Re: [PATCH bpf-next v2] bpftool: Dump map id instead of value for map_of_maps types

[Quentin Monnet <quentin@isovalent.com>]

2023-04-25 14:37 UTC+0800 ~ Xueming Feng <kuro@kuroa.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@kuroa.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