passt: Introduce packet capture implementation

With -DDEBUG, passt now saves guest-side traffic captures in
pcap format at /tmp/passt_<ISO8601 timestamp>.pcap. The timestamp
refers to time and date of start-up.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

pcap.c

@@ -0,0 +1,108 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+/* PASST - Plug A Simple Socket Transport
+ *
+ * pcap.c - Packet capture for PASST
+ *
+ * Copyright (c) 2021 Red Hat GmbH
+ * Author: Stefano Brivio <sbrivio@redhat.com>
+ *
+ */
+
+#include <stdio.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+#include <sys/time.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <time.h>
+#include <net/ethernet.h>
+#include <unistd.h>
+
+#ifdef DEBUG
+
+#define PCAP_PREFIX		"/tmp/passt_"
+#define PCAP_ISO8601_FORMAT	"%FT%H:%M:%SZ"
+#define PCAP_ISO8601_STR	"YYYY-MM-ddTHH:mm:ssZ"
+
+#define PCAP_VERSION_MINOR 4
+
+static int pcap_fd = 1;
+
+/* See pcap.h from libpcap, or pcap-savefile(5) */
+static struct {
+	uint32_t magic;
+#define PCAP_MAGIC		0xa1b2c3d4
+
+	uint16_t major;
+#define PCAP_VERSION_MAJOR	2
+
+	uint16_t minor;
+#define PCAP_VERSION_MINOR	4
+
+	int32_t thiszone;
+	uint32_t sigfigs;
+	uint32_t snaplen;
+
+	uint32_t linktype;
+#define PCAP_LINKTYPE_ETHERNET	1
+} pcap_hdr = {
+	PCAP_MAGIC, PCAP_VERSION_MAJOR, PCAP_VERSION_MINOR, 0, 0, ETH_MAX_MTU,
+	PCAP_LINKTYPE_ETHERNET
+};
+
+struct pcap_pkthdr {
+	uint32_t tv_sec;
+	uint32_t tv_usec;
+	uint32_t caplen;
+	uint32_t len;
+};
+
+void pcap(char *pkt, size_t len)
+{
+	struct pcap_pkthdr h;
+	struct timeval tv;
+
+	if (pcap_fd == -1)
+		return;
+
+	gettimeofday(&tv, NULL);
+	h.tv_sec = tv.tv_sec;
+	h.tv_usec = tv.tv_usec;
+	h.caplen = h.len = len;
+
+	write(pcap_fd, &h, sizeof(h));
+	write(pcap_fd, pkt, len);
+}
+
+void pcap_init(void)
+{
+	char name[] = PCAP_PREFIX PCAP_ISO8601_STR ".pcap";
+	struct timeval tv;
+	struct tm *tm;
+
+	gettimeofday(&tv, NULL);
+	tm = localtime(&tv.tv_sec);
+	strftime(name + strlen(PCAP_PREFIX), sizeof(PCAP_ISO8601_STR) - 1,
+		 PCAP_ISO8601_FORMAT, tm);
+
+	pcap_fd = open(name, O_WRONLY | O_CREAT | O_APPEND | O_DSYNC,
+		       S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
+	if (pcap_fd == -1) {
+		perror("open");
+		return;
+	}
+
+	write(pcap_fd, &pcap_hdr, sizeof(pcap_hdr));
+}
+
+#else /* DEBUG */
+void pcap(char *pkt, size_t len) {
+	(void)pkt;
+	(void)len;
+}
+
+void pcap_init(void) { }
+#endif