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I would like propose new BPF instructions for the misc category: BPF_COP
and BPF_COPX.  It would provide a capability of calling an external
function - think of BPF "coprocessor".  The argument for BPF_COP is an
index to a pre-loaded array of function pointers.  BPF_COPX takes the
function index from the register X rather than a constant.

        BPF_STMT(BPF_MISC+BPF_COP, 0), /* A <- funcs[0](...) */

        typedef uint32_t(*bpf_copfunc_t)(struct mbuf *pkt,
            uint32_t A, uint32_t *M);

        int bpf_set_cop(bpf_ctx_t *c, bpf_copfunc_t funcs[], size_t n);

The arguments passed to a called function would be the packet, accumulator
and the memory store.  The return value would be stored in the accumulator
and the register X would be reset to 0.  Note that the function may also
change the memory store.  If the function index is out of range, then the
register X would be set to 0xffffffff.

Note that bpf_filter(9) would need to take some context structure (which is
preferable in general).



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