Re: [PATCH bpf-next v7 5/8] bpf: Update the struct_ops of a bpf_link.

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

On 3/15/23 7:36 PM, Kui-Feng Lee wrote:
> +static int bpf_struct_ops_map_link_update(struct bpf_link *link, struct bpf_map *new_map)
> +{
> +	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;
> +
> +	st_link = container_of(link, struct bpf_struct_ops_link, link);
> +	st_map = container_of(new_map, struct bpf_struct_ops_map, map);
> +
> +	if (!bpf_struct_ops_valid_to_reg(new_map))
> +		return -EINVAL;
> +
> +	mutex_lock(&update_mutex);
> +
> +	old_map = rcu_dereference_protected(st_link->map, lockdep_is_held(&update_mutex));
> +	old_st_map = container_of(old_map, struct bpf_struct_ops_map, map);
> +	/* The new and old struct_ops must be the same type. */
> +	if (st_map->st_ops != old_st_map->st_ops) {
> +		err = -EINVAL;
> +		goto err_out;
> +	}
> +
> +	err = st_map->st_ops->update(st_map->kvalue.data, old_st_map->kvalue.data);

I don't think it has completely addressed Andrii's comment in v4 regarding 
BPF_F_REPLACE: 
https://lore.kernel.org/bpf/CAEf4BzbK8s+VFG5HefydD7CRLzkRFKg-Er0PKV_-C2-yttfXzA@mail.gmail.com/

For now, tcp_update_congestion_control() enforces the same cc-name. However, it 
is still not the same as what BPF_F_REPLACE intented to do: update only when it 
is the same old-map. Same cc-name does not necessarily mean the same old-map.

> +	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 5a45e3bf34e2..6fa10d108278 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -4676,6 +4676,21 @@ 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;

nit. init zero is unnecessarily.

> +
> +	new_map = bpf_map_get(attr->link_update.new_map_fd);
> +	if (IS_ERR(new_map))
> +		return -EINVAL;
> +
> +	ret = link->ops->update_map(link, new_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)
> @@ -4696,6 +4711,11 @@ static int link_update(union bpf_attr *attr)
>   	if (IS_ERR(link))
>   		return PTR_ERR(link);
>   
> +	if (link->ops->update_map) {
> +		ret = link_update_map(link, attr);
> +		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);
> diff --git a/net/bpf/bpf_dummy_struct_ops.c b/net/bpf/bpf_dummy_struct_ops.c
> index ff4f89a2b02a..158f14e240d0 100644
> --- a/net/bpf/bpf_dummy_struct_ops.c
> +++ b/net/bpf/bpf_dummy_struct_ops.c
> @@ -222,12 +222,18 @@ static void bpf_dummy_unreg(void *kdata)
>   {
>   }
>   
> +static int bpf_dummy_update(void *kdata, void *old_kdata)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
>   struct bpf_struct_ops bpf_bpf_dummy_ops = {
>   	.verifier_ops = &bpf_dummy_verifier_ops,
>   	.init = bpf_dummy_init,
>   	.check_member = bpf_dummy_ops_check_member,
>   	.init_member = bpf_dummy_init_member,
>   	.reg = bpf_dummy_reg,
> +	.update = bpf_dummy_update,

When looking at this together in patch 5, the changes in bpf_dummy_struct_ops.c 
should not be needed.