selftests/bpf: Add BTF_KIND_DECL_TAG typedef example in tag.c

Change value type in progs/tag.c to a typedef with a btf_decl_tag.
With `bpftool btf dump file tag.o`, we have
  ...
  [14] TYPEDEF 'value_t' type_id=17
  [15] DECL_TAG 'tag1' type_id=14 component_idx=-1
  [16] DECL_TAG 'tag2' type_id=14 component_idx=-1
  [17] STRUCT '(anon)' size=8 vlen=2
        'a' type_id=2 bits_offset=0
        'b' type_id=2 bits_offset=32
  ...

The btf_tag selftest also succeeded:
  $ ./test_progs -t tag
    #21 btf_tag:OK
    Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211021195643.4020315-1-yhs@fb.com

diff --git a/tools/testing/selftests/bpf/progs/tag.c b/tools/testing/selftests/bpf/progs/tag.c
index 672d19e7b120b843aa4a64742f1250c40c8b3e3d..1792f4eda09597df3a44725bd4336acac4a7afea 100644 (file)
--- a/tools/testing/selftests/bpf/progs/tag.c
+++ b/tools/testing/selftests/bpf/progs/tag.c
@@ -24,18 +24,23 @@ struct key_t {
        int c;
 } __tag1 __tag2;
 
+typedef struct {
+       int a;
+       int b;
+} value_t __tag1 __tag2;
+
 struct {
        __uint(type, BPF_MAP_TYPE_HASH);
        __uint(max_entries, 3);
        __type(key, struct key_t);
-       __type(value, __u64);
+       __type(value, value_t);
 } hashmap1 SEC(".maps");
 
 
 static __noinline int foo(int x __tag1 __tag2) __tag1 __tag2
 {
        struct key_t key;
-       __u64 val = 1;
+       value_t val = {};
 
        key.a = key.b = key.c = x;
        bpf_map_update_elem(&hashmap1, &key, &val, 0);