Re: [PATCH bpf-next v3 1/5] bpf: Support chain calling multiple BPF programs after each other

[Edward Cree <ecree@solarflare.com>]

On 15/10/2019 17:42, Toke Høiland-Jørgensen wrote:
> Edward Cree <ecree@solarflare.com> writes:
>> On 14/10/2019 19:48, Toke Høiland-Jørgensen wrote:
>>> So that will end up with a single monolithic BPF program being loaded
>>> (from the kernel PoV), right? That won't do; we want to be able to go
>>> back to the component programs, and manipulate them as separate kernel
>>> objects.
>> Why's that? (Since it also applies to the static-linking PoC I'm
>> putting together.) What do you gain by having the components be
>> kernel-visible?
> Because then userspace will have to keep state to be able to answer
> questions like "show me the list of programs that are currently loaded
> (and their call chain)", or do operations like "insert this program into
> the call chain at position X".
Userspace keeps state for stuff all the time.  We call them "daemons" ;)
Now you might have arguments for why putting a given piece of state in
 userspace is a bad idea — there's a reason why not everything is a
 microkernel — but those arguments need to be made.

> We already keep all this state in the kernel,
The kernel keeps the state of "current (monolithic) BPF program loaded
 (against each hook)".  Prior to this patch series, the kernel does
 *not* keep any state on what that BPF program was made of (except in
 the sense of BTF debuginfos, which a linker could combine appropriately).

So if we _don't_ add your chained-programs functionality into the kernel,
 and then _do_ implement userspace linking, then there isn't any
 duplicated functionality or even duplicated state — the userland state
 is "what are my components and what's the linker invocation that glues
 them together", the kernel state is "here is one monolithic BPF blob,
 along with a BTF blob to debug it".  The kernel knows nothing of the
 former, and userspace doesn't store (but knows how to recreate) the
 latter.

(That said, proper dynamic linking is better than static linking OR chain
 calls, because it gives us the full flexibility of linking while giving
 you your 'subprogs as kernel objects & kernel state'.  But I think we'll
 need to prototype things with static linking first so that we can be
 sure of the linker semantics we want, before we try to put a new dynamic
 linker in the kernel.)

-Ed