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

[Xueming Feng <kuro@xxxxxxxx>]

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' : ' ');
+				print_uint(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)