Re: Need a way to modify the section name for a read program object

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

"Eelco Chaudron" <echaudro@redhat.com> writes:

> Hi All,
>
> I'm trying to write an xdpdump like utility and have some missing part 
> in libbpf to change the fentry/FUNCTION section name before loading the 
> trace program.
>
> In short, I have an eBPF program that has a section name like 
> "fentry/FUNCTION" where FUNCTION needs to be replaced by the name of the 
> XDP program loaded in the interfaces its start function.
>
> The code for loading the ftrace part is something like:
>
> 	open_opts.attach_prog_fd = bpf_prog_get_fd_by_id(info.id);
> 	trace_obj = bpf_object__open_file("xdpdump_bpf.o", &open_opts);
>
> 	trace_prog_fentry = bpf_object__find_program_by_title(trace_obj, 
> "fentry/FUNCTION");
>
> 	/* Here I need to replace the trace_prog_fentry->section_name = 
> "fentry/<INTERFACE PROG NAME> */
>
> 	bpf_object__load(trace_obj);
> 	trace_link_fentry = bpf_program__attach_trace(trace_prog_fentry);
>
>
> See the above, I would like to change the section_name but there is no 
> API to do this, and of course, the struct bpf_program is 
> implementation-specific.
>
> Any idea how I would work around this, or what extension to libbpf can 
> be suggested to support this?

I think what's missing is a way for the caller to set the attach_btf_id.
Currently, libbpf always tries to discover this based on the section
name (see libbpf_find_attach_btf_id()). I think the right way to let the
caller specify this is not to change the section name, though, but just
to expose a way to explicitly set the btf_id (which the caller can then
go find on its own).

Not sure if it would be better with a new open_opt (to mirror
attach_prog_fd), or just a setter (bpf_program__set_attach_btf_id()?).
Or maybe both? Andrii, WDYT?

-Toke