monitor: add support for limiting PCAP size/count

This implements support for "rolling captures" by allowing iwmon to
limit the PCAP file size and number of PCAP's that are created.
This is a useful feature when long term monitoring is needed. If
there is some rare behavior requiring iwmon to run for days, months,
or longer the resulting PCAP file would become quite large and fill
up disk space.

When enabled (command line arguments in subsequent patch) the PCAP
file size is checked on each write. If it exceeds the limit a new
PCAP file will be created. Once the number of old PCAP files reaches
the set limit the oldest PCAP will be removed from disk.

monitor/nlmon.c

@@ -43,6 +43,7 @@
 #include <linux/genetlink.h>
 #include <linux/rtnetlink.h>
 #include <linux/filter.h>
+#include <linux/limits.h>
 #include <ell/ell.h>
 
 #ifndef ARPHRD_NETLINK
@@ -94,6 +95,8 @@
 #define BSS_CAPABILITY_APSD		(1<<11)
 #define BSS_CAPABILITY_DSSS_OFDM	(1<<13)
 
+#define BYTES_PER_MB			1000000
+
 struct nlmon *cur_nlmon;
 
 enum msg_type {
@@ -115,6 +118,11 @@ struct nlmon {
 	bool noies;
 	bool read;
 	enum time_format time_format;
+
+	char *file_prefix;
+	unsigned int file_idx;
+	unsigned int max_files;
+	unsigned int max_size;
 };
 
 struct nlmon_req {
@@ -7393,6 +7401,64 @@ static bool nlmon_req_match(const void *a, const void *b)
 	return (req->seq == match->seq && req->pid == match->pid);
 }
 
+/*
+ * Ensures that PCAP names are zero padded when needed. This makes the files
+ * sort correctly.
+ */
+static void next_pcap_name(char *buf, size_t size, const char *prefix,
+				unsigned int idx, unsigned int max)
+{
+	unsigned int ndigits = 1;
+
+	while (max > 9) {
+		max /= 10;
+		ndigits++;
+	}
+
+	snprintf(buf, size, "%s%.*u", prefix, ndigits, idx);
+}
+
+static bool check_pcap(struct nlmon *nlmon, size_t next_size)
+{
+	char path[PATH_MAX];
+
+	if (!nlmon->pcap)
+		return false;
+
+	if (!nlmon->max_size)
+		return true;
+
+	if (pcap_get_size(nlmon->pcap) + next_size <= nlmon->max_size)
+		return true;
+
+	pcap_close(nlmon->pcap);
+
+	/* Exausted the single PCAP file */
+	if (nlmon->max_files < 2) {
+		printf("Reached maximum size of PCAP, exiting\n");
+		nlmon->pcap = NULL;
+		l_main_quit();
+		return false;
+	}
+
+	next_pcap_name(path, sizeof(path), nlmon->file_prefix,
+			++nlmon->file_idx, nlmon->max_files);
+
+	nlmon->pcap = pcap_create(path);
+
+	if (nlmon->max_files > nlmon->file_idx)
+		return true;
+
+	/* Remove oldest PCAP file */
+	next_pcap_name(path, sizeof(path), nlmon->file_prefix,
+		nlmon->file_idx - nlmon->max_files, nlmon->max_files);
+
+	if (remove(path) < 0)
+		printf("Failed to remove old PCAP file %s\n", path);
+
+	return true;
+}
+
 static void store_packet(struct nlmon *nlmon, const struct timeval *tv,
 					uint16_t pkt_type,
 					uint16_t arphrd_type,
@@ -7401,7 +7467,7 @@ static void store_packet(struct nlmon *nlmon, const struct timeval *tv,
 {
 	uint8_t sll_hdr[16], *buf = sll_hdr;
 
-	if (!nlmon->pcap)
+	if (!check_pcap(nlmon, sizeof(sll_hdr) + size))
 		return;
 
 	memset(sll_hdr, 0, sizeof(sll_hdr));
@@ -7527,6 +7593,9 @@ struct nlmon *nlmon_create(uint16_t id, const struct nlmon_config *config)
 	nlmon->noies = config->noies;
 	nlmon->read = config->read_only;
 	nlmon->time_format = config->time_format;
+	nlmon->max_files = config->pcap_file_count;
+	/* Command line expects MB, but use bytes internally */
+	nlmon->max_size = config->pcap_file_size * BYTES_PER_MB;
 
 	return nlmon;
 }
@@ -8554,13 +8623,20 @@ struct nlmon *nlmon_open(uint16_t id, const char *pathname,
 	struct nlmon *nlmon;
 	struct l_io *pae_io;
 	struct pcap *pcap;
+	char path[PATH_MAX];
 
 	pae_io = open_pae();
 	if (!pae_io)
 		return NULL;
 
 	if (pathname) {
-		pcap = pcap_create(pathname);
+		if (config->pcap_file_count > 1)
+			next_pcap_name(path, sizeof(path), pathname,
+					0, config->pcap_file_count);
+		else
+			snprintf(path, sizeof(path), "%s", pathname);
+
+		pcap = pcap_create(path);
 		if (!pcap) {
 			l_io_destroy(pae_io);
 			return NULL;
@@ -8573,6 +8649,7 @@ struct nlmon *nlmon_open(uint16_t id, const char *pathname,
 
 	nlmon->pae_io = pae_io;
 	nlmon->pcap = pcap;
+	nlmon->file_prefix = l_strdup(pathname);
 
 	l_io_set_read_handler(nlmon->pae_io, pae_receive, nlmon, NULL);
 
@@ -8595,5 +8672,7 @@ void nlmon_close(struct nlmon *nlmon)
 	if (nlmon->pcap)
 		pcap_close(nlmon->pcap);
 
+	l_free(nlmon->file_prefix);
+
 	l_free(nlmon);
 }

monitor/nlmon.h

@@ -37,6 +37,10 @@ struct nlmon_config {
 	bool noies;
 	bool read_only;
 	enum time_format time_format;
+
+	/* File size in MB */
+	uint32_t pcap_file_size;
+	uint32_t pcap_file_count;
 };
 
 struct nlmon *nlmon_open(uint16_t id, const char *pathname,