[PATCH] selftests/bpf: improve file descriptor closure handling

[Ma Ke <make24@xxxxxxxxxxx>]

serial_test_fexit_stress() has a non-robust handling of file descriptor
closure. If an error occurs, the function may exit without closing open
file descriptors, potentially causing resource leaks.

Fix the issue by closing file descriptors in reverse order and starting
from the last opened. Ensure proper closure even if an error occurs early.

Fixes: 8fb9fb2f1728 ("selftests/bpf: Query BPF_MAX_TRAMP_LINKS using BTF")
Signed-off-by: Ma Ke <make24@xxxxxxxxxxx>
---
 tools/testing/selftests/bpf/prog_tests/fexit_stress.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/fexit_stress.c b/tools/testing/selftests/bpf/prog_tests/fexit_stress.c
index 596536def43d..b1980bd61583 100644
--- a/tools/testing/selftests/bpf/prog_tests/fexit_stress.c
+++ b/tools/testing/selftests/bpf/prog_tests/fexit_stress.c
@@ -49,11 +49,14 @@ void serial_test_fexit_stress(void)
 	ASSERT_OK(err, "bpf_prog_test_run_opts");
 
 out:
-	for (i = 0; i < bpf_max_tramp_links; i++) {
+	if (i >= bpf_max_tramp_links)
+		i = bpf_max_tramp_links - 1;
+	while (i >= 0) {
 		if (link_fd[i])
 			close(link_fd[i]);
 		if (fexit_fd[i])
 			close(fexit_fd[i]);
+		i--;
 	}
 	free(fd);
 }
-- 
2.25.1