selftests/bpf: Add local kptr stashing test

Add a new selftest, local_kptr_stash, which uses bpf_kptr_xchg to stash
a bpf_obj_new-allocated object in a map. Test the following scenarios:

  * Stash two rb_nodes in an arraymap, don't unstash them, rely on map
    free to destruct them
  * Stash two rb_nodes in an arraymap, unstash the second one in a
    separate program, rely on map free to destruct first
Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230310230743.2320707-4-davemarchevsky@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
+
+#include <test_progs.h>
+#include <network_helpers.h>
+
+#include "local_kptr_stash.skel.h"
+static void test_local_kptr_stash_simple(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts,
+		    .data_in = &pkt_v4,
+		    .data_size_in = sizeof(pkt_v4),
+		    .repeat = 1,
+	);
+	struct local_kptr_stash *skel;
+	int ret;
+
+	skel = local_kptr_stash__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "local_kptr_stash__open_and_load"))
+		return;
+
+	ret = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.stash_rb_nodes), &opts);
+	ASSERT_OK(ret, "local_kptr_stash_add_nodes run");
+	ASSERT_OK(opts.retval, "local_kptr_stash_add_nodes retval");
+
+	local_kptr_stash__destroy(skel);
+}
+
+static void test_local_kptr_stash_unstash(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts,
+		    .data_in = &pkt_v4,
+		    .data_size_in = sizeof(pkt_v4),
+		    .repeat = 1,
+	);
+	struct local_kptr_stash *skel;
+	int ret;
+
+	skel = local_kptr_stash__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "local_kptr_stash__open_and_load"))
+		return;
+
+	ret = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.stash_rb_nodes), &opts);
+	ASSERT_OK(ret, "local_kptr_stash_add_nodes run");
+	ASSERT_OK(opts.retval, "local_kptr_stash_add_nodes retval");
+
+	ret = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.unstash_rb_node), &opts);
+	ASSERT_OK(ret, "local_kptr_stash_add_nodes run");
+	ASSERT_EQ(opts.retval, 42, "local_kptr_stash_add_nodes retval");
+
+	local_kptr_stash__destroy(skel);
+}
+
+void test_local_kptr_stash_success(void)
+{
+	if (test__start_subtest("local_kptr_stash_simple"))
+		test_local_kptr_stash_simple();
+	if (test__start_subtest("local_kptr_stash_unstash"))
+		test_local_kptr_stash_unstash();
+}

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_core_read.h>
+#include "bpf_experimental.h"
+
+struct node_data {
+	long key;
+	long data;
+	struct bpf_rb_node node;
+};
+
+struct map_value {
+	struct prog_test_ref_kfunc *not_kptr;
+	struct prog_test_ref_kfunc __kptr *val;
+	struct node_data __kptr *node;
+};
+
+/* This is necessary so that LLVM generates BTF for node_data struct
+ * If it's not included, a fwd reference for node_data will be generated but
+ * no struct. Example BTF of "node" field in map_value when not included:
+ *
+ * [10] PTR '(anon)' type_id=35
+ * [34] FWD 'node_data' fwd_kind=struct
+ * [35] TYPE_TAG 'kptr_ref' type_id=34
+ *
+ * (with no node_data struct defined)
+ * Had to do the same w/ bpf_kfunc_call_test_release below
+ */
+struct node_data *just_here_because_btf_bug;
+
+extern void bpf_kfunc_call_test_release(struct prog_test_ref_kfunc *p) __ksym;
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, int);
+	__type(value, struct map_value);
+	__uint(max_entries, 2);
+} some_nodes SEC(".maps");
+
+static int create_and_stash(int idx, int val)
+{
+	struct map_value *mapval;
+	struct node_data *res;
+
+	mapval = bpf_map_lookup_elem(&some_nodes, &idx);
+	if (!mapval)
+		return 1;
+
+	res = bpf_obj_new(typeof(*res));
+	if (!res)
+		return 1;
+	res->key = val;
+
+	res = bpf_kptr_xchg(&mapval->node, res);
+	if (res)
+		bpf_obj_drop(res);
+	return 0;
+}
+
+SEC("tc")
+long stash_rb_nodes(void *ctx)
+{
+	return create_and_stash(0, 41) ?: create_and_stash(1, 42);
+}
+
+SEC("tc")
+long unstash_rb_node(void *ctx)
+{
+	struct map_value *mapval;
+	struct node_data *res;
+	long retval;
+	int key = 1;
+
+	mapval = bpf_map_lookup_elem(&some_nodes, &key);
+	if (!mapval)
+		return 1;
+
+	res = bpf_kptr_xchg(&mapval->node, NULL);
+	if (res) {
+		retval = res->key;
+		bpf_obj_drop(res);
+		return retval;
+	}
+	return 1;
+}
+
+SEC("tc")
+long stash_test_ref_kfunc(void *ctx)
+{
+	struct prog_test_ref_kfunc *res;
+	struct map_value *mapval;
+	int key = 0;
+
+	mapval = bpf_map_lookup_elem(&some_nodes, &key);
+	if (!mapval)
+		return 1;
+
+	res = bpf_kptr_xchg(&mapval->val, NULL);
+	if (res)
+		bpf_kfunc_call_test_release(res);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";