It is annoying that the dmesg buffer can overflow and loose messages

undetected during debugging.  To make clear what happens, count the
dropped bytes and write message buffer full to syslogd.  This also
helps to have a reliable log system.
OK deraadt@ millert@ tedu@

diff --git a/sys/kern/subr_log.c b/sys/kern/subr_log.c
index 4c948e0..dc0d90c 100644 (file)
--- a/sys/kern/subr_log.c
+++ b/sys/kern/subr_log.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: subr_log.c,v 1.46 2016/06/08 11:11:47 bluhm Exp $     */
+/*     $OpenBSD: subr_log.c,v 1.47 2016/06/23 13:15:21 bluhm Exp $     */
 /*     $NetBSD: subr_log.c,v 1.11 1996/03/30 22:24:44 christos Exp $   */
 
 /*
@@ -155,6 +155,7 @@ msgbuf_putchar(struct msgbuf *mbp, const char c)
        if (mbp->msg_bufr == mbp->msg_bufx) {
                if (++mbp->msg_bufr >= mbp->msg_bufs)
                        mbp->msg_bufr = 0;
+               mbp->msg_bufd++;
        }
        splx(s);
 }
@@ -201,6 +202,19 @@ logread(dev_t dev, struct uio *uio, int flag)
        }
        logsoftc.sc_state &= ~LOG_RDWAIT;
 
+       if (mbp->msg_bufd > 0) {
+               char buf[64];
+
+               l = snprintf(buf, sizeof(buf),
+                   "<%d>klog: dropped %ld byte%s, message buffer full\n",
+                   LOG_KERN|LOG_WARNING, mbp->msg_bufd,
+                    mbp->msg_bufd == 1 ? "" : "s");
+               error = uiomove(buf, ulmin(l, sizeof(buf) - 1), uio);
+               if (error)
+                       goto out;
+               mbp->msg_bufd = 0;
+       }
+
        while (uio->uio_resid > 0) {
                if (mbp->msg_bufx >= mbp->msg_bufr)
                        l = mbp->msg_bufx - mbp->msg_bufr;

diff --git a/sys/sys/msgbuf.h b/sys/sys/msgbuf.h
index de4e5be..3a53a5a 100644 (file)
--- a/sys/sys/msgbuf.h
+++ b/sys/sys/msgbuf.h
@@ -1,4 +1,4 @@
-/*     $OpenBSD: msgbuf.h,v 1.10 2015/01/13 18:51:27 kettenis Exp $    */
+/*     $OpenBSD: msgbuf.h,v 1.11 2016/06/23 13:15:21 bluhm Exp $       */
 /*     $NetBSD: msgbuf.h,v 1.8 1995/03/26 20:24:27 jtc Exp $   */
 
 /*
@@ -39,6 +39,7 @@ struct        msgbuf {
        long    msg_bufr;               /* read pointer */
        long    msg_bufs;               /* real msg_bufc size (bytes) */
        long    msg_bufl;               /* # chars, <= msg_bufs */
+       long    msg_bufd;               /* number of dropped bytes */
        char    msg_bufc[1];            /* buffer */
 };
 #ifdef _KERNEL