Messages in this thread | | | Date | Thu, 21 May 2015 09:53:21 -0700 | From | Alexei Starovoitov <> | Subject | Re: [PATCH net-next 1/4] bpf: allow bpf programs to tail-call other bpf programs |
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On 5/21/15 9:43 AM, Andy Lutomirski wrote: > 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.
well, I don't see a use case for 'pointer to tail-call target', but more generic 'pointer to skb' indeed is a useful concept. I was thinking more like 'pointer to structure of the type X', then we can natively support 'pointer to task_struct', 'pointer to inode', etc which will help tracing programs to be written in more convenient way. Right now pointer walking has to be done via bpf_probe_read() helper as demonstrated in tracex1_kern.c example. With this future 'pointer to struct of type X' knowledge in verifier we'll be able to do 'ptr->field' natively with higher performance.
> We'd still need some way to stick fds into a map, but that's not > really the verifier's problem.
well, they both need to be aware of that. When it comes to safety generalization suffers. Have to do extra checks both in map_update_elem and in verifier. No way around that.
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