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
