selftests: bpf: add testmod kfunc for nullable params

Add special test to be sure that only __nullable BTF params can be
replaced by NULL. This patch adds fake kfuncs in bpf_testmod to
properly test different params.
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Vadim Fedorenko <vadfed@meta.com>
Link: https://lore.kernel.org/r/20240613211817.1551967-6-vadfed@meta.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -154,6 +154,11 @@ __bpf_kfunc void bpf_kfunc_common_test(void)
 {
 }
 
+__bpf_kfunc void bpf_kfunc_dynptr_test(struct bpf_dynptr *ptr,
+				       struct bpf_dynptr *ptr__nullable)
+{
+}
+
 struct bpf_testmod_btf_type_tag_1 {
 	int a;
 };
@@ -363,6 +368,7 @@ BTF_ID_FLAGS(func, bpf_iter_testmod_seq_new, KF_ITER_NEW)
 BTF_ID_FLAGS(func, bpf_iter_testmod_seq_next, KF_ITER_NEXT | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_iter_testmod_seq_destroy, KF_ITER_DESTROY)
 BTF_ID_FLAGS(func, bpf_kfunc_common_test)
+BTF_ID_FLAGS(func, bpf_kfunc_dynptr_test)
 BTF_KFUNCS_END(bpf_testmod_common_kfunc_ids)
 
 static const struct btf_kfunc_id_set bpf_testmod_common_kfunc_set = {

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod_kfunc.h

@@ -134,4 +134,5 @@ int bpf_kfunc_call_sock_sendmsg(struct sendmsg_args *args) __ksym;
 int bpf_kfunc_call_kernel_getsockname(struct addr_args *args) __ksym;
 int bpf_kfunc_call_kernel_getpeername(struct addr_args *args) __ksym;
 
+void bpf_kfunc_dynptr_test(struct bpf_dynptr *ptr, struct bpf_dynptr *ptr__nullable) __ksym;
 #endif /* _BPF_TESTMOD_KFUNC_H */

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

+// SPDX-License-Identifier: GPL-2.0
+
+/* Copyright (c) 2024 Meta Platforms, Inc */
+
+#include <test_progs.h>
+#include "test_kfunc_param_nullable.skel.h"
+
+void test_kfunc_param_nullable(void)
+{
+	RUN_TESTS(test_kfunc_param_nullable);
+}

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Meta Platforms, Inc */
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+#include "bpf_kfuncs.h"
+#include "../bpf_testmod/bpf_testmod_kfunc.h"
+
+SEC("tc")
+int kfunc_dynptr_nullable_test1(struct __sk_buff *skb)
+{
+	struct bpf_dynptr data;
+
+	bpf_dynptr_from_skb(skb, 0, &data);
+	bpf_kfunc_dynptr_test(&data, NULL);
+
+	return 0;
+}
+
+SEC("tc")
+int kfunc_dynptr_nullable_test2(struct __sk_buff *skb)
+{
+	struct bpf_dynptr data;
+
+	bpf_dynptr_from_skb(skb, 0, &data);
+	bpf_kfunc_dynptr_test(&data, &data);
+
+	return 0;
+}
+
+SEC("tc")
+__failure __msg("expected pointer to stack or dynptr_ptr")
+int kfunc_dynptr_nullable_test3(struct __sk_buff *skb)
+{
+	struct bpf_dynptr data;
+
+	bpf_dynptr_from_skb(skb, 0, &data);
+	bpf_kfunc_dynptr_test(NULL, &data);
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";