Re: bpf: ability to attach freplace to multiple parents

Alan Maguire [off-list ref] writes:

On Tue, 12 May 2020, Toke Høiland-Jørgensen wrote:

quoted

Alexei Starovoitov [off-list ref] writes:

quoted

quoted

quoted

Currently fentry/fexit/freplace progs have single prog->aux->linked_prog pointer.
It just needs to become a linked list.
The api extension could be like this:
bpf_raw_tp_open(prog_fd, attach_prog_fd, attach_btf_id);
(currently it's just bpf_raw_tp_open(prog_fd))
The same pair of (attach_prog_fd, attach_btf_id) is already passed into prog_load
to hold the linked_prog and its corresponding btf_id.
I'm proposing to extend raw_tp_open with this pair as well to
attach existing fentry/fexit/freplace prog to another target.
Internally the kernel verify that btf of current linked_prog
exactly matches to btf of another requested linked_prog and
if they match it will attach the same prog to two target programs (in case of freplace)
or two kernel functions (in case of fentry/fexit).

API-wise this was exactly what I had in mind as well.

perfect!

Apologies in advance if I've missed a way to do this, but
for fentry/fexit, if we could attach the same program to
multiple kernel functions, it would be great it we could
programmatically access the BTF func proto id for the
attached function (prog->aux->attach_btf_id I think?).

Yes! I pushed for adding this to the GET_LINK_INFO operation, but it
wasn't included the first time around; I still think it ought to be added.

Actually in general, getting the btf_id of the currently running
function for any type of BPF program would be good; e.g., for xdpdump we
want to attach to a running XDP program, but we need the btf_id of the
main function. And because the name can be truncated to BPF_OBJ_NAME_LEN
when returned from the kernel, we have to walk the BTF info for the
program, and basically guess...

-Toke