selftests/bpf: Add tests for KF_IMPLICIT_ARGS

Add trivial end-to-end tests to validate that KF_IMPLICIT_ARGS flag is
properly handled by both resolve_btfids and the verifier.

Declare kfuncs in bpf_testmod. Check that bpf_prog_aux pointer is set
in the kfunc implementation. Verify that calls with implicit args and
a legacy case all work.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260120222638.3976562-7-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include <test_progs.h>
+#include "kfunc_implicit_args.skel.h"
+
+void test_kfunc_implicit_args(void)
+{
+	RUN_TESTS(kfunc_implicit_args);
+}

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

@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+extern int bpf_kfunc_implicit_arg(int a) __weak __ksym;
+extern int bpf_kfunc_implicit_arg_impl(int a, struct bpf_prog_aux *aux) __weak __ksym; /* illegal */
+extern int bpf_kfunc_implicit_arg_legacy(int a, int b) __weak __ksym;
+extern int bpf_kfunc_implicit_arg_legacy_impl(int a, int b, struct bpf_prog_aux *aux) __weak __ksym;
+
+char _license[] SEC("license") = "GPL";
+
+SEC("syscall")
+__retval(5)
+int test_kfunc_implicit_arg(void *ctx)
+{
+	return bpf_kfunc_implicit_arg(5);
+}
+
+SEC("syscall")
+__failure __msg("cannot find address for kernel function bpf_kfunc_implicit_arg_impl")
+int test_kfunc_implicit_arg_impl_illegal(void *ctx)
+{
+	return bpf_kfunc_implicit_arg_impl(5, NULL);
+}
+
+SEC("syscall")
+__retval(7)
+int test_kfunc_implicit_arg_legacy(void *ctx)
+{
+	return bpf_kfunc_implicit_arg_legacy(3, 4);
+}
+
+SEC("syscall")
+__retval(11)
+int test_kfunc_implicit_arg_legacy_impl(void *ctx)
+{
+	return bpf_kfunc_implicit_arg_legacy_impl(5, 6, NULL);
+}

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

@@ -1142,6 +1142,10 @@ __bpf_kfunc int bpf_kfunc_st_ops_inc10(struct st_ops_args *args)
 __bpf_kfunc int bpf_kfunc_multi_st_ops_test_1(struct st_ops_args *args, u32 id);
 __bpf_kfunc int bpf_kfunc_multi_st_ops_test_1_impl(struct st_ops_args *args, void *aux_prog);
 
+__bpf_kfunc int bpf_kfunc_implicit_arg(int a, struct bpf_prog_aux *aux);
+__bpf_kfunc int bpf_kfunc_implicit_arg_legacy(int a, int b, struct bpf_prog_aux *aux);
+__bpf_kfunc int bpf_kfunc_implicit_arg_legacy_impl(int a, int b, struct bpf_prog_aux *aux);
+
 BTF_KFUNCS_START(bpf_testmod_check_kfunc_ids)
 BTF_ID_FLAGS(func, bpf_testmod_test_mod_kfunc)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test1)
@@ -1184,6 +1188,9 @@ BTF_ID_FLAGS(func, bpf_kfunc_st_ops_test_pro_epilogue, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_kfunc_st_ops_inc10)
 BTF_ID_FLAGS(func, bpf_kfunc_multi_st_ops_test_1)
 BTF_ID_FLAGS(func, bpf_kfunc_multi_st_ops_test_1_impl)
+BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg, KF_IMPLICIT_ARGS)
+BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg_legacy, KF_IMPLICIT_ARGS)
+BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg_legacy_impl)
 BTF_KFUNCS_END(bpf_testmod_check_kfunc_ids)
 
 static int bpf_testmod_ops_init(struct btf *btf)
@@ -1675,6 +1682,25 @@ int bpf_kfunc_multi_st_ops_test_1_impl(struct st_ops_args *args, void *aux__prog
 	return ret;
 }
 
+int bpf_kfunc_implicit_arg(int a, struct bpf_prog_aux *aux)
+{
+	if (aux && a > 0)
+		return a;
+	return -EINVAL;
+}
+
+int bpf_kfunc_implicit_arg_legacy(int a, int b, struct bpf_prog_aux *aux)
+{
+	if (aux)
+		return a + b;
+	return -EINVAL;
+}
+
+int bpf_kfunc_implicit_arg_legacy_impl(int a, int b, struct bpf_prog_aux *aux)
+{
+	return bpf_kfunc_implicit_arg_legacy(a, b, aux);
+}
+
 static int multi_st_ops_reg(void *kdata, struct bpf_link *link)
 {
 	struct bpf_testmod_multi_st_ops *st_ops =