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

["Eelco Chaudron" <echaudro@xxxxxxxxxx>]

On 4 Feb 2020, at 20:32, Andrii Nakryiko wrote:

> On Tue, Feb 4, 2020 at 11:27 AM Toke Høiland-Jørgensen 
> <toke@xxxxxxxxxx> wrote:
> > Andrii Nakryiko <andriin@xxxxxx> writes:
> >
> > > On 2/4/20 2:19 AM, Toke Høiland-Jørgensen wrote:
> > > > "Eelco Chaudron" <echaudro@xxxxxxxxxx> 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).
>
> great! and, ha-ha, too object-oriented thinking ;)

Thanks for your feedback, assuming you are not working on it, I’ll 
implement/test it and sent out a patch.

//Eelco