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

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Date:   Fri, 21 Apr 2023 23:43:57 +0100
From:   Quentin Monnet <quentin@...valent.com>
To:     Xueming Feng <kuro@...oa.me>
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] Dump map id instead of value for map_of_maps types

On Fri, 21 Apr 2023 at 11:12, Xueming Feng <kuro@...oa.me> 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 disruption scripted behavior. Since user

s/disruption/disrupting/ ?

> could dump the inner map with id, but need to convert value.
>
> Signed-off-by: Xueming Feng <kuro@...oa.me>

Thanks for this patch! It looks good, with some minor comments below.

> ---
>  tools/bpf/bpftool/main.c | 16 ++++++++++++++++
>  tools/bpf/bpftool/main.h |  1 +
>  tools/bpf/bpftool/map.c  |  9 +++++++--
>  3 files changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/tools/bpf/bpftool/main.c b/tools/bpf/bpftool/main.c
> index 08d0ac543c67..d297200c91f7 100644
> --- a/tools/bpf/bpftool/main.c
> +++ b/tools/bpf/bpftool/main.c
> @@ -251,6 +251,22 @@ int detect_common_prefix(const char *arg, ...)
>         return 0;
>  }
>
> +void fprint_uint(FILE *f, void *arg, unsigned int n)

I suppose you based this function on fprint_hex(). But for your
function, let's remove the first argument? We always print to stdout.
We can always turn it to a "fprint" version again if we need to print
elsewhere in the future.

Also, "arg" should probably be a "const"?

Can you please rename "n" to "arg_size" or something similar?

> +{
> +       unsigned char *data = arg;
> +       unsigned int data_uint = 0;
> +
> +       for (unsigned int i = 0; i < n && i < 4; i++) {

Please move the declaration for "i" to the top of the function, for consistency.

Also, why stop at i == 4? Couldn't this function be used for 8-byte
long integers too? It should be up to the caller function to set "n"
correctly.

> +       #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> +               data_uint |= data[i] << (i * 8);
> +       #else
> +               data_uint |= data[i] << ((n - i - 1) * 8);
> +       #endif
> +       }
> +
> +       fprintf(f, "%d", data_uint);
> +}
> +
>  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..7488ef38e7a9 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 fprint_uint(FILE *f, void *arg, unsigned int n);
>  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..638bd8de8135 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' : ' ');
> +                               fprint_uint(stdout, value, info->value_size);
> +                       } else {
> +                               printf("value:%c", break_names ? '\n' : ' ');
> +                               fprint_hex(stdout, value, info->value_size, " ");
> +                       }
>                 }
>
>                 printf("\n");
> --
> 2.37.1 (Apple Git-137.1)
>