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

[Prashant Bhole]

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...@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