selftests/bpf: Add testcase for static kfunc with unused arg

kfuncs are allowed to be static, or not use one or more of their
arguments. For example, bpf_xdp_metadata_rx_hash() in net/core/xdp.c is
meant to be implemented by drivers, with the default implementation just
returning -EOPNOTSUPP. As described in [0], such kfuncs can have their
arguments elided, which can cause BTF encoding to be skipped. The new
__bpf_kfunc macro should address this, and this patch adds a selftest
which verifies that a static kfunc with at least one unused argument can
still be encoded and invoked by a BPF program.
Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230201173016.342758-5-void@manifault.com

net/bpf/test_run.c

@@ -741,6 +741,11 @@ __bpf_kfunc void bpf_kfunc_call_test_destructive(void)
 {
 }
 
+__bpf_kfunc static u32 bpf_kfunc_call_test_static_unused_arg(u32 arg, u32 unused)
+{
+	return arg;
+}
+
 __diag_pop();
 
 BTF_SET8_START(bpf_test_modify_return_ids)
@@ -779,6 +784,7 @@ BTF_ID_FLAGS(func, bpf_kfunc_call_test_mem_len_fail1)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_mem_len_fail2)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_ref, KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_destructive, KF_DESTRUCTIVE)
+BTF_ID_FLAGS(func, bpf_kfunc_call_test_static_unused_arg)
 BTF_SET8_END(test_sk_check_kfunc_ids)
 
 static void *bpf_test_init(const union bpf_attr *kattr, u32 user_size,

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

@@ -77,6 +77,7 @@ static struct kfunc_test_params kfunc_tests[] = {
 	TC_TEST(kfunc_call_test_get_mem, 42),
 	SYSCALL_TEST(kfunc_syscall_test, 0),
 	SYSCALL_NULL_CTX_TEST(kfunc_syscall_test_null, 0),
+	TC_TEST(kfunc_call_test_static_unused_arg, 0),
 };
 
 struct syscall_test_args {

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

@@ -17,6 +17,7 @@ extern void bpf_kfunc_call_test_mem_len_pass1(void *mem, int len) __ksym;
 extern void bpf_kfunc_call_test_mem_len_fail2(__u64 *mem, int len) __ksym;
 extern int *bpf_kfunc_call_test_get_rdwr_mem(struct prog_test_ref_kfunc *p, const int rdwr_buf_size) __ksym;
 extern int *bpf_kfunc_call_test_get_rdonly_mem(struct prog_test_ref_kfunc *p, const int rdonly_buf_size) __ksym;
+extern u32 bpf_kfunc_call_test_static_unused_arg(u32 arg, u32 unused) __ksym;
 
 SEC("tc")
 int kfunc_call_test4(struct __sk_buff *skb)
@@ -181,4 +182,14 @@ int kfunc_call_test_get_mem(struct __sk_buff *skb)
 	return ret;
 }
 
+SEC("tc")
+int kfunc_call_test_static_unused_arg(struct __sk_buff *skb)
+{
+
+	u32 expected = 5, actual;
+
+	actual = bpf_kfunc_call_test_static_unused_arg(expected, 0xdeadbeef);
+	return actual != expected ? -1 : 0;
+}
+
 char _license[] SEC("license") = "GPL";