On Tue, Mar 7, 2023 at 3:33 PM Kui-Feng Lee <kuifeng@xxxxxxxx> wrote:
>
> BPF struct_ops maps are employed directly to register TCP Congestion
> Control algorithms. Unlike other BPF programs that terminate when
> their links gone. The link of a BPF struct_ops map provides a uniform
> experience akin to other types of BPF programs.
>
> bpf_links are responsible for registering their associated
> struct_ops. You can only use a struct_ops that has the BPF_F_LINK flag
> set to create a bpf_link, while a structs without this flag behaves in
> the same manner as before and is registered upon updating its value.
>
> The BPF_LINK_TYPE_STRUCT_OPS serves a dual purpose. Not only is it
> used to craft the links for BPF struct_ops programs, but also to
> create links for BPF struct_ops them-self. Since the links of BPF
> struct_ops programs are only used to create trampolines internally,
> they are never seen in other contexts. Thus, they can be reused for
> struct_ops themself.
>
> To maintain a reference to the map supporting this link, we add
> bpf_struct_ops_link as an additional type. The pointer of the map is
> RCU and won't be necessary until later in the patchset.
>
> Signed-off-by: Kui-Feng Lee <kuifeng@xxxxxxxx>
> ---
> include/linux/bpf.h | 11 +++
> include/uapi/linux/bpf.h | 12 +++-
> kernel/bpf/bpf_struct_ops.c | 119 +++++++++++++++++++++++++++++++--
> kernel/bpf/syscall.c | 23 ++++---
> tools/include/uapi/linux/bpf.h | 12 +++-
> 5 files changed, 163 insertions(+), 14 deletions(-)
>
[...]
> +int bpf_struct_ops_link_create(union bpf_attr *attr)
> +{
> + struct bpf_struct_ops_link *link = NULL;
> + struct bpf_link_primer link_primer;
> + struct bpf_struct_ops_map *st_map;
> + struct bpf_map *map;
> + int err;
> +
> + map = bpf_map_get(attr->link_create.map_fd);
> + if (!map)
> + return -EINVAL;
> +
> + st_map = (struct bpf_struct_ops_map *)map;
> +
> + if (map->map_type != BPF_MAP_TYPE_STRUCT_OPS || !(map->map_flags & BPF_F_LINK) ||
> + /* Pair with smp_store_release() during map_update */
> + smp_load_acquire(&st_map->kvalue.state) != BPF_STRUCT_OPS_STATE_READY) {
> + err = -EINVAL;
> + goto err_out;
> + }
> +
> + link = kzalloc(sizeof(*link), GFP_USER);
> + if (!link) {
> + err = -ENOMEM;
> + goto err_out;
> + }
> + bpf_link_init(&link->link, BPF_LINK_TYPE_STRUCT_OPS, &bpf_struct_ops_map_lops, NULL);
> + RCU_INIT_POINTER(link->map, map);
> +
> + err = bpf_link_prime(&link->link, &link_primer);
> + if (err)
> + goto err_out;
> +
> + err = st_map->st_ops->reg(st_map->kvalue.data);
> + if (err) {
> + bpf_link_cleanup(&link_primer);
link = NULL to avoid kfree()-ing it, see bpf_tracing_prog_attach() for
similar approach
> + goto err_out;
> + }
> +
> + return bpf_link_settle(&link_primer);
> +
> +err_out:
> + bpf_map_put(map);
> + kfree(link);
> + return err;
> +}
> +
[...]
