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

On Sat, Jun 5, 2021 at 4:11 AM Jiri Olsa [off-list ref] 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?