Re: [PATCH 13/19] bpf: Add support to link multi func tracing program

Alexei Starovoitov
2021-06-08T15:43:03Z

On Sat, Jun 5, 2021 at 4:11 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding support to attach multiple functions to tracing program
> by using the link_create/link_update interface.
>
> Adding multi_btf_ids/multi_btf_ids_cnt pair to link_create struct
> API, that define array of functions btf ids that will be attached
> to prog_fd.
>
> The prog_fd needs to be multi prog tracing program (BPF_F_MULTI_FUNC).
>
> The new link_create interface creates new BPF_LINK_TYPE_TRACING_MULTI
> link type, which creates separate bpf_trampoline and registers it
> as direct function for all specified btf ids.
>
> The new bpf_trampoline is out of scope (bpf_trampoline_lookup) of
> standard trampolines, so all registered functions need to be free
> of direct functions, otherwise the link fails.

Overall the api makes sense to me.
The restriction of multi vs non-multi is too severe though.
The multi trampoline can serve normal fentry/fexit too.
If ip is moved to the end (instead of start) the trampoline
will be able to call into multi and normal fentry/fexit progs. Right?