[PATCH v3 4/7] log, conf: Add support for logging to file

In some environments, such as KubeVirt pods, we might not have a
system logger available. We could choose to run in foreground, but
this takes away the convenient synchronisation mechanism derived from
forking to background when interfaces are ready.

Add optional logging to file with -l/--log-file and --log-size.

Unfortunately, this means we need to duplicate features that are more
appropriately implemented by a system logger, such as rotation. Keep
that reasonably simple, by using fallocate() with range collapsing
where supported (Linux kernel >= 3.15, extent-based ext4 and XFS) and
falling back to an unsophisticated block-by-block moving of entries
toward the beginning of the file once we reach the (mandatory) size
limit.

While at it, clarify the role of LOG_EMERG in passt.c.

Signed-off-by: Stefano Brivio <sbrivio(a)redhat.com>
---
v3:
  - don't use FALLOC_FL_COLLAPSE_RANGE if not defined in headers (e.g.
    glibc < 2.18), and add fallocate() to syscall list optionally using
    EXTRA_SYSCALLS in Makefile, depending on availability of that flag
  - reported by clang-tidy: perform multiplication for
    LOGFILE_SIZE_DEFAULT as unsigned long

v2:
  - Make truncation on different failure modes in logfile_rotate_move()
    consistent (David Gibson)
  - Actually define default maximum log size as 1 MiB, not 1 MB (David
    Gibson)

 Makefile  |  15 ++--
 README.md |   2 +-
 conf.c    |  49 ++++++++++-
 log.c     | 258 ++++++++++++++++++++++++++++++++++++++++++++++++++----
 log.h     |   6 ++
 passt.1   |  14 ++-
 passt.c   |   1 +
 7 files changed, 316 insertions(+), 29 deletions(-)

diff --git a/Makefile b/Makefile
index 74bbfeb..e4c64fe 100644
--- a/Makefile
+++ b/Makefile
@@ -41,7 +41,7 @@ SRCS = $(PASST_SRCS) $(QRAP_SRCS)
 MANPAGES = passt.1 pasta.1 qrap.1
 
 PASST_HEADERS = arch.h arp.h checksum.h conf.h dhcp.h dhcpv6.h icmp.h \
-	isolation.h lineread.h ndp.h netlink.h packet.h passt.h pasta.h \
+	isolation.h lineread.h log.h ndp.h netlink.h packet.h passt.h pasta.h \
 	pcap.h port_fwd.h siphash.h tap.h tcp.h tcp_splice.h udp.h util.h
 HEADERS = $(PASST_HEADERS) seccomp.h
 
@@ -90,6 +90,11 @@ ifeq ($(shell :|$(CC) -fstack-protector-strong -S -xc - -o -
>/dev/null 2>&1; ec
 	FLAGS += -fstack-protector-strong
 endif
 
+C := \#define _GNU_SOURCE\n\#include <fcntl.h>\nx = FALLOC_FL_COLLAPSE_RANGE;
+ifeq ($(shell printf "$(C)" | $(CC) -S -xc - -o - >/dev/null 2>&1;
echo $$?),0)
+	EXTRA_SYSCALLS += fallocate
+endif
+
 prefix		?= /usr/local
 exec_prefix	?= $(prefix)
 bindir		?= $(exec_prefix)/bin
@@ -110,7 +115,7 @@ static: FLAGS += -static -DGLIBC_NO_STATIC_NSS
 static: clean all
 
 seccomp.h: seccomp.sh $(PASST_SRCS) $(PASST_HEADERS)
-	@ EXTRA_SYSCALLS=$(EXTRA_SYSCALLS) ./seccomp.sh $(PASST_SRCS) $(PASST_HEADERS)
+	@ EXTRA_SYSCALLS="$(EXTRA_SYSCALLS)" ./seccomp.sh $(PASST_SRCS)
$(PASST_HEADERS)
 
 passt: $(PASST_SRCS) $(HEADERS)
 	$(CC) $(FLAGS) $(CFLAGS) $(PASST_SRCS) -o passt $(LDFLAGS)
@@ -128,9 +133,9 @@ pasta.avx2 pasta.1 pasta: pasta%: passt%
 qrap: $(QRAP_SRCS) passt.h
 	$(CC) $(FLAGS) $(CFLAGS) $(QRAP_SRCS) -o qrap $(LDFLAGS)
 
-valgrind: EXTRA_SYSCALLS="rt_sigprocmask rt_sigtimedwait rt_sigaction \
-			  getpid gettid kill clock_gettime mmap munmap open \
-			  unlink gettimeofday futex"
+valgrind: EXTRA_SYSCALLS += rt_sigprocmask rt_sigtimedwait rt_sigaction	\
+			    getpid gettid kill clock_gettime mmap	\
+			    munmap open unlink gettimeofday futex
 valgrind: FLAGS:=-g -O0 $(filter-out -O%,$(FLAGS))
 valgrind: all
 
diff --git a/README.md b/README.md
index 2c522b2..3dc4fc5 100644
--- a/README.md
+++ b/README.md
@@ -289,7 +289,7 @@ speeding up local connections, and usually requiring NAT. _pasta_:
 * ✅ all capabilities dropped, other than `CAP_NET_BIND_SERVICE` (if granted)
 * ✅ with default options, user, mount, IPC, UTS, PID namespaces are detached
 * ✅ no external dependencies (other than a standard C library)
-* ✅ restrictive seccomp profiles (26 syscalls allowed for _passt_, 40 for
+* ✅ restrictive seccomp profiles (30 syscalls allowed for _passt_, 41 for
   _pasta_ on x86_64)
 * ✅ examples of [AppArmor](/passt/tree/contrib/apparmor) and
   [SELinux](/passt/tree/contrib/selinux) profiles available
diff --git a/conf.c b/conf.c
index e9cd4a7..f22940b 100644
--- a/conf.c
+++ b/conf.c
@@ -633,6 +633,9 @@ static void usage(const char *name)
 	info(   "    default: run in background if started from a TTY");
 	info(   "  -e, --stderr		Log to stderr too");
 	info(   "    default: log to system logger only if started from a TTY");
+	info(   "  -l, --log-file PATH	Log (only) to given file");
+	info(   "  --log-size BYTES	Maximum size of log file");
+	info(   "    default: 1 MiB");
 	info(   "  --runas UID|UID:GID 	Run as given UID, GID, which can be");
 	info(   "    numeric, or login and group names");
 	info(   "    default: drop to user \"nobody\"");
@@ -994,6 +997,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		{"quiet",	no_argument,		NULL,		'q' },
 		{"foreground",	no_argument,		NULL,		'f' },
 		{"stderr",	no_argument,		NULL,		'e' },
+		{"log-file",	required_argument,	NULL,		'l' },
 		{"help",	no_argument,		NULL,		'h' },
 		{"socket",	required_argument,	NULL,		's' },
 		{"ns-ifname",	required_argument,	NULL,		'I' },
@@ -1034,26 +1038,28 @@ void conf(struct ctx *c, int argc, char **argv)
 		{"no-netns-quit", no_argument,		NULL,		10 },
 		{"trace",	no_argument,		NULL,		11 },
 		{"runas",	required_argument,	NULL,		12 },
+		{"log-size",	required_argument,	NULL,		13 },
 		{ 0 },
 	};
 	struct get_bound_ports_ns_arg ns_ports_arg = { .c = c };
 	char userns[PATH_MAX] = { 0 }, netns[PATH_MAX] = { 0 };
 	bool v4_only = false, v6_only = false;
+	char *runas = NULL, *logfile = NULL;
 	struct in6_addr *dns6 = c->ip6.dns;
 	struct fqdn *dnss = c->dns_search;
 	uint32_t *dns4 = c->ip4.dns;
 	int name, ret, mask, b, i;
 	const char *optstring;
 	unsigned int ifi = 0;
-	char *runas = NULL;
+	size_t logsize = 0;
 	uid_t uid;
 	gid_t gid;
 
 	if (c->mode == MODE_PASTA) {
 		c->no_dhcp_dns = c->no_dhcp_dns_search = 1;
-		optstring = "dqfehI:p:P:m:a:n:M:g:i:D:S:46t:u:T:U:";
+		optstring = "dqfel:hI:p:P:m:a:n:M:g:i:D:S:46t:u:T:U:";
 	} else {
-		optstring = "dqfehs:p:P:m:a:n:M:g:i:D:S:46t:u:";
+		optstring = "dqfel:hs:p:P:m:a:n:M:g:i:D:S:46t:u:";
 	}
 
 	c->tcp.fwd_in.mode = c->tcp.fwd_out.mode = 0;
@@ -1177,6 +1183,20 @@ void conf(struct ctx *c, int argc, char **argv)
 
 			runas = optarg;
 			break;
+		case 13:
+			if (logsize) {
+				err("Multiple --log-size options given");
+				usage(argv[0]);
+			}
+
+			errno = 0;
+			logsize = strtol(optarg, NULL, 0);
+
+			if (logsize < LOGFILE_SIZE_MIN || errno) {
+				err("Invalid --log-size: %s", optarg);
+				usage(argv[0]);
+			}
+			break;
 		case 'd':
 			if (c->debug) {
 				err("Multiple --debug options given");
@@ -1192,6 +1212,11 @@ void conf(struct ctx *c, int argc, char **argv)
 			c->foreground = 1;
 			break;
 		case 'e':
+			if (logfile) {
+				err("Can't log to both file and stderr");
+				usage(argv[0]);
+			}
+
 			if (c->stderr) {
 				err("Multiple --stderr options given");
 				usage(argv[0]);
@@ -1199,6 +1224,19 @@ void conf(struct ctx *c, int argc, char **argv)
 
 			c->stderr = 1;
 			break;
+		case 'l':
+			if (c->stderr) {
+				err("Can't log to both stderr and file");
+				usage(argv[0]);
+			}
+
+			if (logfile) {
+				err("Multiple --log-file options given");
+				usage(argv[0]);
+			}
+
+			logfile = optarg;
+			break;
 		case 'q':
 			if (c->quiet) {
 				err("Multiple --quiet options given");
@@ -1460,6 +1498,11 @@ void conf(struct ctx *c, int argc, char **argv)
 	if (ret)
 		usage(argv[0]);
 
+	if (logfile) {
+		logfile_init(c->mode == MODE_PASST ? "passt" : "pasta",
+			     logfile, logsize);
+	}
+
 	if (c->mode == MODE_PASTA) {
 		if (conf_pasta_ns(&netns_only, userns, netns,
 				  optind, argc, argv) < 0)
diff --git a/log.c b/log.c
index 54a7cbb..d769a33 100644
--- a/log.c
+++ b/log.c
@@ -12,7 +12,12 @@
  * Author: Stefano Brivio &lt;sbrivio(a)redhat.com&gt;
  */
 
+#include <arpa/inet.h>
+#include <limits.h>
+#include <errno.h>
+#include <fcntl.h>
 #include <stdio.h>
+#include <stdint.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <string.h>
@@ -20,36 +25,49 @@
 #include <syslog.h>
 #include <stdarg.h>
 #include <sys/socket.h>
-#include <sys/un.h>
 
 #include "log.h"
+#include "util.h"
+#include "passt.h"
 
-/* For __openlog() and __setlogmask() wrappers, and passt_vsyslog() */
-static int	log_mask;
-static int	log_sock = -1;
-static char	log_ident[BUFSIZ];
-static int	log_opt;
-static time_t	log_debug_start;
-int		log_trace;
+static int	log_sock = -1;		/* Optional socket to system logger */
+static char	log_ident[BUFSIZ];	/* Identifier string for openlog() */
+static int	log_mask;		/* Current log priority mask */
+static int	log_opt;		/* Options for openlog() */
+
+static int	log_file = -1;		/* Optional log file descriptor */
+static size_t	log_size;		/* Maximum log file size in bytes */
+static size_t	log_written;		/* Currently used bytes in log file */
+static size_t	log_cut_size;		/* Bytes to cut at start on rotation */
+static char	log_header[BUFSIZ];	/* File header, written back on cuts */
+
+static time_t	log_start;		/* Start timestamp */
+int		log_trace;		/* --trace mode enabled */
 
 #define logfn(name, level)						\
 void name(const char *format, ...) {					\
 	struct timespec tp;						\
 	va_list args;							\
 									\
-	if (setlogmask(0) & LOG_MASK(LOG_DEBUG)) {			\
+	if (setlogmask(0) & LOG_MASK(LOG_DEBUG) && log_file == -1) {	\
 		clock_gettime(CLOCK_REALTIME, &tp);			\
 		fprintf(stderr, "%li.%04li: ",				\
-			tp.tv_sec - log_debug_start,			\
+			tp.tv_sec - log_start,				\
 			tp.tv_nsec / (100L * 1000));			\
-	} else {							\
+	}								\
+									\
+	if ((LOG_MASK(LOG_PRI(level)) & log_mask) ||			\
+	    setlogmask(0) == LOG_MASK(LOG_EMERG)) {			\
 		va_start(args, format);					\
-		passt_vsyslog(level, format, args);			\
+		if (log_file != -1)					\
+			logfile_write(level, format, args);		\
+		else if (!(setlogmask(0) & LOG_MASK(LOG_DEBUG)))	\
+			passt_vsyslog(level, format, args);		\
 		va_end(args);						\
 	}								\
 									\
-	if (setlogmask(0) & LOG_MASK(LOG_DEBUG) ||			\
-	    setlogmask(0) == LOG_MASK(LOG_EMERG)) {			\
+	if ((setlogmask(0) & LOG_MASK(LOG_DEBUG) ||			\
+	     setlogmask(0) == LOG_MASK(LOG_EMERG)) && log_file == -1) {	\
 		va_start(args, format);					\
 		(void)vfprintf(stderr, format, args); 			\
 		va_end(args);						\
@@ -58,6 +76,16 @@ void name(const char *format, ...) {					\
 	}								\
 }
 
+/* Prefixes for log file messages, indexed by priority */
+const char *logfile_prefix[] = {
+	NULL, NULL, NULL,	/* Unused: LOG_EMERG, LOG_ALERT, LOG_CRIT */
+	"ERROR:   ",
+	"WARNING: ",
+	NULL,			/* Unused: LOG_NOTICE */
+	"info:    ",
+	"         ",		/* LOG_DEBUG */
+};
+
 logfn(err,   LOG_ERR)
 logfn(warn,  LOG_WARNING)
 logfn(info,  LOG_INFO)
@@ -79,7 +107,7 @@ void __openlog(const char *ident, int option, int facility)
 	struct timespec tp;
 
 	clock_gettime(CLOCK_REALTIME, &tp);
-	log_debug_start = tp.tv_sec;
+	log_start = tp.tv_sec;
 
 	if (log_sock < 0) {
 		struct sockaddr_un a = { .sun_family = AF_UNIX, };
@@ -124,9 +152,6 @@ void passt_vsyslog(int pri, const char *format, va_list ap)
 	char buf[BUFSIZ];
 	int n;
 
-	if (!(LOG_MASK(LOG_PRI(pri)) & log_mask))
-		return;
-
 	/* Send without name and timestamp, the system logger should add them */
 	n = snprintf(buf, BUFSIZ, "<%i> ", pri);
 
@@ -141,3 +166,200 @@ void passt_vsyslog(int pri, const char *format, va_list ap)
 	if (send(log_sock, buf, n, 0) != n)
 		fprintf(stderr, "Failed to send %i bytes to syslog\n", n);
 }
+
+/**
+ * logfile_init() - Open log file and write header with PID and path
+ * @name:	Identifier for header: passt or pasta
+ * @path:	Path to log file
+ * @size:	Maximum size of log file: log_cut_size is calculatd here
+ */
+void logfile_init(const char *name, const char *path, size_t size)
+{
+	char nl = '\n', exe[PATH_MAX] = { 0 };
+	int n;
+
+	if (readlink("/proc/self/exe", exe, PATH_MAX - 1) < 0) {
+		perror("readlink /proc/self/exe");
+		exit(EXIT_FAILURE);
+	}
+
+	log_file = open(path, O_CREAT | O_TRUNC | O_APPEND | O_RDWR | O_CLOEXEC,
+			S_IRUSR | S_IWUSR);
+	if (log_file == -1) {
+		err("Couldn't open log file %s: %s", path, strerror(errno));
+		exit(EXIT_FAILURE);
+	}
+
+	log_size = size ? size : LOGFILE_SIZE_DEFAULT;
+
+	n = snprintf(log_header, sizeof(log_header), "%s: %s (%i)",
+		     name, exe, getpid());
+
+	if (write(log_file, log_header, n) <= 0 ||
+	    write(log_file, &nl, 1) <= 0) {
+		perror("Couldn't write to log file\n");
+		exit(EXIT_FAILURE);
+	}
+
+	/* For FALLOC_FL_COLLAPSE_RANGE: VFS block size can be up to one page */
+	log_cut_size = ROUND_UP(log_size * LOGFILE_CUT_RATIO / 100, PAGE_SIZE);
+}
+
+#ifdef FALLOC_FL_COLLAPSE_RANGE
+/**
+ * logfile_rotate_fallocate() - Write header, set log_written after fallocate()
+ * @fd:		Log file descriptor
+ * @ts:		Current timestamp
+ *
+ * #syscalls lseek ppc64le:_llseek ppc64:_llseek armv6l:_llseek armv7l:_llseek
+ */
+static void logfile_rotate_fallocate(int fd, struct timespec *ts)
+{
+	char buf[BUFSIZ], *nl;
+	int n;
+
+	if (lseek(fd, 0, SEEK_SET) == -1)
+		return;
+	if (read(fd, buf, BUFSIZ) == -1)
+		return;
+
+	n = snprintf(buf, BUFSIZ,
+		     "%s - log truncated at %li.%04li", log_header,
+		     ts->tv_sec - log_start, ts->tv_nsec / (100L * 1000));
+
+	/* Avoid partial lines by padding the header with spaces */
+	nl = memchr(buf + n + 1, '\n', BUFSIZ - n - 1);
+	if (nl)
+		memset(buf + n, ' ', nl - (buf + n));
+
+	if (lseek(fd, 0, SEEK_SET) == -1)
+		return;
+	if (write(fd, buf, BUFSIZ) == -1)
+		return;
+
+	log_written -= log_cut_size;
+}
+#endif /* FALLOC_FL_COLLAPSE_RANGE */
+
+/**
+ * logfile_rotate_move() - Fallback: move recent entries toward start, then cut
+ * @fd:		Log file descriptor
+ * @ts:		Current timestamp
+ *
+ * #syscalls lseek ppc64le:_llseek ppc64:_llseek armv6l:_llseek armv7l:_llseek
+ * #syscalls ftruncate
+ */
+static void logfile_rotate_move(int fd, struct timespec *ts)
+{
+	int header_len, write_offset, end, discard, n;
+	char buf[BUFSIZ], *nl;
+
+	header_len = snprintf(buf, BUFSIZ,
+			      "%s - log truncated at %li.%04li\n", log_header,
+			      ts->tv_sec - log_start,
+			      ts->tv_nsec / (100L * 1000));
+	if (lseek(fd, 0, SEEK_SET) == -1)
+		return;
+	if (write(fd, buf, header_len) == -1)
+		return;
+
+	end = write_offset = header_len;
+	discard = log_cut_size + header_len;
+
+	/* Try to cut cleanly at newline */
+	if (lseek(fd, discard, SEEK_SET) == -1)
+		goto out;
+	if ((n = read(fd, buf, BUFSIZ)) <= 0)
+		goto out;
+	if ((nl = memchr(buf, '\n', n)))
+		discard += (nl - buf) + 1;
+
+	/* Go to first block to be moved */
+	if (lseek(fd, discard, SEEK_SET) == -1)
+		goto out;
+
+	while ((n = read(fd, buf, BUFSIZ)) > 0) {
+		end = header_len;
+
+		if (lseek(fd, write_offset, SEEK_SET) == -1)
+			goto out;
+		if ((n = write(fd, buf, n)) == -1)
+			goto out;
+		write_offset += n;
+
+		if ((n = lseek(fd, 0, SEEK_CUR)) == -1)
+			goto out;
+
+		if (lseek(fd, discard - header_len, SEEK_CUR) == -1)
+			goto out;
+
+		end = n;
+	}
+
+out:
+	if (ftruncate(fd, end))
+		return;
+
+	log_written = end;
+}
+
+/**
+ * logfile_rotate() - "Rotate" log file once it's full
+ * @fd:		Log file descriptor
+ * @ts:		Current timestamp
+ *
+ * Return: 0 on success, negative error code on failure
+ *
+ * #syscalls fcntl
+ *
+ * fallocate() passed as EXTRA_SYSCALL only if FALLOC_FL_COLLAPSE_RANGE is there
+ */
+static int logfile_rotate(int fd, struct timespec *ts)
+{
+	if (fcntl(fd, F_SETFL, O_RDWR /* Drop O_APPEND: explicit lseek() */))
+		return -errno;
+
+#ifdef FALLOC_FL_COLLAPSE_RANGE
+	/* Only for Linux >= 3.15, extent-based ext4 or XFS, glibc >= 2.18 */
+	if (!fallocate(fd, FALLOC_FL_COLLAPSE_RANGE, 0, log_cut_size))
+		logfile_rotate_fallocate(fd, ts);
+	else
+#endif
+		logfile_rotate_move(fd, ts);
+
+	if (fcntl(fd, F_SETFL, O_RDWR | O_APPEND))
+		return -errno;
+
+	return 0;
+}
+
+/**
+ * logfile_write() - Write entry to log file, trigger rotation if full
+ * @pri:	Facility and level map, same as priority for vsyslog()
+ * @format:	Same as vsyslog() format
+ * @ap:		Same as vsyslog() ap
+ */
+void logfile_write(int pri, const char *format, va_list ap)
+{
+	struct timespec ts;
+	char buf[BUFSIZ];
+	int n;
+
+	if (clock_gettime(CLOCK_REALTIME, &ts))
+		return;
+
+	n = snprintf(buf, BUFSIZ, "%li.%04li: %s",
+		     ts.tv_sec - log_start, ts.tv_nsec / (100L * 1000),
+		     logfile_prefix[pri]);
+
+	n += vsnprintf(buf + n, BUFSIZ - n, format, ap);
+
+	if (format[strlen(format)] != '\n')
+		n += snprintf(buf + n, BUFSIZ - n, "\n");
+
+	if ((log_written + n >= log_size) && logfile_rotate(log_file, &ts))
+		return;
+
+	if ((n = write(log_file, buf, n)) >= 0)
+		log_written += n;
+}
diff --git a/log.h b/log.h
index 70cad6f..f92394c 100644
--- a/log.h
+++ b/log.h
@@ -6,6 +6,10 @@
 #ifndef LOG_H
 #define LOG_H
 
+#define LOGFILE_SIZE_DEFAULT		(1024 * 1024UL)
+#define LOGFILE_CUT_RATIO		30	/* When full, cut ~30% size */
+#define LOGFILE_SIZE_MIN		(5UL * MAX(BUFSIZ, PAGE_SIZE))
+
 void err(const char *format, ...);
 void warn(const char *format, ...);
 void info(const char *format, ...);
@@ -20,7 +24,9 @@ void trace_init(int enable);
 	} while (0)
 
 void __openlog(const char *ident, int option, int facility);
+void logfile_init(const char *name, const char *path, size_t size);
 void passt_vsyslog(int pri, const char *format, va_list ap);
+void logfile_write(int pri, const char *format, va_list ap);
 void __setlogmask(int mask);
 
 #endif /* LOG_H */
diff --git a/passt.1 b/passt.1
index 83395bc..64236b6 100644
--- a/passt.1
+++ b/passt.1
@@ -79,7 +79,7 @@ for performance reasons.
 
 .TP
 .BR \-d ", " \-\-debug
-Be verbose, don't run in background.
+Be verbose, don't run in background, don't log to the system logger.
 
 .TP
 .BR \-\-trace
@@ -99,9 +99,19 @@ Default is to fork into background.
 .TP
 .BR \-e ", " \-\-stderr
 Log to standard error too.
-Default is to log to system logger only, if started from an interactive
+Default is to log to the system logger only, if started from an interactive
 terminal, and to both system logger and standard error otherwise.
 
+.TP
+.BR \-l ", " \-\-log-file " " \fIPATH\fR
+Log to file \fIPATH\fR, not to standard error, and not to the system logger.
+
+.TP
+.BR \-\-log-size " " \fISIZE\fR
+Limit log file size to \fISIZE\fR bytes. When the log file is full, make room
+for new entries by removing old ones at the beginning. This limit is mandatory.
+Default is 1048576 (1 MiB).
+
 .TP
 .BR \-\-runas " " \fIUID\fR|\fIUID:GID\fR|\fILOGIN\fR|\fILOGIN:GROUP\fR
 Attempt to change to given UID and corresponding group if UID is given,
diff --git a/passt.c b/passt.c
index 0c561b2..7589b05 100644
--- a/passt.c
+++ b/passt.c
@@ -220,6 +220,7 @@ int main(int argc, char **argv)
 
 	__openlog(log_name, 0, LOG_DAEMON);
 
+	/* Meaning we don't know yet: log everything. LOG_EMERG is unused */
 	__setlogmask(LOG_MASK(LOG_EMERG));
 
 	c.epollfd = epoll_create1(EPOLL_CLOEXEC);
-- 
2.35.1