On Wed, Mar 15, 2023 at 7:37 PM Kui-Feng Lee <kuifeng@xxxxxxxx> wrote:
>
> bpf_map__attach_struct_ops() was creating a dummy bpf_link as a
> placeholder, but now it is constructing an authentic one by calling
> bpf_link_create() if the map has the BPF_F_LINK flag.
>
> You can flag a struct_ops map with BPF_F_LINK by calling
> bpf_map__set_map_flags().
>
> Signed-off-by: Kui-Feng Lee <kuifeng@xxxxxxxx>
> ---
> tools/lib/bpf/libbpf.c | 90 +++++++++++++++++++++++++++++++-----------
> 1 file changed, 66 insertions(+), 24 deletions(-)
>
[...]
> - if (!prog)
> - continue;
> + link->link.detach = bpf_link__detach_struct_ops;
>
> - prog_fd = bpf_program__fd(prog);
> - kern_data = st_ops->kern_vdata + st_ops->kern_func_off[i];
> - *(unsigned long *)kern_data = prog_fd;
> + if (!(map->def.map_flags & BPF_F_LINK)) {
> + /* w/o a real link */
> + link->link.fd = map->fd;
> + link->map_fd = -1;
> + return &link->link;
> }
>
> - err = bpf_map_update_elem(map->fd, &zero, st_ops->kern_vdata, 0);
> - if (err) {
> - err = -errno;
> + fd = bpf_link_create(map->fd, -1, BPF_STRUCT_OPS, NULL);
pass 0, not -1. BPF APIs have a convention that fd=0 means "no fd was
provided". And actually kernel should have rejected this -1, so please
check why that didn't happen in your testing, we might be missing some
kernel validation.
> + if (fd < 0) {
> free(link);
> - return libbpf_err_ptr(err);
> + return libbpf_err_ptr(fd);
> }
>
> - link->detach = bpf_link__detach_struct_ops;
> - link->fd = map->fd;
> + link->link.fd = fd;
> + link->map_fd = map->fd;
>
> - return link;
> + return &link->link;
> }
>
> typedef enum bpf_perf_event_ret (*bpf_perf_event_print_t)(struct perf_event_header *hdr,
> --
> 2.34.1
>
