netdev - Re: [PATCH bpf-next] libbpf: try to add a name for bpftool self-created maps

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Date:   Mon, 8 Aug 2022 14:45:03 +0100
From:   Quentin Monnet <quentin@...valent.com>
To:     Hangbin Liu <liuhangbin@...il.com>, netdev@...r.kernel.org
Cc:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next] libbpf: try to add a name for bpftool
 self-created maps

On 08/08/2022 10:33, Hangbin Liu wrote:
> As discussed before[1], the bpftool self-created maps can appear in final
> map show output due to deferred removal in kernel. These maps don't have
> a name, which would make users confused about where it comes from.
> 
> Adding names for these maps could make users know what these maps used for.
> It also could make some tests (like test_offload.py, which skip base maps
> without names as a workaround) filter them out.
> 
> As Quentin suggested, add a small wrapper to fall back with no name
> if kernel is not supported.
> 
> [1] https://lore.kernel.org/bpf/CAEf4BzY66WPKQbDe74AKZ6nFtZjq5e+G3Ji2egcVytB9R6_sGQ@mail.gmail.com/
> 
> Suggested-by: Quentin Monnet <quentin@...valent.com>
> Signed-off-by: Hangbin Liu <liuhangbin@...il.com>
> ---
>  tools/lib/bpf/libbpf.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 77e3797cf75a..db4f1a02b9e0 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4423,6 +4423,22 @@ static int probe_kern_prog_name(void)
>  	return probe_fd(ret);
>  }
>  
> +static int probe_kern_map_name(enum bpf_map_type map_type,
> +			       const char *map_name, __u32 key_size,
> +			       __u32 value_size, __u32 max_entries,
> +			       const struct bpf_map_create_opts *opts)
> +{
> +	int map;
> +
> +	map = bpf_map_create(map_type, map_name, key_size, value_size, max_entries, opts);
> +	if (map < 0 && errno == EINVAL) {
> +		/* Retry without name */
> +		map = bpf_map_create(map_type, NULL, key_size, value_size, max_entries, opts);
> +	}
> +
> +	return map;
> +}
> +
>  static int probe_kern_global_data(void)
>  {
>  	char *cp, errmsg[STRERR_BUFSIZE];
> @@ -4434,7 +4450,7 @@ static int probe_kern_global_data(void)
>  	};
>  	int ret, map, insn_cnt = ARRAY_SIZE(insns);
>  
> -	map = bpf_map_create(BPF_MAP_TYPE_ARRAY, NULL, sizeof(int), 32, 1, NULL);
> +	map = probe_kern_map_name(BPF_MAP_TYPE_ARRAY, "global_data", sizeof(int), 32, 1, NULL);

Thanks! Some comments on the naming: It reads strange here to "probe"
for the maps, given that we still need to compare the return value
below. Maybe use something else instead of "probe_kern_map_name()"?
Maybe "map_create_adjust_name()" or "map_create_compat()" (or something
else)?

Regarding "global_data": If the intent is to filter out these maps from
the output of bpftool for example, should we use a common prefix for the
three of them? "libbpf_" or "probe_"? Or even something shorter? I know
we're limited to 15 characters.

Quentin