On Wed, 10 Oct 2012 21:26:36 +0100 Ben Hutchings <[email protected]> wrote:
> 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? They exist in the RFC. GRE implementation mixes bits on the wire with bits from ioctl(). > > > 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. > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
