On 3/7/23 16:37, Andrii Nakryiko wrote:
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
Got it! Thank you!
+ goto err_out;
+ }
+
+ return bpf_link_settle(&link_primer);
+
+err_out:
+ bpf_map_put(map);
+ kfree(link);
+ return err;
+}
+
[...]
