selftests/bpf: Add variable-length data concatenation pattern test

Add selftest that validates variable-length data reading and concatentation
with one big shared data array. This is a common pattern in production use for
monitoring and tracing applications, that potentially can read a lot of data,
but overall read much less. Such pattern allows to determine precisely what
amount of data needs to be sent over perfbuf/ringbuf and maximize efficiency.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200623032224.4020118-2-andriin@fb.com

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+
+#include <test_progs.h>
+#include <time.h>
+#include "test_varlen.skel.h"
+
+#define CHECK_VAL(got, exp) \
+	CHECK((got) != (exp), "check", "got %ld != exp %ld\n", \
+	      (long)(got), (long)(exp))
+
+void test_varlen(void)
+{
+	int duration = 0, err;
+	struct test_varlen* skel;
+	struct test_varlen__bss *bss;
+	struct test_varlen__data *data;
+	const char str1[] = "Hello, ";
+	const char str2[] = "World!";
+	const char exp_str[] = "Hello, \0World!\0";
+	const int size1 = sizeof(str1);
+	const int size2 = sizeof(str2);
+
+	skel = test_varlen__open_and_load();
+	if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
+		return;
+	bss = skel->bss;
+	data = skel->data;
+
+	err = test_varlen__attach(skel);
+	if (CHECK(err, "skel_attach", "skeleton attach failed: %d\n", err))
+		goto cleanup;
+
+	bss->test_pid = getpid();
+
+	/* trigger everything */
+	memcpy(bss->buf_in1, str1, size1);
+	memcpy(bss->buf_in2, str2, size2);
+	bss->capture = true;
+	usleep(1);
+	bss->capture = false;
+
+	CHECK_VAL(bss->payload1_len1, size1);
+	CHECK_VAL(bss->payload1_len2, size2);
+	CHECK_VAL(bss->total1, size1 + size2);
+	CHECK(memcmp(bss->payload1, exp_str, size1 + size2), "content_check",
+	      "doesn't match!");
+
+	CHECK_VAL(data->payload2_len1, size1);
+	CHECK_VAL(data->payload2_len2, size2);
+	CHECK_VAL(data->total2, size1 + size2);
+	CHECK(memcmp(data->payload2, exp_str, size1 + size2), "content_check",
+	      "doesn't match!");
+cleanup:
+	test_varlen__destroy(skel);
+}

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>
+
+#define MAX_LEN 256
+
+char buf_in1[MAX_LEN] = {};
+char buf_in2[MAX_LEN] = {};
+
+int test_pid = 0;
+bool capture = false;
+
+/* .bss */
+long payload1_len1 = 0;
+long payload1_len2 = 0;
+long total1 = 0;
+char payload1[MAX_LEN + MAX_LEN] = {};
+
+/* .data */
+int payload2_len1 = -1;
+int payload2_len2 = -1;
+int total2 = -1;
+char payload2[MAX_LEN + MAX_LEN] = { 1 };
+
+SEC("raw_tp/sys_enter")
+int handler64(void *regs)
+{
+	int pid = bpf_get_current_pid_tgid() >> 32;
+	void *payload = payload1;
+	u64 len;
+
+	/* ignore irrelevant invocations */
+	if (test_pid != pid || !capture)
+		return 0;
+
+	len = bpf_probe_read_kernel_str(payload, MAX_LEN, &buf_in1[0]);
+	if (len <= MAX_LEN) {
+		payload += len;
+		payload1_len1 = len;
+	}
+
+	len = bpf_probe_read_kernel_str(payload, MAX_LEN, &buf_in2[0]);
+	if (len <= MAX_LEN) {
+		payload += len;
+		payload1_len2 = len;
+	}
+
+	total1 = payload - (void *)payload1;
+
+	return 0;
+}
+
+SEC("tp_btf/sys_enter")
+int handler32(void *regs)
+{
+	int pid = bpf_get_current_pid_tgid() >> 32;
+	void *payload = payload2;
+	u32 len;
+
+	/* ignore irrelevant invocations */
+	if (test_pid != pid || !capture)
+		return 0;
+
+	len = bpf_probe_read_kernel_str(payload, MAX_LEN, &buf_in1[0]);
+	if (len <= MAX_LEN) {
+		payload += len;
+		payload2_len1 = len;
+	}
+
+	len = bpf_probe_read_kernel_str(payload, MAX_LEN, &buf_in2[0]);
+	if (len <= MAX_LEN) {
+		payload += len;
+		payload2_len2 = len;
+	}
+
+	total2 = payload - (void *)payload2;
+
+	return 0;
+}
+
+SEC("tp_btf/sys_exit")
+int handler_exit(void *regs)
+{
+	long bla;
+
+	if (bpf_probe_read_kernel(&bla, sizeof(bla), 0))
+		return 1;
+	else
+		return 0;
+}
+
+char LICENSE[] SEC("license") = "GPL";