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Date: Tue, 25 Apr 2023 09:57:17 +0100
From: Quentin Monnet <quentin@...valent.com>
To: Xueming Feng <kuro@...oa.me>, yhs@...a.com
Cc: andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
daniel@...earbox.net, haoluo@...gle.com, john.fastabend@...il.com,
jolsa@...nel.org, kpsingh@...nel.org, linux-kernel@...r.kernel.org,
martin.lau@...ux.dev, sdf@...gle.com, song@...nel.org, yhs@...com
Subject: Re: [PATCH bpf-next v2] bpftool: Dump map id instead of value for
map_of_maps types
2023-04-25 14:37 UTC+0800 ~ Xueming Feng <kuro@...oa.me>
>> On 4/24/23 9:10 PM, Xueming Feng wrote:
>>>> 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' : ' ');
>>>> 1> + print_uint(value, info->value_size);
>>>
>>> On Mon, 24 Apr 2023 18:07:27 -0700, Yonghong Song wrote:
>>>> 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);
>>>
>>> That is true, maybe the "id" could also be changed to "map_id" to follow the
>>> convention. Do you think that `print_uint` could be useful in the future?
>>> If that is the case, should I keep using it here as an example usage, and to
>>> avoid dead code? Or should I just remove it?
This makes me think we could also have something similar for prog_array
maps (but not necessarily as part of your patchset).
>
> On Mon, 24 Apr 2023 22:58:10 -0700, Yonghong Song wrote:
>> Maybe, "inner_map_id" is a better choice. For array of maps, some array
>> element value could be 0, implying "inner_map_id 0", but I think it is
>> okay, people should know a real inner_map_id (or any map_id) should
>> never be 0.
>>
>> Function "print_uint" is not needed any more. Please remove it.
>
> Will reflect this in v3.
>
>>
>> Please add the command line to dump map values triggering the above
>> change, also the actual dumps with and without this patch.
>
> $ 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
> ```
Thanks! Please add those sample outputs to the commit description for
v3. Can you please also add an example with JSON ("-p")?
Quentin
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