Re: [RFC PATCH v2 04/13] bpf: Define a few bpf_link_ops for BPF_TRACE_MAP

[Eric Dumazet <eric.dumazet@gmail.com>]

On 9/29/21 4:59 PM, Joe Burton wrote:
> From: Joe Burton <jevburton@google.com>
> 
> Define release, dealloc, and update_prog for the new tracing programs.
> Updates are protected by a single global mutex.
> 
> Signed-off-by: Joe Burton <jevburton@google.com>
> ---
>  kernel/bpf/map_trace.c | 71 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 71 insertions(+)
> 
> diff --git a/kernel/bpf/map_trace.c b/kernel/bpf/map_trace.c
> index 7776b8ccfe88..35906d59ba3c 100644
> --- a/kernel/bpf/map_trace.c
> +++ b/kernel/bpf/map_trace.c
> @@ -14,6 +14,14 @@ struct bpf_map_trace_target_info {
>  static struct list_head targets = LIST_HEAD_INIT(targets);
>  static DEFINE_MUTEX(targets_mutex);
>  
> +struct bpf_map_trace_link {
> +	struct bpf_link link;
> +	struct bpf_map *map;
> +	struct bpf_map_trace_target_info *tinfo;
> +};
> +
> +static DEFINE_MUTEX(link_mutex);
> +
>  int bpf_map_trace_reg_target(const struct bpf_map_trace_reg *reg_info)
>  {
>  	struct bpf_map_trace_target_info *tinfo;
> @@ -77,3 +85,66 @@ int bpf_map_initialize_trace_progs(struct bpf_map *map)
>  	return 0;
>  }
>  
> +static void bpf_map_trace_link_release(struct bpf_link *link)
> +{
> +	struct bpf_map_trace_link *map_trace_link =
> +			container_of(link, struct bpf_map_trace_link, link);
> +	enum bpf_map_trace_type trace_type =
> +			map_trace_link->tinfo->reg_info->trace_type;
> +	struct bpf_map_trace_prog *cur_prog;
> +	struct bpf_map_trace_progs *progs;
> +
> +	progs = map_trace_link->map->trace_progs;
> +	mutex_lock(&progs->mutex);
> +	list_for_each_entry(cur_prog, &progs->progs[trace_type].list, list) {

You might consider using list_for_each_entry_safe(), or ...

> +		if (cur_prog->prog == link->prog) {
> +			progs->length[trace_type] -= 1;
> +			list_del_rcu(&cur_prog->list);
> +			kfree_rcu(cur_prog, rcu);

or add a break; if you do not expect to find multiple entries.

> +		}
> +	}
> +	mutex_unlock(&progs->mutex);
> +	bpf_map_put_with_uref(map_trace_link->map);
> +}
> +
> +static void bpf_map_trace_link_dealloc(struct bpf_link *link)
> +{
> +	struct bpf_map_trace_link *map_trace_link =
> +			container_of(link, struct bpf_map_trace_link, link);
> +
> +	kfree(map_trace_link);
> +}
> +
> +static int bpf_map_trace_link_replace(struct bpf_link *link,
> +				      struct bpf_prog *new_prog,
> +				      struct bpf_prog *old_prog)
> +{
> +	int ret = 0;
> +
> +	mutex_lock(&link_mutex);
> +	if (old_prog && link->prog != old_prog) {
> +		ret = -EPERM;
> +		goto out_unlock;
> +	}
> +
> +	if (link->prog->type != new_prog->type ||
> +	    link->prog->expected_attach_type != new_prog->expected_attach_type ||
> +	    link->prog->aux->attach_btf_id != new_prog->aux->attach_btf_id) {
> +		ret = -EINVAL;
> +		goto out_unlock;
> +	}
> +
> +	old_prog = xchg(&link->prog, new_prog);
> +	bpf_prog_put(old_prog);
> +
> +out_unlock:
> +	mutex_unlock(&link_mutex);
> +	return ret;
> +}
> +
> +static const struct bpf_link_ops bpf_map_trace_link_ops = {
> +	.release = bpf_map_trace_link_release,
> +	.dealloc = bpf_map_trace_link_dealloc,
> +	.update_prog = bpf_map_trace_link_replace,
> +};
> +
>