Messages in this thread | | | Subject | Re: [RFC] ip tunnel flag byte order issue | From | Ben Hutchings <> | Date | Wed, 10 Oct 2012 21:26:36 +0100 |
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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.
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