linux-kernel - Re: [PATCH bpf-next v2] bpftool: Dump map id instead of value for map_of

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Date:   Mon, 24 Apr 2023 18:07:27 -0700
From:   Yonghong Song <yhs@...a.com>
To:     Xueming Feng <kuro@...oa.me>,
        Quentin Monnet <quentin@...valent.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        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, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2] bpftool: Dump map id instead of value for
 map_of_maps types



On 4/24/23 2:09 AM, Xueming Feng wrote:
> When using `bpftool map dump` in plain format, it is usually
> more convenient to show the inner map id instead of raw value.
> Changing this behavior would help with quick debugging with
> `bpftool`, without disrupting scripted behavior. Since user
> could dump the inner map with id, and need to convert value.
> 
> Signed-off-by: Xueming Feng <kuro@...oa.me>
> ---
> 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.
> 
> Thanks for the review and suggestions! I have changed my patch accordingly.
> There is a possibility that `arg_size` is larger than `unsigned long`,
> but previous review suggested that it should be up to the caller function to
> set `arg_size` correctly. So I didn't add check for that, should I?
> 
>   tools/bpf/bpftool/main.c | 15 +++++++++++++++
>   tools/bpf/bpftool/main.h |  1 +
>   tools/bpf/bpftool/map.c  |  9 +++++++--
>   3 files changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/main.c b/tools/bpf/bpftool/main.c
> index 08d0ac543c67..810c0dc10ecb 100644
> --- a/tools/bpf/bpftool/main.c
> +++ b/tools/bpf/bpftool/main.c
> @@ -251,6 +251,21 @@ int detect_common_prefix(const char *arg, ...)
>   	return 0;
>   }
>   
> +void print_uint(const void *arg, unsigned int arg_size)
> +{
> +	const unsigned char *data = arg;
> +	unsigned long val = 0ul;
> +
> +	#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> +		memcpy(&val, data, arg_size);
> +	#else
> +		memcpy((unsigned char *)&val + sizeof(val) - arg_size,
> +		       data, arg_size);
> +	#endif
> +
> +	fprintf(stdout, "%lu", val);
> +}
> +
>   void fprint_hex(FILE *f, void *arg, unsigned int n, const char *sep)
>   {
>   	unsigned char *data = arg;
> diff --git a/tools/bpf/bpftool/main.h b/tools/bpf/bpftool/main.h
> index 0ef373cef4c7..0de671423431 100644
> --- a/tools/bpf/bpftool/main.h
> +++ b/tools/bpf/bpftool/main.h
> @@ -90,6 +90,7 @@ void __printf(1, 2) p_info(const char *fmt, ...);
>   
>   bool is_prefix(const char *pfx, const char *str);
>   int detect_common_prefix(const char *arg, ...);
> +void print_uint(const void *arg, unsigned int arg_size);
>   void fprint_hex(FILE *f, void *arg, unsigned int n, const char *sep);
>   void usage(void) __noreturn;
>   
> diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
> index aaeb8939e137..f5be4c0564cf 100644
> --- a/tools/bpf/bpftool/map.c
> +++ b/tools/bpf/bpftool/map.c
> @@ -259,8 +259,13 @@ 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("id:%c", break_names ? '\n' : ' ');
> +				print_uint(value, info->value_size);

For all map_in_map types, the inner map value size is 32bit int which 
represents a fd (for map creation) and a id (for map info), e.g., in
show_prog_maps() in prog.c. So maybe we can simplify the code as below:
	printf("id: %u", *(unsigned int *)value);


> +			} else {
> +				printf("value:%c", break_names ? '\n' : ' ');
> +				fprint_hex(stdout, value, info->value_size, " ");
> +			}
>   		}
>   
>   		printf("\n");