Index: usr.sbin/syslogd/syslog.conf.5
===================================================================
--- usr.sbin/syslogd/syslog.conf.5
+++ usr.sbin/syslogd/syslog.conf.5
@@ -28,7 +28,7 @@
 .\"     @(#)syslog.conf.5	8.1 (Berkeley) 6/9/93
 .\" $FreeBSD$
 .\"
-.Dd November 1, 2016
+.Dd June 25, 2019
 .Dt SYSLOG.CONF 5
 .Os
 .Sh NAME
@@ -342,6 +342,21 @@
 .Dq -
 option may improve performance,
 especially if the kernel is logging many messages.
+.Pp
+Options may be specified after the pathname.
+The options, delimited by whitespace, take the form of a letter,
+potentially followed by an associated value.
+The available options are:
+.Bl -tag -indent
+.It Ar R size
+Limit the growth of the file to the given size.
+If the file exceeds that size, enter
+.Dq recycle
+mode, in which the last 32 KBytes of the file become a circular buffer
+which is continually overwritten, preserving the bulk of the original file.
+The size is specified in bytes, with the usual power-of-two suffixes
+recognized.
+.El
 .It
 A hostname (preceded by an at
 .Pq Dq @
Index: usr.sbin/syslogd/syslogd.c
===================================================================
--- usr.sbin/syslogd/syslogd.c
+++ usr.sbin/syslogd/syslogd.c
@@ -199,6 +199,24 @@
 static STAILQ_HEAD(, socklist) shead = STAILQ_HEAD_INITIALIZER(shead);
 
 /*
+ * If a file has a size limit specified, and that limit is exceeded, we treat
+ * the last few kbytes of the file as a circular buffer and repeatedly overwrite
+ * it.  When that happens we write a msg to the file saying that we're doing so.
+ * This effectively turns the end of the file into a small circular buffer but
+ * preserves the bulk of the file (and thus hopefully preserves some clue as to
+ * what began spewing to the log to create this condition).
+ *
+ */
+#define LIMIT_CIRCBUF_SIZE      (32 * 1024)
+#define LIMIT_CIRCBUF_MESSAGE                                                       \
+"\n"                                                                                \
+"\n*******************************************************************************" \
+"\nsyslogd: Configured file size limit has been exceeded; data has been lost."      \
+"\nsyslogd: From here down the file is being recycled in circular-buffer mode."     \
+"\n*******************************************************************************" \
+"\n"
+
+/*
  * Flags to logmsg().
  */
 
@@ -242,8 +260,11 @@
 			struct addrinfo *f_addr;
 
 		} f_forw;		/* forwarding address */
-		char	f_fname[MAXPATHLEN];
 		struct {
+			char	f_fname[MAXPATHLEN];
+			off_t   f_fsizelimit;
+		} f_logfile;
+		struct {
 			char	f_pname[MAXPATHLEN];
 			pid_t	f_pid;
 		} f_pipe;
@@ -251,7 +272,8 @@
 #define	fu_uname	f_un.f_uname
 #define	fu_forw_hname	f_un.f_forw.f_hname
 #define	fu_forw_addr	f_un.f_forw.f_addr
-#define	fu_fname	f_un.f_fname
+#define	fu_fname	f_un.f_logfile.f_fname
+#define	fu_fsizelimit	f_un.f_logfile.f_fsizelimit
 #define	fu_pipe_pname	f_un.f_pipe.f_pname
 #define	fu_pipe_pid	f_un.f_pipe.f_pid
 	char	f_prevline[MAXSVLINE];		/* last message logged */
@@ -1727,6 +1749,18 @@
 	case F_FILE:
 		dprintf(" %s\n", f->fu_fname);
 		iovlist_append(il, "\n");
+		/*
+		 * If this file has a size limit, and we're about to write to a
+		 * location beyond that limit, seek to limit-CIRCBUF_SIZE before
+		 * writing and drop a message into the file to say we lost data.
+		 */
+		if (f->fu_fsizelimit != 0 && f->fu_fsizelimit <
+		    lseek(f->f_file, 0, SEEK_CUR)) {
+			lseek(f->f_file, f->fu_fsizelimit - LIMIT_CIRCBUF_SIZE,
+			    SEEK_SET);
+			write(f->f_file, LIMIT_CIRCBUF_MESSAGE,
+			    sizeof(LIMIT_CIRCBUF_MESSAGE) - 1); 
+		}
 		if (writev(f->f_file, il->iov, il->iovcnt) < 0) {
 			/*
 			 * If writev(2) fails for potentially transient errors
@@ -2529,6 +2563,42 @@
 }
 
 /*
+ * Parse a filename config line to extract the filename and any options that may
+ * follow it on the line, storing the values into f->f_un.logfile.
+ */
+static void
+parse_fline(const char *p, struct filed *f)
+{
+	char *q, *r, *s;
+	uint64_t sizelimit;
+
+	(void)strlcpy(f->fu_fname, p, sizeof(f->fu_fname));
+
+	/* If there are no spaces on the line, no options follow the name. */
+	if ((q = strpbrk(f->fu_fname, "\t ")) == NULL)
+		return;
+
+	/* The only option we support right now is 'R' for recycle. */
+	r = q + strspn(q, "\t ");
+	if (*r++ != 'R')
+		return;
+
+	/* The thing following the R must be numeric. */
+	s = r + strspn(r, "\t ");
+	if (expand_number(s, &sizelimit) == -1)
+		return;
+
+	/*
+	 * Looks like a valid R <size> option is present.  Terminate fu_name at
+	 * the first space and store the limit associated with the file.
+	 */
+	*q = 0;
+	f->fu_fsizelimit = sizelimit;
+	if (f->fu_fsizelimit < LIMIT_CIRCBUF_SIZE)
+		f->fu_fsizelimit = LIMIT_CIRCBUF_SIZE;
+}
+
+/*
  * Crack a configuration file line
  */
 static struct filed *
@@ -2746,9 +2816,14 @@
 		break;
 
 	case '/':
-		if ((f->f_file = open(p, logflags, 0600)) < 0) {
+		/* Parse the filename and any options into f->f_un.f_logfile. */
+		parse_fline(p, f);
+		if (f->fu_fsizelimit != 0) {
+			logflags &= ~O_APPEND;
+		}
+		if ((f->f_file = open(f->fu_fname, logflags, 0600)) < 0) {
 			f->f_type = F_UNUSED;
-			logerror(p);
+			logerror(f->fu_fname);
 			break;
 		}
 		if (syncfile)
@@ -2761,7 +2836,12 @@
 			(void)strlcpy(f->fu_fname, p + sizeof(_PATH_DEV) - 1,
 			    sizeof(f->fu_fname));
 		} else {
-			(void)strlcpy(f->fu_fname, p, sizeof(f->fu_fname));
+			/*
+			 * O_APPEND isn't used when limiting file size; limiting
+			 * requires seeking.  Start by seeking to end of file.
+			 */
+			if (f->fu_fsizelimit != 0)
+				lseek(f->f_file, 0, SEEK_END);
 			f->f_type = F_FILE;
 		}
 		break;