Re: [PATCH bpf-next v5 5/8] bpf: Update the struct_ops of a bpf_link.

Martin KaFai Lau
2023-03-08T22:22:21Z

On 3/7/23 4:50 PM, Kui-Feng Lee wrote:
> By improving the BPF_LINK_UPDATE command of bpf(), it should allow you
> to conveniently switch between different struct_ops on a single
> bpf_link. This would enable smoother transitions from one struct_ops
> to another.
>
> The struct_ops maps passing along with BPF_LINK_UPDATE should have the
> BPF_F_LINK flag.
>
> Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
> ---
> include/linux/bpf.h | 1 +
> include/uapi/linux/bpf.h | 8 ++++--
> kernel/bpf/bpf_struct_ops.c | 46 ++++++++++++++++++++++++++++++++++
> kernel/bpf/syscall.c | 43 ++++++++++++++++++++++++++++---
> tools/include/uapi/linux/bpf.h | 7 +++++-
> 5 files changed, 98 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index afca6c526fe4..29d555a82bad 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1470,6 +1470,7 @@ struct bpf_link_ops {
> void (*show_fdinfo)(const struct bpf_link *link, struct seq_file *seq);
> int (*fill_link_info)(const struct bpf_link *link,
> struct bpf_link_info *info);
> + int (*update_map)(struct bpf_link *link, struct bpf_map *new_map);
> };
>
> struct bpf_tramp_link {
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index f9fc7b8af3c4..edef9cf7d596 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1555,8 +1555,12 @@ union bpf_attr {
>
> struct { /* struct used by BPF_LINK_UPDATE command */
> __u32 link_fd; /* link fd */
> - /* new program fd to update link with */
> - __u32 new_prog_fd;
> + union {
> + /* new program fd to update link with */
> + __u32 new_prog_fd;
> + /* new struct_ops map fd to update link with */
> + __u32 new_map_fd;
> + };
> __u32 flags; /* extra flags */
> /* expected link's program fd; is specified only if
> * BPF_F_REPLACE flag is set in flags */
> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
> index 5a7e86cf67b5..79e663869e51 100644
> --- a/kernel/bpf/bpf_struct_ops.c
> +++ b/kernel/bpf/bpf_struct_ops.c
> @@ -775,10 +775,56 @@ static int bpf_struct_ops_map_link_fill_link_info(const struct bpf_link *link,
> return 0;
> }
>
> +static int bpf_struct_ops_map_link_update(struct bpf_link *link, struct bpf_map *new_map)
> +{
> + struct bpf_struct_ops_value *kvalue;
> + struct bpf_struct_ops_map *st_map, *old_st_map;
> + struct bpf_struct_ops_link *st_link;
> + struct bpf_map *old_map;
> + int err = 0;
> +
> + if (new_map->map_type != BPF_MAP_TYPE_STRUCT_OPS ||
> + !(new_map->map_flags & BPF_F_LINK))
> + return -EINVAL;
> +
> + mutex_lock(&update_mutex);
> +
> + st_link = container_of(link, struct bpf_struct_ops_link, link);
> +
> + /* The new and old struct_ops must be the same type. */
> + st_map = container_of(new_map, struct bpf_struct_ops_map, map);

nit. move the st_link and st_map init out of the lock.

> +
> + old_map = st_link->map;

rcu_dereference_protected(...)

> + old_st_map = container_of(old_map, struct bpf_struct_ops_map, map);
> + if (st_map->st_ops != old_st_map->st_ops ||
> + /* Pair with smp_store_release() during map_update */
> + smp_load_acquire(&st_map->kvalue.state) != BPF_STRUCT_OPS_STATE_READY) {

nit. test the smp_load_acquire(&st_map...) outside of the lock.
Do it together with the new_map checking at the beginning of the func.

> + err = -EINVAL;
> + goto err_out;
> + }
> +
> + kvalue = &st_map->kvalue;
> +
> + err = st_map->st_ops->update(kvalue->data, old_st_map->kvalue.data);
> + if (err)
> + goto err_out;
> +
> + bpf_map_inc(new_map);
> + rcu_assign_pointer(st_link->map, new_map);
> +
> + bpf_map_put(old_map);
> +
> +err_out:
> + mutex_unlock(&update_mutex);
> +
> + return err;
> +}
> +
> static const struct bpf_link_ops bpf_struct_ops_map_lops = {
> .dealloc = bpf_struct_ops_map_link_dealloc,
> .show_fdinfo = bpf_struct_ops_map_link_show_fdinfo,
> .fill_link_info = bpf_struct_ops_map_link_fill_link_info,
> + .update_map = bpf_struct_ops_map_link_update,
> };
>
> int bpf_struct_ops_link_create(union bpf_attr *attr)
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 3a4503987a48..c087dd2e2c08 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -4658,6 +4658,30 @@ static int link_create(union bpf_attr *attr, bpfptr_t uattr)
> return ret;
> }
>
> +static int link_update_map(struct bpf_link *link, union bpf_attr *attr)
> +{
> + struct bpf_map *new_map;
> + int ret = 0;
> +
> + new_map = bpf_map_get(attr->link_update.new_map_fd);
> + if (IS_ERR(new_map))
> + return -EINVAL;
> +
> + if (new_map->map_type != BPF_MAP_TYPE_STRUCT_OPS) {

This is unnecessary test. The individual '.update_map()' should test for its own
map_type and the new bpf_struct_ops_map_link_update() does test it.

> + ret = -EINVAL;
> + goto out_put_map;
> + }
> +
> + if (link->ops->update_map)

This has just been tested in link_update() before calling link_update_map().

> + ret = link->ops->update_map(link, new_map);
> + else
> + ret = -EINVAL;
> +
> +out_put_map:
> + bpf_map_put(new_map);
> + return ret;
> +}
> +
> #define BPF_LINK_UPDATE_LAST_FIELD link_update.old_prog_fd
>
> static int link_update(union bpf_attr *attr)
> @@ -4670,14 +4694,25 @@ static int link_update(union bpf_attr *attr)
> if (CHECK_ATTR(BPF_LINK_UPDATE))
> return -EINVAL;
>
> - flags = attr->link_update.flags;
> - if (flags & ~BPF_F_REPLACE)
> - return -EINVAL;
> -
> link = bpf_link_get_from_fd(attr->link_update.link_fd);
> if (IS_ERR(link))
> return PTR_ERR(link);
>
> + flags = attr->link_update.flags;
> +
> + if (link->ops->update_map) {
> + if (flags) /* always replace the existing one */
> + ret = -EINVAL;
> + else
> + ret = link_update_map(link, attr);
> + goto out_put_link;
> + }
> +
> + if (flags & ~BPF_F_REPLACE) {
> + ret = -EINVAL;
> + goto out_put_link;
> + }
> +
> new_prog = bpf_prog_get(attr->link_update.new_prog_fd);
> if (IS_ERR(new_prog)) {
> ret = PTR_ERR(new_prog);