netdev - Re: [PATCH libbpf] libbpf: check if pin

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Date:   Fri, 2 Oct 2020 13:49:36 +0200
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     Hangbin Liu <liuhangbin@...il.com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>, netdev@...r.kernel.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: [PATCH libbpf] libbpf: check if pin_path was set even map fd
 exist

On Fri, Oct 02, 2020 at 03:57:50PM +0800, Hangbin Liu wrote:
> Say a user reuse map fd after creating a map manually and set the
> pin_path, then load the object via libbpf.
> 
> In libbpf bpf_object__create_maps(), bpf_object__reuse_map() will
> return 0 if there is no pinned map in map->pin_path. Then after
> checking if map fd exist, we should also check if pin_path was set
> and do bpf_map__pin() instead of continue the loop.
> 
> Fix it by creating map if fd not exist and continue checking pin_path
> after that.
> 
> Suggested-by: Andrii Nakryiko <andrii.nakryiko@...il.com>
> Signed-off-by: Hangbin Liu <liuhangbin@...il.com>
> ---
>  tools/lib/bpf/libbpf.c | 75 +++++++++++++++++++++---------------------
>  1 file changed, 37 insertions(+), 38 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index e493d6048143..d4149585a76c 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -3861,50 +3861,49 @@ bpf_object__create_maps(struct bpf_object *obj)
>  			}
>  		}
>  
> -		if (map->fd >= 0) {
> -			pr_debug("map '%s': skipping creation (preset fd=%d)\n",
> -				 map->name, map->fd);
> -			continue;
> -		}
> -
> -		err = bpf_object__create_map(obj, map);
> -		if (err)
> -			goto err_out;
> -
> -		pr_debug("map '%s': created successfully, fd=%d\n", map->name,
> -			 map->fd);
> -
> -		if (bpf_map__is_internal(map)) {
> -			err = bpf_object__populate_internal_map(obj, map);
> -			if (err < 0) {
> -				zclose(map->fd);
> +		if (map->fd < 0) {
> +			err = bpf_object__create_map(obj, map);
> +			if (err)
>  				goto err_out;
> -			}
> -		}
> -
> -		if (map->init_slots_sz) {
> -			for (j = 0; j < map->init_slots_sz; j++) {
> -				const struct bpf_map *targ_map;
> -				int fd;
>  
> -				if (!map->init_slots[j])
> -					continue;
> +			pr_debug("map '%s': created successfully, fd=%d\n", map->name,
> +				 map->fd);
>  
> -				targ_map = map->init_slots[j];
> -				fd = bpf_map__fd(targ_map);
> -				err = bpf_map_update_elem(map->fd, &j, &fd, 0);
> -				if (err) {
> -					err = -errno;
> -					pr_warn("map '%s': failed to initialize slot [%d] to map '%s' fd=%d: %d\n",
> -						map->name, j, targ_map->name,
> -						fd, err);
> +			if (bpf_map__is_internal(map)) {
> +				err = bpf_object__populate_internal_map(obj, map);
> +				if (err < 0) {
> +					zclose(map->fd);
>  					goto err_out;
>  				}
> -				pr_debug("map '%s': slot [%d] set to map '%s' fd=%d\n",
> -					 map->name, j, targ_map->name, fd);
>  			}
> -			zfree(&map->init_slots);
> -			map->init_slots_sz = 0;
> +
> +			if (map->init_slots_sz) {

Couldn't we flatten the code by inverting the logic here and using goto?

	if (!map->init_slot_sz) {
		pr_debug("map '%s': skipping creation (preset fd=%d)\n",
			 map->name, map->fd);
		goto map_pin;
	}

	(...)
map_pin:
	if (map->pin_path && !map->pinned) {

If I'm reading this right.

> +				for (j = 0; j < map->init_slots_sz; j++) {
> +					const struct bpf_map *targ_map;
> +					int fd;
> +
> +					if (!map->init_slots[j])
> +						continue;
> +
> +					targ_map = map->init_slots[j];
> +					fd = bpf_map__fd(targ_map);
> +					err = bpf_map_update_elem(map->fd, &j, &fd, 0);
> +					if (err) {
> +						err = -errno;
> +						pr_warn("map '%s': failed to initialize slot [%d] to map '%s' fd=%d: %d\n",
> +							map->name, j, targ_map->name,
> +							fd, err);
> +						goto err_out;
> +					}
> +					pr_debug("map '%s': slot [%d] set to map '%s' fd=%d\n",
> +						map->name, j, targ_map->name, fd);
> +				}
> +				zfree(&map->init_slots);
> +				map->init_slots_sz = 0;
> +			}
> +		} else {
> +			pr_debug("map '%s': skipping creation (preset fd=%d)\n",
> +				 map->name, map->fd);
>  		}
>  
>  		if (map->pin_path && !map->pinned) {
> -- 
> 2.25.4
>