Subject: RE: [PATCH bpf-next 3/8] libbpf: further decouple feature checking logic from bpf_object

From: John Fastabend <[email-masked]>
Date: Mon, 11 Dec 2023 13:41:00 -0800

Andrii Nakryiko wrote:
> Add feat_supported() helper that accepts feature cache instead of
> bpf_object. This allows low-level code in bpf.c to not know or care
> about higher-level concept of bpf_object, yet it will be able to utilize
> custom feature checking in cases where BPF token might influence the
> outcome.
> 
> Signed-off-by: Andrii Nakryiko <[email-masked]>
> ---

...

> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index a6b8d6f70918..af5e777efcbd 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -637,6 +637,7 @@ struct elf_state {
>  };
>  
>  struct usdt_manager;
> +struct kern_feature_cache;
>  
>  struct bpf_object {
>  	char name[BPF_OBJ_NAME_LEN];
> @@ -5063,17 +5064,14 @@ static struct kern_feature_desc {
>  	},
>  };
>  
> -bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id)
> +bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)
>  {
>  	struct kern_feature_desc *feat = &feature_probes[feat_id];
> -	struct kern_feature_cache *cache = &feature_cache;
>  	int ret;
>  
> -	if (obj && obj->gen_loader)
> -		/* To generate loader program assume the latest kernel
> -		 * to avoid doing extra prog_load, map_create syscalls.
> -		 */
> -		return true;
> +	/* assume global feature cache, unless custom one is provided */
> +	if (!cache)
> +		cache = &feature_cache;

Why expose a custom cache at all? Where would that be used? I guess we are
looking at libbpf_internal APIs so maybe not a big deal.

>  
>  	if (READ_ONCE(cache->res[feat_id]) == FEAT_UNKNOWN) {
>  		ret = feat->probe();
> @@ -5090,6 +5088,17 @@ bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id)
>  	return READ_ONCE(cache->res[feat_id]) == FEAT_SUPPORTED;
>  }

Acked-by: John Fastabend <[email-masked]>