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

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

Andrii Nakryiko <andriin@fb.com> writes:

> On 2/4/20 2:19 AM, Toke Høiland-Jørgensen wrote:
>> "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).
>
> Yes, I agree, section name should be treated as an immutable identifier 
> and a (overrideable) hint to libbpf.
>
>> 
>> 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?
>
> open_opts is definitely wrong way to do this, because open_opts apply to 
> all BPF programs, while this should be per-program.

Yes, of course; silly me :)

> I'm also not sure having API that allows to specify BTF type ID is the
> best, probably better to let libbpf perform the search by name. So I'd
> say something like this:
>
> int bpf_program__set_attach_target(int attach_prog_fd, const char 
> *attach_func_name)
>
> This should handle customizing all the tp_btf/fentry/fexit/freplace BPF 
> programs we have.

Right, that makes sense; I think that would cover it (apart from your
function signature missing a struct bpf_program argument).

> We might add extra attach_target_ops for future extensibility, if we
> anticipate that we'll need more knobs in the future, I haven't thought
> too much about that.

Good question, me neither. Will see if I can think of anything...

-Toke