tools/bpf: bpftool, print strerror when map lookup error occurs

Since map lookup error can be ENOENT or EOPNOTSUPP, let's print
strerror() as error message in normal and JSON output.

This patch adds helper function print_entry_error() to print
entry from lookup error occurs

Example: Following example dumps a map which does not support lookup.

Output before:
root# bpftool map -jp dump id 40
[
    "key": ["0x0a","0x00","0x00","0x00"
    ],
    "value": {
        "error": "can\'t lookup element"
    },
    "key": ["0x0b","0x00","0x00","0x00"
    ],
    "value": {
        "error": "can\'t lookup element"
    }
]

root# bpftool map dump id 40
can't lookup element with key:
0a 00 00 00
can't lookup element with key:
0b 00 00 00
Found 0 elements

Output after changes:
root# bpftool map dump -jp  id 45
[
    "key": ["0x0a","0x00","0x00","0x00"
    ],
    "value": {
        "error": "Operation not supported"
    },
    "key": ["0x0b","0x00","0x00","0x00"
    ],
    "value": {
        "error": "Operation not supported"
    }
]

root# bpftool map dump id 45
key:
0a 00 00 00
value:
Operation not supported
key:
0b 00 00 00
value:
Operation not supported
Found 0 elements
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/map.c

@@ -336,6 +336,25 @@ static void print_entry_json(struct bpf_map_info *info, unsigned char *key,
 	jsonw_end_object(json_wtr);
 }
 
+static void print_entry_error(struct bpf_map_info *info, unsigned char *key,
+			      const char *value)
+{
+	int value_size = strlen(value);
+	bool single_line, break_names;
+
+	break_names = info->key_size > 16 || value_size > 16;
+	single_line = info->key_size + value_size <= 24 && !break_names;
+
+	printf("key:%c", break_names ? '\n' : ' ');
+	fprint_hex(stdout, key, info->key_size, " ");
+
+	printf(single_line ? "  " : "\n");
+
+	printf("value:%c%s", break_names ? '\n' : ' ', value);
+
+	printf("\n");
+}
+
 static void print_entry_plain(struct bpf_map_info *info, unsigned char *key,
 			      unsigned char *value)
 {
@@ -663,6 +682,7 @@ static int dump_map_elem(int fd, void *key, void *value,
 			 json_writer_t *btf_wtr)
 {
 	int num_elems = 0;
+	int lookup_errno;
 
 	if (!bpf_map_lookup_elem(fd, key, value)) {
 		if (json_output) {
@@ -685,6 +705,8 @@ static int dump_map_elem(int fd, void *key, void *value,
 	}
 
 	/* lookup error handling */
+	lookup_errno = errno;
+
 	if (map_is_map_of_maps(map_info->type) ||
 	    map_is_map_of_progs(map_info->type))
 		return 0;
@@ -694,13 +716,10 @@ static int dump_map_elem(int fd, void *key, void *value,
 		print_hex_data_json(key, map_info->key_size);
 		jsonw_name(json_wtr, "value");
 		jsonw_start_object(json_wtr);
-		jsonw_string_field(json_wtr, "error",
-				   "can't lookup element");
+		jsonw_string_field(json_wtr, "error", strerror(lookup_errno));
 		jsonw_end_object(json_wtr);
 	} else {
-		p_info("can't lookup element with key: ");
-		fprint_hex(stderr, key, map_info->key_size, " ");
-		fprintf(stderr, "\n");
+		print_entry_error(map_info, key, strerror(lookup_errno));
 	}
 
 	return 0;