Re: [PATCH v3 bpf-next 15/18] bpf: Support attaching tracing BPF program to other BPF programs

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Alexei Starovoitov <ast@kernel.org> writes:

> Allow FENTRY/FEXIT BPF programs to attach to other BPF programs of any type
> including their subprograms. This feature allows snooping on input and output
> packets in XDP, TC programs including their return values. In order to do that
> the verifier needs to track types not only of vmlinux, but types of other BPF
> programs as well. The verifier also needs to translate uapi/linux/bpf.h types
> used by networking programs into kernel internal BTF types used by FENTRY/FEXIT
> BPF programs. In some cases LLVM optimizations can remove arguments from BPF
> subprograms without adjusting BTF info that LLVM backend knows. When BTF info
> disagrees with actual types that the verifiers sees the BPF trampoline has to
> fallback to conservative and treat all arguments as u64. The FENTRY/FEXIT
> program can still attach to such subprograms, but won't be able to recognize
> pointer types like 'struct sk_buff *' into won't be able to pass them to
> bpf_skb_output() for dumping to user space.
>
> The BPF_PROG_LOAD command is extended with attach_prog_fd field. When it's set
> to zero the attach_btf_id is one vmlinux BTF type ids. When attach_prog_fd
> points to previously loaded BPF program the attach_btf_id is BTF type id of
> main function or one of its subprograms.
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

This is cool! Certainly solves the xdpdump use case; thanks!

I do have a few questions (thinking about whether it can also be used
for running multiple XDP programs):

- Can a FEXIT function loaded this way only *observe* the return code of
  the BPF program it attaches to, or can it also change it?

- Is it possible to attach multiple FENTRY/FEXIT programs to the same
  XDP program and/or to recursively attach FENTRY/FEXIT programs to each
  other?

- Could it be possible for an FENTRY/FEXIT program to call into another
  XDP program (i.e., one that has the regular XDP program type)?

-Toke