Re: [PATCH bpf-next v2 1/2] bpftool: Fail map dump for maps that can't be iterated

[Quentin Monnet <qmo@kernel.org>]

2026-09-25 19:20 UTC+0300 ~ Maxim Skokov <skokovmaksimevg@gmail.com>
> "bpftool map dump" walks the map with bpf_map_get_next_key() and treats
> ENOENT as the end of the map. Map types that can't be iterated fail that
> call on the very first key: local storage and ringbuf with ENOTSUPP,
> bloom filter and arena with EOPNOTSUPP, queue and stack with EINVAL.
> bpftool then ends the walk without a word and prints what it prints for
> an empty map, "[]" with BTF or "Found 0 elements" without, and only the
> exit status tells that the map was not read. On a socket storage map
> that holds storage for a socket:
> 
>   # bpftool map dump pinned /sys/fs/bpf/sk_storage
>   []
> 
> Ask for the first key before printing anything, and when that fails with
> anything but ENOENT, fail with the map type and the error instead of a
> dump:
> 
>   # bpftool map dump pinned /sys/fs/bpf/sk_storage
>   Error: can't dump sk_storage map: Unknown error 524
>   # bpftool -j map dump pinned /sys/fs/bpf/sk_storage
>   {"error":"can't dump sk_storage map: Unknown error 524"}
> 
> An error later in the walk is now reported too, as "bpftool map getnext"
> reports it, and the element count is printed only after a complete walk.
> 
> Assisted-by: LLM
> Signed-off-by: Maxim Skokov <skokovmaksimevg@gmail.com>
> ---
>  tools/bpf/bpftool/map.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
> index 20d59eab09..626882f207 100644
> --- a/tools/bpf/bpftool/map.c
> +++ b/tools/bpf/bpftool/map.c
> @@ -856,6 +856,25 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
>  		}
>  	}
>  
> +	/*
> +	 * Map types that can't be iterated fail on the very first key. Say so
> +	 * before printing anything, so that the output isn't an empty dump.
> +	 */


Or simply:

  /* Fail early to avoid an empty dump if map type cannot be iterated */

But that's a minor nit, the patch looks good to me, thank you!

Acked-by: Quentin Monnet <qmo@kernel.org>

Looks like you need to address Sashiko's findings on patch 2, though.


> +	if (bpf_map_get_next_key(fd, NULL, key) && errno != ENOENT) {
> +		const char *map_type_str;
> +		int saved_errno = errno;
> +
> +		map_type_str = libbpf_bpf_map_type_str(info->type);
> +		if (map_type_str)
> +			p_err("can't dump %s map: %s", map_type_str,
> +			      strerror(saved_errno));
> +		else
> +			p_err("can't dump map of type %u: %s", info->type,
> +			      strerror(saved_errno));
> +		err = -1;
> +		goto exit_free;
> +	}


Tianyi, note that this conflicts with your series ("bpftool: Add
recursive map dumping"), one of you will have to rebase.


> +
>  	if (wtr) {
>  		err = get_map_kv_btf(info, &btf);
>  		if (err) {
> @@ -883,8 +902,11 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
>  	while (true) {
>  		err = bpf_map_get_next_key(fd, prev_key, key);
>  		if (err) {
> -			if (errno == ENOENT)
> +			if (errno == ENOENT) {
>  				err = 0;
> +				break;
> +			}
> +			p_err("can't get next key: %s", strerror(errno));
>  			break;
>  		}
>  		if (!dump_map_elem(fd, key, value, info, btf, wtr,
> @@ -897,7 +919,7 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
>  		jsonw_end_array(wtr);	/* elements */
>  		if (show_header)
>  			jsonw_end_object(wtr);	/* map object */
> -	} else {
> +	} else if (!err) {
>  		printf("Found %u element%s\n", num_elems,
>  		       num_elems != 1 ? "s" : "");
>  	}