Re: [PATCH bpf-next v5 6/8] libbpf: Update a bpf_link with another struct_ops.

[Martin KaFai Lau <martin.lau@linux.dev>]

On 3/7/23 4:50 PM, Kui-Feng Lee wrote:
> Introduce bpf_link__update_struct_ops(), which will allow you to
> effortlessly transition the struct_ops map of any given bpf_link into
> an alternative.
> 
> Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
> ---
>   tools/lib/bpf/libbpf.c   | 36 ++++++++++++++++++++++++++++++++++++
>   tools/lib/bpf/libbpf.h   |  1 +
>   tools/lib/bpf/libbpf.map |  1 +
>   3 files changed, 38 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index f70b55c0f40e..0406d0e00e1f 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -11657,6 +11657,42 @@ struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map)
>   	return &link->link;
>   }
>   
> +/*
> + * Swap the back struct_ops of a link with a new struct_ops map.
> + */
> +int bpf_link__update_map(struct bpf_link *link, const struct bpf_map *map)
> +{
> +	struct bpf_link_struct_ops *st_ops_link;
> +	__u32 zero = 0;
> +	int err, fd;
> +
> +	if (!bpf_map__is_struct_ops(map) || map->fd == -1)
> +		return -EINVAL;
> +
> +	st_ops_link = container_of(link, struct bpf_link_struct_ops, link);
> +	/* Ensure the type of a link is correct */
> +	if (st_ops_link->map_fd < 0)
> +		return -EINVAL;
> +
> +	err = bpf_map_update_elem(map->fd, &zero, map->st_ops->kern_vdata, 0);
> +	if (err && errno != EBUSY) {
> +		err = -errno;
> +		free(link);

hmm... this looks wrong to free here.

> +		return err;
> +	}
> +
> +	fd = bpf_link_update(link->fd, map->fd, NULL);
> +	if (fd < 0) {
> +		err = -errno;
> +		free(link);

same here.

> +		return err;
> +	}
> +
> +	st_ops_link->map_fd = map->fd;
> +
> +	return 0;
> +}
> +
>   typedef enum bpf_perf_event_ret (*bpf_perf_event_print_t)(struct perf_event_header *hdr,
>   							  void *private_data);
>   
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index db4992a036f8..1615e55e2e79 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -719,6 +719,7 @@ bpf_program__attach_freplace(const struct bpf_program *prog,
>   struct bpf_map;
>   
>   LIBBPF_API struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map);
> +LIBBPF_API int bpf_link__update_map(struct bpf_link *link, const struct bpf_map *map);
>   
>   struct bpf_iter_attach_opts {
>   	size_t sz; /* size of this struct for forward/backward compatibility */
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 50dde1f6521e..cc05be376257 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -387,6 +387,7 @@ LIBBPF_1.2.0 {
>   	global:
>   		bpf_btf_get_info_by_fd;
>   		bpf_link_get_info_by_fd;
> +		bpf_link__update_map;
>   		bpf_map_get_info_by_fd;
>   		bpf_prog_get_info_by_fd;
>   } LIBBPF_1.1.0;