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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
