bpf: Ensure type tags precede modifiers in BTF

It is guaranteed that for modifiers, clang always places type tags before other modifiers, and then the base type. We would like to rely on this guarantee inside the kernel to make it simple to parse type tags from BTF. However, a user would be allowed to construct a BTF without such guarantees. Hence, add a pass to check that in modifier chains, type tags only occur at the head of the chain, and then don't occur later in the chain. If we see a type tag, we can have one or more type tags preceding other modifiers that then never have another type tag. If we see other modifiers, all modifiers following them should never be a type tag. Instead of having to walk chains we verified previously, we can remember the last good modifier type ID which headed a good chain. At that point, we must have verified all other chains headed by type IDs less than it. This makes the verification process less costly, and it becomes a simple O(n) pass.

KKKumar Kartikeya Dwivedi committed 4 years ago