netdev - Re: [PATCH bpf-next v2 2/8] libbpf: Add a helper for retrieving a prog via index

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Date:   Wed, 23 Jan 2019 11:41:11 +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 2/8] libbpf: Add a helper for retrieving a
 prog via index

On 01/21/2019 10:10 AM, Maciej Fijalkowski wrote:
> xdp_redirect_cpu has a 6 different XDP programs that can be attached to
> network interface. This sample has a option --prognum that allows user
> for specifying which particular program from a given set will be
> attached to network interface.
> In order to make it easier when converting the mentioned sample to
> libbpf usage, add a function to libbpf that will return program's fd for
> a given index.
> 
> Note that there is already a bpf_object__find_prog_by_idx, which could
> be exported and might be used for that purpose, but it operates on the
> number of ELF section and here we need an index from a programs array
> within the bpf_object.

Series in general looks good to me. Few minor comments, mainly in relation
to the need for libbpf extensions.

Would it not be a better interface to the user to instead choose the prog
based on section name and then retrieve it via bpf_object__find_program_by_title()
instead of prognum (which feels less user friendly) at least?

> Signed-off-by: Maciej Fijalkowski <maciejromanfijalkowski@...il.com>
> Reviewed-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> ---
>  tools/lib/bpf/libbpf.c   | 8 ++++++++
>  tools/lib/bpf/libbpf.h   | 3 +++
>  tools/lib/bpf/libbpf.map | 1 +
>  3 files changed, 12 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index dc838bea403f..21c84d0f6128 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -935,6 +935,14 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
>  	return err;
>  }
>  
> +int
> +bpf_object__get_prog_fd_by_num(struct bpf_object *obj, int idx)
> +{
> +	if (idx >= 0 && idx < obj->nr_programs)
> +		return bpf_program__fd(&obj->programs[idx]);
> +	return -ENOENT;
> +}
> +
>  static struct bpf_program *
>  bpf_object__find_prog_by_idx(struct bpf_object *obj, int idx)
>  {
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 7f10d36abdde..ca1b381cb3ad 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -95,6 +95,9 @@ LIBBPF_API int bpf_object__btf_fd(const struct bpf_object *obj);
>  LIBBPF_API struct bpf_program *
>  bpf_object__find_program_by_title(struct bpf_object *obj, const char *title);
>  
> +LIBBPF_API int
> +bpf_object__get_prog_fd_by_num(struct bpf_object *obj, int idx);
> +
>  LIBBPF_API struct bpf_object *bpf_object__next(struct bpf_object *prev);
>  #define bpf_object__for_each_safe(pos, tmp)			\
>  	for ((pos) = bpf_object__next(NULL),		\
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 7c59e4f64082..871d2fc07150 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -127,4 +127,5 @@ LIBBPF_0.0.1 {
>  LIBBPF_0.0.2 {
>  	global:
>  		bpf_object__find_map_fd_by_name;
> +		bpf_object__get_prog_fd_by_num;
>  } LIBBPF_0.0.1;
>