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. 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. 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.
-Toke
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