netdev - Re: [PATCH v3 bpf-next 1/2] bpf: adding support for map in map in libbpf

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Date:   Tue, 20 Nov 2018 15:18:47 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Nikita V. Shirokov" <tehnerd@...nerd.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH v3 bpf-next 1/2] bpf: adding support for map in map in
 libbpf

On Mon, Nov 19, 2018 at 10:42:21PM -0800, Nikita V. Shirokov wrote:
> idea is pretty simple. for specified map (pointed by struct bpf_map)
> we would provide descriptor of already loaded map, which is going to be
> used as a prototype for inner map. proposed workflow:
> 1) open bpf's object (bpf_object__open)
> 2) create bpf's map which is going to be used as a prototype
> 3) find (by name) map-in-map which you want to load and update w/
> descriptor of inner map w/ a new helper from this patch
> 4) load bpf program w/ bpf_object__load
> 
> inner_map_fd is ignored by any other maps aside from (hash|array) of
> maps
> 
> Signed-off-by: Nikita V. Shirokov <tehnerd@...nerd.com>
> Acked-by: Yonghong Song <yhs@...com>
> ---
>  tools/lib/bpf/libbpf.c | 11 ++++++++++-
>  tools/lib/bpf/libbpf.h |  2 ++
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index a01eb9584e52..7e130e0c8fc9 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -163,6 +163,7 @@ struct bpf_map {
>  	char *name;
>  	size_t offset;
>  	int map_ifindex;
> +	int inner_map_fd;
>  	struct bpf_map_def def;
>  	__u32 btf_key_type_id;
>  	__u32 btf_value_type_id;
> @@ -653,8 +654,10 @@ bpf_object__init_maps(struct bpf_object *obj, int flags)
>  	 * fd (fd=0 is stdin) when failure (zclose won't close
>  	 * negative fd)).
>  	 */
> -	for (i = 0; i < nr_maps; i++)
> +	for (i = 0; i < nr_maps; i++) {
>  		obj->maps[i].fd = -1;
> +		obj->maps[i].inner_map_fd = -1;
> +	}
>  
>  	/*
>  	 * Fill obj->maps using data in "maps" section.
> @@ -1146,6 +1149,7 @@ bpf_object__create_maps(struct bpf_object *obj)
>  		create_attr.btf_fd = 0;
>  		create_attr.btf_key_type_id = 0;
>  		create_attr.btf_value_type_id = 0;
> +		create_attr.inner_map_fd = map->inner_map_fd;
>  
>  		if (obj->btf && !bpf_map_find_btf_info(map, obj->btf)) {
>  			create_attr.btf_fd = btf__fd(obj->btf);
> @@ -2562,6 +2566,11 @@ void bpf_map__set_ifindex(struct bpf_map *map, __u32 ifindex)
>  	map->map_ifindex = ifindex;
>  }
>  
> +void bpf_map__add_inner_map_fd(struct bpf_map *map, int fd)
> +{
> +	map->inner_map_fd = fd;

I think the name bpf_map__set_inner_map_fd() would be more appropriate
and it should check that map->def->type == map-in-map && map->inner_map_fd == -1
before assigning new one.

Also the behavior of bpf_object__create_maps() is not great.
If nothing is set the function will be passing
create_attr.inner_map_fd == -1 to the kernel.
For regular maps that field is sadly ignored by kernel.
Only for map-in-map the value of -1 will be triggering map_create error.
Imo bpf_object__create_maps() should be doing:
if (create_attr.map_type == map-in-map && map->inner_map_fd >= 0)
 create_attr.inner_map_fd = map->inner_map_fd;
// otherwise keep it zero inited