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/progs/test_varlen.c

@@ -0,0 +1,96 @@
+// 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";