[PATCH bpf-next] selftests/bpf: Fix the issue where the error code is 0

[Feng Yang]

From: Feng Yang <yangf...@kylinos.cn>

The error message printed here only uses the previous err value,
which results in it being printed as 0.
When bpf_map__attach_struct_ops encounters an error,
it uses libbpf_err_ptr(err) to set errno = -err and returns NULL.
Therefore, strerror(errno) can be used to fix this issue.

Fix before:
run_subtest:FAIL:1019 bpf_map__attach_struct_ops failed for map pro_epilogue: 
err=0

Fix after:
run_subtest:FAIL:1019 bpf_map__attach_struct_ops failed for map pro_epilogue: 
Bad file descriptor

Signed-off-by: Feng Yang <yangf...@kylinos.cn>
---
 tools/testing/selftests/bpf/test_loader.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/test_loader.c 
b/tools/testing/selftests/bpf/test_loader.c
index f361c8aa1daf..686a7d7f87b1 100644
--- a/tools/testing/selftests/bpf/test_loader.c
+++ b/tools/testing/selftests/bpf/test_loader.c
@@ -1008,8 +1008,8 @@ void run_subtest(struct test_loader *tester,
                        }
                        link = bpf_map__attach_struct_ops(map);
                        if (!link) {
-                               PRINT_FAIL("bpf_map__attach_struct_ops failed 
for map %s: err=%d\n",
-                                          bpf_map__name(map), err);
+                               PRINT_FAIL("bpf_map__attach_struct_ops failed 
for map %s: %s\n",
+                                          bpf_map__name(map), strerror(errno));
                                goto tobj_cleanup;
                        }
                        links[links_cnt++] = link;
-- 
2.27.0