netdev - Re: [PATCH bpf-next v2 1/8] libbpf: Add a helper for retrieving a map fd for a given name

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Date:   Wed, 23 Jan 2019 11:54:41 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Maciej Fijalkowski <maciejromanfijalkowski@...il.com>,
        ast@...nel.org
Cc:     netdev@...r.kernel.org, jakub.kicinski@...ronome.com,
        brouer@...hat.com
Subject: Re: [PATCH bpf-next v2 1/8] libbpf: Add a helper for retrieving a map
 fd for a given name

On 01/21/2019 10:10 AM, Maciej Fijalkowski wrote:
> XDP samples are mostly cooperating with eBPF maps through their file
> descriptors. In case of a eBPF program that contains multiple maps it
> might be tiresome to iterate through them and call bpf_map__fd for each
> one. Add a helper mostly based on bpf_object__find_map_by_name, but
> instead of returning the struct bpf_map pointer, return map fd.
> 
> Bump libbpf ABI version to 0.0.2.
> 
> Suggested-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> Signed-off-by: Maciej Fijalkowski <maciejromanfijalkowski@...il.com>
> Reviewed-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> ---
>  tools/lib/bpf/libbpf.c   | 12 ++++++++++++
>  tools/lib/bpf/libbpf.h   |  3 +++
>  tools/lib/bpf/libbpf.map |  4 ++++
>  3 files changed, 19 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 169e347c76f6..dc838bea403f 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -2840,6 +2840,18 @@ bpf_object__find_map_by_name(struct bpf_object *obj, const char *name)
>  	return NULL;
>  }

Application could just do: bpf_map__fd(bpf_object__find_map_by_name(...)) or
bpf_object__find_map_by_name(...)->fd as both are exposed via library, though
I guess it may be okay to have a helper for it as it feels this might be needed
in many cases.

> +int
> +bpf_object__find_map_fd_by_name(struct bpf_object *obj, const char *name)
> +{
> +	struct bpf_map *pos;
> +
> +	bpf_map__for_each(pos, obj) {
> +		if (pos->name && !strcmp(pos->name, name))
> +			return bpf_map__fd(pos);
> +	}
> +	return -ENOENT;

Can we instead just do:

int
bpf_object__find_map_fd_by_name(struct bpf_object *obj, const char *name)
{
        return bpf_map__fd(bpf_object__find_map_by_name(obj, name));
}

> +}
> +
>  struct bpf_map *
>  bpf_object__find_map_by_offset(struct bpf_object *obj, size_t offset)
>  {
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 5f68d7b75215..7f10d36abdde 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -264,6 +264,9 @@ struct bpf_map;
>  LIBBPF_API struct bpf_map *
>  bpf_object__find_map_by_name(struct bpf_object *obj, const char *name);
>  
> +LIBBPF_API int
> +bpf_object__find_map_fd_by_name(struct bpf_object *obj, const char *name);
> +
>  /*
>   * Get bpf_map through the offset of corresponding struct bpf_map_def
>   * in the BPF object file.
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index cd02cd4e2cc3..7c59e4f64082 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -124,3 +124,7 @@ LIBBPF_0.0.1 {
>  	local:
>  		*;
>  };
> +LIBBPF_0.0.2 {
> +	global:
> +		bpf_object__find_map_fd_by_name;
> +} LIBBPF_0.0.1;
>