selftests/bpf: Add test relying only on CO-RE and no recent kernel features

Add a test that relies on CO-RE, but doesn't expect any of the recent
features, not available on old kernels. This is useful for Travis CI tests
running against very old kernels (e.g., libbpf has 4.9 kernel testing now), to
verify that CO-RE still works, even if kernel itself doesn't support BTF yet,
as long as there is .BTF embedded into vmlinux image by pahole. Given most of
CO-RE doesn't require any kernel awareness of BTF, it is a useful test to
validate that libbpf's BTF sanitization is working well even with ancient
kernels.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200708015318.3827358-5-andriin@fb.com

1 files changed, 30 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/test_core_retro.c b/tools/testing/selftests/bpf/progs/test_core_retro.c
new file mode 100644
index 000000000000..75c60c3c29cf
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_core_retro.c
@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2020 Facebook
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_core_read.h>
+
+struct task_struct {
+	int tgid;
+} __attribute__((preserve_access_index));
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, int);
+} results SEC(".maps");
+
+SEC("tp/raw_syscalls/sys_enter")
+int handle_sys_enter(void *ctx)
+{
+	struct task_struct *task = (void *)bpf_get_current_task();
+	int tgid = BPF_CORE_READ(task, tgid);
+	int zero = 0;
+
+	bpf_map_update_elem(&results, &zero, &tgid, 0);
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";