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

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Message-Id: <20230425060938.51020-1-kuro@kuroa.me>
Date:   Tue, 25 Apr 2023 14:09:38 +0800
From:   Xueming Feng <kuro@...oa.me>
To:     sinquersw@...il.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, kuro@...oa.me,
        linux-kernel@...r.kernel.org, martin.lau@...ux.dev,
        quentin@...valent.com, 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

>On 4/24/23 20:58, Xueming Feng wrote:
>>> On 4/24/23 02:09, 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
>> 
>> On Mon, 24 Apr 2023 09:44:18 -0700, Kui-Feng Lee wrote:
>>> Is it possible that arg_size is bigger than sizeof(val)?
>> 
>> Yes it is possible, I had the thought of adding a check. But as I mentioned
>> before the diff section, previous review
>> https://lore.kernel.org/bpf/20230421101154.23690-1-kuro@kuroa.me/ suggested that
>> I should leave it to the caller function to behave. If I were to add a check,
>> what action do you recommend if the check fails? Print a '-1', do nothing,
>> or just use the first sizeof(val) bytes?

On Mon, 24 Apr 2023 22:19:52 -0700, Kui-Feng Lee wrote:
> In the previous patch, it may have integer overflow, but it is never 
> buffer underrun.  This version uses memcpy and may cause buffer underrun 
> if arg_size is bigger than sizeof(val).  I would say that at least 
> prevent buffer underrun from happening.

Thanks for the advice! This function is pending remove in the next version of 
this patch. But I will remember this for future patches.

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