1512491661.26976.19.camel@sipsolutions.net> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: linux-wireless@vger.kernel.org, netdev@vger.kernel.org, j@w1.fi, dsahern@gmail.com To: johannes@sipsolutions.net Return-path: Received: from shards.monkeyblade.net ([184.105.139.130]:36810 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751550AbdLEQlI (ORCPT ); Tue, 5 Dec 2017 11:41:08 -0500 In-Reply-To: <1512491661.26976.19.camel@sipsolutions.net> Sender: netdev-owner@vger.kernel.org List-ID: From: Johannes Berg Date: Tue, 05 Dec 2017 17:34:21 +0100 > On Tue, 2017-12-05 at 11:31 -0500, David Miller wrote: >> >> > We could try to fix up the big endian problem here, but we >> > don't know *how* userspace misbehaved - if using nla_put_u32 >> > then we could, but we also found a debug tool (which we'll >> > ignore for the purposes of this regression) that was putting >> > the padding into the length. > >> We're stuck with this thing forever... I'd like to consider other >> options. >> >> I've seen this problem at least one time before, therefore I >> suggest when we see a U8 attribute with a U32's length: >> >> 1) We access it as a u32, this takes care of all endianness >> issues. > > Possible, but as I said above, I've seen at least one tool (a debug > only script) now that will actually emit a U8 followed by 3 bytes of > padding to make it netlink-aligned, but set the length to 4. That would > be broken by making this change. There is no reasonable interpretation for what that application is doing, so I think we can safely call that case as buggy. We are only trying to handle the situation where a U8 attribute is presented as a bonafide U32 or a correct U8. Does this make sense?