selftests/bpf: Add selftest for multi-prog sections and bpf-to-bpf calls

Add a selftest excercising bpf-to-bpf subprogram calls, as well as multiple
entry-point BPF programs per section. Also make sure that BPF CO-RE works for
such set ups both for sub-programs and for multi-entry sections.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200903203542.15944-8-andriin@fb.com

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include <test_progs.h>
+#include <time.h>
+#include "test_subprogs.skel.h"
+
+static int duration;
+
+void test_subprogs(void)
+{
+	struct test_subprogs *skel;
+	int err;
+
+	skel = test_subprogs__open_and_load();
+	if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
+		return;
+
+	err = test_subprogs__attach(skel);
+	if (CHECK(err, "skel_attach", "failed to attach skeleton: %d\n", err))
+		goto cleanup;
+
+	usleep(1);
+
+	CHECK(skel->bss->res1 != 12, "res1", "got %d, exp %d\n", skel->bss->res1, 12);
+	CHECK(skel->bss->res2 != 17, "res2", "got %d, exp %d\n", skel->bss->res2, 17);
+	CHECK(skel->bss->res3 != 19, "res3", "got %d, exp %d\n", skel->bss->res3, 19);
+	CHECK(skel->bss->res4 != 36, "res4", "got %d, exp %d\n", skel->bss->res4, 36);
+
+cleanup:
+	test_subprogs__destroy(skel);
+}

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

+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_core_read.h>
+
+const char LICENSE[] SEC("license") = "GPL";
+
+__noinline int sub1(int x)
+{
+	return x + 1;
+}
+
+static __noinline int sub5(int v);
+
+__noinline int sub2(int y)
+{
+	return sub5(y + 2);
+}
+
+static __noinline int sub3(int z)
+{
+	return z + 3 + sub1(4);
+}
+
+static __noinline int sub4(int w)
+{
+	return w + sub3(5) + sub1(6);
+}
+
+/* sub5() is an identitify function, just to test weirder functions layout and
+ * call patterns
+ */
+static __noinline int sub5(int v)
+{
+	return sub1(v) - 1; /* compensates sub1()'s + 1 */
+}
+
+/* unfortunately verifier rejects `struct task_struct *t` as an unkown pointer
+ * type, so we need to accept pointer as integer and then cast it inside the
+ * function
+ */
+__noinline int get_task_tgid(uintptr_t t)
+{
+	/* this ensures that CO-RE relocs work in multi-subprogs .text */
+	return BPF_CORE_READ((struct task_struct *)(void *)t, tgid);
+}
+
+int res1 = 0;
+int res2 = 0;
+int res3 = 0;
+int res4 = 0;
+
+SEC("raw_tp/sys_enter")
+int prog1(void *ctx)
+{
+	/* perform some CO-RE relocations to ensure they work with multi-prog
+	 * sections correctly
+	 */
+	struct task_struct *t = (void *)bpf_get_current_task();
+
+	if (!BPF_CORE_READ(t, pid) || !get_task_tgid((uintptr_t)t))
+		return 1;
+
+	res1 = sub1(1) + sub3(2); /* (1 + 1) + (2 + 3 + (4 + 1)) = 12 */
+	return 0;
+}
+
+SEC("raw_tp/sys_exit")
+int prog2(void *ctx)
+{
+	struct task_struct *t = (void *)bpf_get_current_task();
+
+	if (!BPF_CORE_READ(t, pid) || !get_task_tgid((uintptr_t)t))
+		return 1;
+
+	res2 = sub2(3) + sub3(4); /* (3 + 2) + (4 + 3 + (4 + 1)) = 17 */
+	return 0;
+}
+
+/* prog3 has the same section name as prog1 */
+SEC("raw_tp/sys_enter")
+int prog3(void *ctx)
+{
+	struct task_struct *t = (void *)bpf_get_current_task();
+
+	if (!BPF_CORE_READ(t, pid) || !get_task_tgid((uintptr_t)t))
+		return 1;
+
+	res3 = sub3(5) + 6; /* (5 + 3 + (4 + 1)) + 6 = 19 */
+	return 0;
+}
+
+/* prog4 has the same section name as prog2 */
+SEC("raw_tp/sys_exit")
+int prog4(void *ctx)
+{
+	struct task_struct *t = (void *)bpf_get_current_task();
+
+	if (!BPF_CORE_READ(t, pid) || !get_task_tgid((uintptr_t)t))
+		return 1;
+
+	res4 = sub4(7) + sub1(8); /* (7 + (5 + 3 + (4 + 1)) + (6 + 1)) + (8 + 1) = 36 */
+	return 0;
+}