[RFC bpf-next 4/4] tools/bpf: handle EOPNOTSUPP when map lookup is failed

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

Let's add a check for EOPNOTSUPP error when map lookup is failed.
Also in case map doesn't support lookup, the output of map dump is
changed from "can't lookup element" to "lookup not supported for
this map".

Patch adds function print_entry_error() function to print the error
value.

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": "lookup not supported for this map"
    },
    "key": ["0x0b","0x00","0x00","0x00"
    ],
    "value": {
        "error": "lookup not supported for this map"
    }
]

root# bpftool map dump id 45
key:
0a 00 00 00
value:
lookup not supported for this map
key:
0b 00 00 00
value:
lookup not supported for this map
Found 0 elements

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
---
 tools/bpf/bpftool/main.h |  5 +++++
 tools/bpf/bpftool/map.c  | 35 ++++++++++++++++++++++++++++++-----
 2 files changed, 35 insertions(+), 5 deletions(-)

diff --git a/tools/bpf/bpftool/main.h b/tools/bpf/bpftool/main.h
index 40492cdc4e53..1a8c683f949b 100644
--- a/tools/bpf/bpftool/main.h
+++ b/tools/bpf/bpftool/main.h
@@ -46,6 +46,11 @@
 
 #include "json_writer.h"
 
+#define ERR_CANNOT_LOOKUP \
+	"can't lookup element"
+#define ERR_LOOKUP_NOT_SUPPORTED \
+	"lookup not supported for this map"
+
 #define ptr_to_u64(ptr)	((__u64)(unsigned long)(ptr))
 
 #define NEXT_ARG()	({ argc--; argv++; if (argc < 0) usage(); })
diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
index 284e12a289c0..2faccd2098c9 100644
--- a/tools/bpf/bpftool/map.c
+++ b/tools/bpf/bpftool/map.c
@@ -333,6 +333,25 @@ 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,
+			      char *value)
+{
+	bool single_line, break_names;
+	int value_size = strlen(value);
+
+	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)
 {
@@ -660,6 +679,8 @@ static int dump_map_elem(int fd, void *key, void *value,
 			 json_writer_t *btf_wtr)
 {
 	int num_elems = 0;
+	int lookup_errno;
+	char *errstr;
 
 	if (!bpf_map_lookup_elem(fd, key, value)) {
 		if (json_output) {
@@ -682,22 +703,26 @@ 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))
 		goto out;
 
+	if (lookup_errno == EOPNOTSUPP)
+		errstr = ERR_LOOKUP_NOT_SUPPORTED;
+	else
+		errstr = ERR_CANNOT_LOOKUP;
+
 	if (json_output) {
 		jsonw_name(json_wtr, "key");
 		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", errstr);
 		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, errstr);
 	}
 out:
 	return num_elems;
-- 
2.17.1