Re: [PATCH bpf-next v7 1/2] bpftool: Add recursive map dumping

[Quentin Monnet <qmo@kernel.org>]

2026-09-25 01:14 UTC+0900 ~ Tianyi Chen <hi@tychen.cc>
> Dumping a map-of-maps currently shows inner map IDs without their
> contents. Add a recursive keyword to map dump to include referenced
> inner maps, leaving the default output unchanged. Show selected maps
> followed by distinct inner maps, preserving plain and BTF formatting
> and using an array of map objects for JSON output.
> 
> Holding every discovered inner-map FD open would make descriptor use
> grow with the number of maps and could exhaust RLIMIT_NOFILE. Keep the
> selected map FDs open, queue distinct inner map IDs, and open, dump and
> close each queued map in turn. This needs only one additional map FD.
> The tradeoff is deferred ID resolution: concurrently removed inner
> maps can disappear before they are opened, so the dump is not atomic.
> 
> Report failure when an inner map cannot be opened, retain per-entry
> lookup errors, and close JSON containers before returning.
> 
> Link: https://github.com/libbpf/bpftool/issues/58
> Assisted-by: LLM
> Signed-off-by: Tianyi Chen <hi@tychen.cc>
> ---

> diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
> index 20d59eab09a1..ef47de7da200 100644
> --- a/tools/bpf/bpftool/map.c
> +++ b/tools/bpf/bpftool/map.c

> @@ -907,21 +960,37 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
>  	free(value);
>  	free(cpu_ids);
>  	free_map_kv_btf(btf);
> +	if (plain_btf_wtr)
> +		jsonw_destroy(&plain_btf_wtr);
>  
>  	return err;
>  }
>  
>  static int do_dump(int argc, char **argv)
>  {
> +	LIBBPF_OPTS(bpf_get_fd_by_id_opts, opts,
> +		    .open_flags = BPF_F_RDONLY,
> +	);
>  	json_writer_t *wtr = NULL, *btf_wtr = NULL;
>  	struct bpf_map_info info = {};
> +	struct map_dump_ctx ctx = {};
> +	bool recursive_dump = false;
>  	int nb_fds, i = 0;
>  	__u32 len = sizeof(info);
>  	int *fds = NULL;
>  	int err = -1;
> +	size_t j;
>  
> -	if (argc != 2)
> +	if (argc != 2 && argc != 3)
>  		usage();
> +	if (argc == 3) {
> +		if (!*argv[2] || !is_prefix(argv[2], "recursive")) {
> +			p_err("expected 'recursive', got: '%s'", argv[2]);
> +			return -1;
> +		}
> +		recursive_dump = true;
> +		argc--;
> +	}


If we put the keyword first, we'll get a confusing message:

    # bpftool map dump recursive id 1337
    Error: expected 'recursive', got: '1337'

Can we align it with what most other subcommands in bpftool do, please?

	if (!REQ_ARGS(2))
		return -1;

	fds = malloc(sizeof(int));
	if (!fds) {
		p_err("mem alloc failed");
		return -1;
	}
	nb_fds = map_parse_fds(&argc, &argv, &fds, BPF_F_RDONLY);
	if (nb_fds < 1)
		goto exit_free;

	while (argc > 0) {
		if (is_prefix(*argv, "recursive")) {
			NEXT_ARG();
			recursive_dump = true;
		} else {
			p_err("expected 'recursive', got: '%s'", *argv);
			goto exit_free;
		}
	}

The rest looks good, thank you.

Quentin