untrusted comment: verify with openbsd-68-base.pub
RWQZj25CSG5R2qa51GQoCWnUxwiJ9TEiQIOeVuan6C6DmqWoH3dWRB+pKk/1e+6tXAgtcpDItVu2ZZLuOfvAseAfangOp+FG0Ac=

OpenBSD 6.8 errata 026, July 25, 2021:

On mips64, the strchr/index/strrchr/rindex functions in libc handled
signed characters incorrectly.

Apply by doing:
   signify -Vep /etc/signify/openbsd-68-base.pub -x 026_strchr.patch.sig \
       -m - | (cd /usr/src && patch -p0)

And then rebuild and install libc, and statically linked binaries:
   cd /usr/src/lib/libc
   make obj && make && make install
   cd /usr/src/bin
   make obj && make && make install
   cd /usr/src/sbin
   make obj && make && make install
   cd /usr/src/usr.bin/kdump
   make obj && make && make install
   cd /usr/src/usr.bin/ktrace
   make obj && make && make install
   cd /usr/src/usr.sbin/chroot
   make obj && make && make install
   cd /usr/src/usr.sbin/installboot
   make obj && make && make install
   cd /usr/src/usr.sbin/watchdogd
   make obj && make && make install

Index: lib/libc/arch/mips64/string/strchr.S
===================================================================
RCS file: /cvs/src/lib/libc/arch/mips64/string/strchr.S,v
diff -u -p -u -r1.3 strchr.S
--- lib/libc/arch/mips64/string/strchr.S        31 Aug 2015 02:53:56 -0000      1.3
+++ lib/libc/arch/mips64/string/strchr.S        24 Jul 2021 14:56:36 -0000
@@ -37,6 +37,7 @@ WEAK_ALIAS(index, strchr)

LEAF(strchr, 0)
       .set    reorder
+       andi    a1, 0xff
0:
       lbu     a2, 0(a0)               # get a byte
       daddu   a0, a0, 1
Index: lib/libc/arch/mips64/string/strrchr.S
===================================================================
RCS file: /cvs/src/lib/libc/arch/mips64/string/strrchr.S,v
diff -u -p -u -r1.3 strrchr.S
--- lib/libc/arch/mips64/string/strrchr.S       31 Aug 2015 02:53:56 -0000      1.3
+++ lib/libc/arch/mips64/string/strrchr.S       24 Jul 2021 14:56:36 -0000
@@ -38,6 +38,7 @@ WEAK_ALIAS(rindex, strrchr)
LEAF(strrchr, 0)
       .set    reorder
       move    v0, zero                # default if not found
+       andi    a1, 0xff
1:
       lbu     a3, 0(a0)               # get a byte
       daddu   a0, a0, 1