selftest/bpf: Add a recursion test

Add recursive non-sleepable fentry program as a test.
All attach points where sleepable progs can execute are non recursive so far.
The recursion protection mechanism for sleepable cannot be activated yet.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210210033634.62081-6-alexei.starovoitov@gmail.com

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+#include <test_progs.h>
+#include "recursion.skel.h"
+
+void test_recursion(void)
+{
+	struct recursion *skel;
+	int key = 0;
+	int err;
+
+	skel = recursion__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	err = recursion__attach(skel);
+	if (!ASSERT_OK(err, "skel_attach"))
+		goto out;
+
+	ASSERT_EQ(skel->bss->pass1, 0, "pass1 == 0");
+	bpf_map_lookup_elem(bpf_map__fd(skel->maps.hash1), &key, 0);
+	ASSERT_EQ(skel->bss->pass1, 1, "pass1 == 1");
+	bpf_map_lookup_elem(bpf_map__fd(skel->maps.hash1), &key, 0);
+	ASSERT_EQ(skel->bss->pass1, 2, "pass1 == 2");
+
+	ASSERT_EQ(skel->bss->pass2, 0, "pass2 == 0");
+	bpf_map_lookup_elem(bpf_map__fd(skel->maps.hash2), &key, 0);
+	ASSERT_EQ(skel->bss->pass2, 1, "pass2 == 1");
+	bpf_map_lookup_elem(bpf_map__fd(skel->maps.hash2), &key, 0);
+	ASSERT_EQ(skel->bss->pass2, 2, "pass2 == 2");
+out:
+	recursion__destroy(skel);
+}

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, long);
+} hash1 SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, long);
+} hash2 SEC(".maps");
+
+int pass1 = 0;
+int pass2 = 0;
+
+SEC("fentry/__htab_map_lookup_elem")
+int BPF_PROG(on_lookup, struct bpf_map *map)
+{
+	int key = 0;
+
+	if (map == (void *)&hash1) {
+		pass1++;
+		return 0;
+	}
+	if (map == (void *)&hash2) {
+		pass2++;
+		/* htab_map_gen_lookup() will inline below call
+		 * into direct call to __htab_map_lookup_elem()
+		 */
+		bpf_map_lookup_elem(&hash2, &key);
+		return 0;
+	}
+
+	return 0;
+}