Re: [PATCH bpf-next v3] bpftool: Dump map id instead of value for map_of_maps types

[Yonghong Song <yhs@meta.com>]

On 4/26/23 9:19 PM, Xueming Feng wrote:
> When using `bpftool map dump` with map_of_maps, it is usually
> more convenient to show the inner map id instead of raw value.
> 
> We are changing the plain print behavior to show inner_map_id
> instead of hex value, this would help with quick look up of
> inner map with `bpftool map dump id <inner_map_id>`.
> To avoid disrupting scripted behavior, we will add a new
> `inner_map_id` field to json output instead of replacing value.
> 
> plain print:
> ```
> $ bpftool map dump id 138
> 
> Without Patch:
> key:
> fc 00 00 00 00 00 00 00  00 00 00 00 00 00 00 05
> 27 16 06 00
> value:
> 8b 00 00 00
> Found 1 element
> 
> With Patch:
> key:
> fc 00 00 00 00 00 00 00  00 00 00 00 00 00 00 05
> 27 16 06 00
> inner_map_id:
> 139
> Found 1 element
> ```
> 
> json print:
> ```
> $ bpftool -p map dump id 567
> 
> Without Patch:
> [{
>          "key": ["0xc0","0x00","0x02","0x05","0x27","0x16","0x06","0x00"
>          ],
>          "value": ["0x38","0x02","0x00","0x00"
>          ]
>      }
> ]
> 
> With Patch:
> [{
>          "key": ["0xc0","0x00","0x02","0x05","0x27","0x16","0x06","0x00"
>          ],
>          "value": ["0x38","0x02","0x00","0x00"
>          ],
>          "inner_map_id": 568
>      }
> ]
> ```
> 
> Signed-off-by: Xueming Feng <kuro@kuroa.me>

Ack with a small nit below.

Acked-by: Yonghong Song <yhs@fb.com>

> ---
> 
> Changes in v3:
>   - In plain print, use printf() directly since inner map id is always a 32bit int.
>   - Remove unused print_uint() function.
>   - Rename `id` to `inner_map_id` in plain print output for clearness.
>   - Add a new `inner_map_id` field to json output.
>   - Add example output to commit message.
> 
> Changes in v2:
>   - Fix commit message grammar.
>   - Change `print_uint` to only print to stdout, make `arg` const,
>     and rename `n` to `arg_size`.
>   - Make `print_uint` able to take any size of argument up to `unsigned long`,
>     and print it as unsigned decimal.
> 
> Sorry for taking this long, my compiling machine is acting weird. And thanks for
> the reviews and suggestions!
> 
>   tools/bpf/bpftool/map.c | 13 +++++++++++--
>   1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
> index aaeb8939e137..afcd8455db74 100644
> --- a/tools/bpf/bpftool/map.c
> +++ b/tools/bpf/bpftool/map.c
> @@ -139,6 +139,10 @@ static void print_entry_json(struct bpf_map_info *info, unsigned char *key,
>   		print_hex_data_json(key, info->key_size);
>   		jsonw_name(json_wtr, "value");
>   		print_hex_data_json(value, info->value_size);
> +		if (map_is_map_of_maps(info->type)) {
> +			jsonw_uint_field(json_wtr, "inner_map_id",
> +					 *(unsigned int *)value);
> +		}

There is only statement under 'if' condition. The above
parenthesis ('{}') is not needed.

>   		if (btf) {
>   			struct btf_dumper d = {
>   				.btf = btf,
> @@ -259,8 +263,13 @@ static void print_entry_plain(struct bpf_map_info *info, unsigned char *key,
>   		}
>   
>   		if (info->value_size) {
> -			printf("value:%c", break_names ? '\n' : ' ');
> -			fprint_hex(stdout, value, info->value_size, " ");
> +			if (map_is_map_of_maps(info->type)) {
> +				printf("inner_map_id:%c", break_names ? '\n' : ' ');
> +				printf("%u ", *(unsigned int *)value);
> +			} else {
> +				printf("value:%c", break_names ? '\n' : ' ');
> +				fprint_hex(stdout, value, info->value_size, " ");
> +			}
>   		}
>   
>   		printf("\n");