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

[Yonghong Song]

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@xxxxxxxx>
> > > ---
> > > 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?

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.

Please add the command line to dump map values triggering the above 
change, also the actual dumps with and without this patch.

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