Alexei Starovoitov [off-list ref] 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