selftests/bpf: add test of __weak unknown virtual __kconfig extern

Exercise libbpf's logic for unknown __weak virtual __kconfig externs.
USDT selftests are already excercising non-weak known virtual extern
already (LINUX_HAS_BPF_COOKIE), so no need to add explicit tests for it.

Tested-by: Alan Maguire <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/prog_tests/core_extern.c

···
 		       "CONFIG_STR=\"abracad\"\n"
 		       "CONFIG_MISSING=0",
 		.data = {
+			.unkn_virt_val = 0,
 			.bpf_syscall = false,
 			.tristate_val = TRI_MODULE,
 			.bool_val = true,
···
 void test_core_extern(void)
 {
 	const uint32_t kern_ver = get_kernel_version();
-	int err, duration = 0, i, j;
+	int err, i, j;
 	struct test_core_extern *skel = NULL;
 	uint64_t *got, *exp;
 	int n = sizeof(*skel->data) / sizeof(uint64_t);
···
 			continue;
 
 		skel = test_core_extern__open_opts(&opts);
-		if (CHECK(!skel, "skel_open", "skeleton open failed\n"))
+		if (!ASSERT_OK_PTR(skel, "skel_open"))
 			goto cleanup;
 		err = test_core_extern__load(skel);
 		if (t->fails) {
-			CHECK(!err, "skel_load",
-			      "shouldn't succeed open/load of skeleton\n");
+			ASSERT_ERR(err, "skel_load_should_fail");
 			goto cleanup;
-		} else if (CHECK(err, "skel_load",
-				 "failed to open/load skeleton\n")) {
+		} else if (!ASSERT_OK(err, "skel_load")) {
 			goto cleanup;
 		}
 		err = test_core_extern__attach(skel);
-		if (CHECK(err, "attach_raw_tp", "failed attach: %d\n", err))
+		if (!ASSERT_OK(err, "attach_raw_tp"))
 			goto cleanup;
 
 		usleep(1);
···
 		got = (uint64_t *)skel->data;
 		exp = (uint64_t *)&t->data;
 		for (j = 0; j < n; j++) {
-			CHECK(got[j] != exp[j], "check_res",
-			      "result #%d: expected %llx, but got %llx\n",
-			       j, (__u64)exp[j], (__u64)got[j]);
+			ASSERT_EQ(got[j], exp[j], "result");
 		}
 cleanup:
 		test_core_extern__destroy(skel);

tools/testing/selftests/bpf/progs/test_core_extern.c

···
 static int (*bpf_missing_helper)(const void *arg1, int arg2) = (void *) 999;
 
 extern int LINUX_KERNEL_VERSION __kconfig;
+extern int LINUX_UNKNOWN_VIRTUAL_EXTERN __kconfig __weak;
 extern bool CONFIG_BPF_SYSCALL __kconfig; /* strong */
 extern enum libbpf_tristate CONFIG_TRISTATE __kconfig __weak;
 extern bool CONFIG_BOOL __kconfig __weak;
···
 extern uint64_t CONFIG_MISSING __kconfig __weak;
 
 uint64_t kern_ver = -1;
+uint64_t unkn_virt_val = -1;
 uint64_t bpf_syscall = -1;
 uint64_t tristate_val = -1;
 uint64_t bool_val = -1;
···
 	int i;
 
 	kern_ver = LINUX_KERNEL_VERSION;
+	unkn_virt_val = LINUX_UNKNOWN_VIRTUAL_EXTERN;
 	bpf_syscall = CONFIG_BPF_SYSCALL;
 	tristate_val = CONFIG_TRISTATE;
 	bool_val = CONFIG_BOOL;