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

On Tue, Jun 08, 2021 at 08:42:32AM -0700, Alexei Starovoitov wrote:

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

quoted

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.

so multi trampoline gets called from all the registered functions,
so there would need to be filter for specific ip before calling the
standard program.. single cmp/jnz might not be that bad, I'll check

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?

we could just skip ip arg when generating entry for normal programs 
and start from first argument address for %rdi

and it'd need to be transparent for current trampolines user API,
so I wonder there will be some hiccup ;-) let's see

thanks,
jirka