samples: bpf: Add basic infrastructure for XDP samples

This file implements some common helpers to consolidate differences in
features and functionality between the various XDP samples and give them
a consistent look, feel, and reporting capabilities.

This commit only adds support for receive statistics, which does not
rely on any tracepoint, but on the XDP program installed on the device
by each XDP redirect sample.

Some of the key features are:
 * A concise output format accompanied by helpful text explaining its
   fields.
 * An elaborate output format building upon the concise one, and folding
   out details in case of errors and staying out of view otherwise.
 * Printing driver names for devices redirecting packets.
 * Getting mac address for interface.
 * Printing summarized total statistics for the entire session.
 * Ability to dynamically switch between concise and verbose mode, using
   SIGQUIT (Ctrl + \).

In later patches, the support will be extended for each tracepoint with
its own custom output in concise and verbose mode.
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210821002010.845777-4-memxor@gmail.com

samples/bpf/xdp_sample_shared.h

+// SPDX-License-Identifier: GPL-2.0-only
+#ifndef _XDP_SAMPLE_SHARED_H
+#define _XDP_SAMPLE_SHARED_H
+
+struct datarec {
+	size_t processed;
+	size_t dropped;
+	size_t issue;
+	union {
+		size_t xdp_pass;
+		size_t info;
+	};
+	size_t xdp_drop;
+	size_t xdp_redirect;
+} __attribute__((aligned(64)));
+
+#endif

samples/bpf/xdp_sample_user.h

+// SPDX-License-Identifier: GPL-2.0-only
+#ifndef XDP_SAMPLE_USER_H
+#define XDP_SAMPLE_USER_H
+
+#include <bpf/libbpf.h>
+#include <linux/compiler.h>
+
+#include "xdp_sample_shared.h"
+
+enum stats_mask {
+	_SAMPLE_REDIRECT_MAP        = 1U << 0,
+	SAMPLE_RX_CNT               = 1U << 1,
+};
+
+/* Exit return codes */
+#define EXIT_OK			0
+#define EXIT_FAIL		1
+#define EXIT_FAIL_OPTION	2
+#define EXIT_FAIL_XDP		3
+#define EXIT_FAIL_BPF		4
+#define EXIT_FAIL_MEM		5
+
+int sample_setup_maps(struct bpf_map **maps);
+int __sample_init(int mask);
+void sample_exit(int status);
+int sample_run(int interval, void (*post_cb)(void *), void *ctx);
+
+void sample_switch_mode(void);
+int sample_install_xdp(struct bpf_program *xdp_prog, int ifindex, bool generic,
+		       bool force);
+void sample_usage(char *argv[], const struct option *long_options,
+		  const char *doc, int mask, bool error);
+
+const char *get_driver_name(int ifindex);
+int get_mac_addr(int ifindex, void *mac_addr);
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstringop-truncation"
+__attribute__((unused))
+static inline char *safe_strncpy(char *dst, const char *src, size_t size)
+{
+	if (!size)
+		return dst;
+	strncpy(dst, src, size - 1);
+	dst[size - 1] = '\0';
+	return dst;
+}
+#pragma GCC diagnostic pop
+
+#define DEFINE_SAMPLE_INIT(name)                                               \
+	static int sample_init(struct name *skel, int mask)                    \
+	{                                                                      \
+		int ret;                                                       \
+		ret = __sample_init(mask);                                     \
+		if (ret < 0)                                                   \
+			return ret;                                            \
+		return 0;                                                      \
+	}
+
+#endif