Re: [PATCH net-next 1/4] bpf: allow bpf programs to tail-call other bpf programs

[Andy Lutomirski <luto@amacapital.net>]

On Thu, May 21, 2015 at 9:40 AM, Alexei Starovoitov <ast@plumgrid.com> wrote:
> On 5/21/15 9:20 AM, Andy Lutomirski wrote:
>>
>>
>> What I mean is: why do we need the interface to be "look up this index
>> in an array and just to what it references" as a single atomic
>> instruction?  Can't we break it down into first "look up this index in
>> an array" and then "do this tail call"?
>
>
> I've actually considered to do this split and do first part as map lookup
> and 2nd as 'tail call to this ptr' insn, but it turned out to be
> painful: verifier gets more complicated, ctx pointer needs to kept
> somewhere, JITs need to special case two things instead of one.
> Also I couldn't see a use case for exposing program pointer to the
> program itself. I've explored this path only because it felt more
> traditional 'goto *ptr' like, but adding new PTR_TO_PROG type to
> verifier looked wasteful.

At some point, I think that it would be worth extending the verifier
to support more general non-integral scalar types. "Pointer to
tail-call target" would be just one of them.  "Pointer to skb" might
be nice as a real first-class scalar type that lives in a register as
opposed to just being magic typed context.

We'd still need some way to stick fds into a map, but that's not
really the verifier's problem.

--Andy