untrusted comment: signature from openbsd 6.1 base secret key
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OpenBSD 6.1 errata 016, August 3, 2017:

A missing length check in sendsyslog() may result in a kernel panic.

Apply by doing:
   signify -Vep /etc/signify/openbsd-61-base.pub -x 016_sendsyslog.patch.sig \
       -m - | (cd /usr/src && patch -p0)

And then rebuild and install a new kernel:
   KK=`sysctl -n kern.osversion | cut -d# -f1`
   cd /usr/src/sys/arch/`machine`/compile/$KK
   make obj
   make config
   make
   make install

Index: sys/kern/subr_log.c
===================================================================
RCS file: /cvs/src/sys/kern/subr_log.c,v
--- sys/kern/subr_log.c 27 Mar 2017 11:45:49 -0000      1.50
+++ sys/kern/subr_log.c 18 Jul 2017 22:22:19 -0000      1.51
@@ -417,6 +417,9 @@ dosendsyslog(struct proc *p, const char
       struct uio auio;
       size_t i, len;
       int error;
+
+       if (nbyte > 8192)
+               nbyte = 8192;

       /* Global variable syslogf may change during sleep, use local copy. */
       fp = syslogf;