From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ben Hutchings Subject: Re: [RFC] ip tunnel flag byte order issue Date: Wed, 10 Oct 2012 21:26:36 +0100 Message-ID: <1349900796.2691.32.camel@bwh-desktop.uk.solarflarecom.com> References: <20121010120630.5e9f2c2c@nehalam.linuxnetplumber.net> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: David Miller , Saurabh Mohan , , To: Stephen Hemminger Return-path: Received: from webmail.solarflare.com ([12.187.104.25]:7045 "EHLO ocex02.SolarFlarecom.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1756582Ab2JJU0k (ORCPT ); Wed, 10 Oct 2012 16:26:40 -0400 In-Reply-To: <20121010120630.5e9f2c2c@nehalam.linuxnetplumber.net> Sender: netdev-owner@vger.kernel.org List-ID: On Wed, 2012-10-10 at 12:06 -0700, Stephen Hemminger wrote: > Sparse found a real problem with the ABI for tunnelling. > > The SIT and VTI tunnel ioctl's both overload the i_flags field in the > ip_tunnel parameters structure. This field is defined as big endian > (be16) and the various GRE_XXX macros do the necessary byte swapping. > > The problem is that both SIT and VTI are using an additional flag bit > that is defined in host byte order, and is then or'd in. It happens to > work because both possible locations hit holes in the current usage of > GRE. For big endian cpu's it overlaps the GRE_VERSION which is always > zero, and for little endian it overlaps the GRE recursion field also > always zero. Why do these fields exist if they're always going to be 0? > Having the field in different places on different CPU architectures > was a mistake. The problem is fixing it will break the ABI on one or > the other architecture. I choose to break big endian since it the > minority. Or we can define the 'flag' to have both bits set (0x0101, with a __cpu_to_be16 to keep sparse happy) while accepting either set on input. > Also both VTI and SIT are overloading the same bit which is an > accident waiting to happen. Since VTI is newer, I propose giving a > different bit to VTI. Indeed VTI is new in 3.6, so there is still a short window in which it's fairly safe to tweak its ABI. > The other alternative is keeping the same ABI, but putting a big note > as to why it works in spite of our stupidity. [...] Does it even matter that different tunnel types have different meanings for flags? Ben. -- Ben Hutchings, Staff Engineer, Solarflare Not speaking for my employer; that's the marketing department's job. They asked us to note that Solarflare product names are trademarked.