[RFC v2 bpf-next 4/5] tools/bpf: bpftool, print strerror when map lookup error occurs

[Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>]

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>
---
 tools/bpf/bpftool/map.c | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
index 28d365435fea..9f5de48f8a99 100644
--- a/tools/bpf/bpftool/map.c
+++ b/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;
-- 
2.17.1