diff --git a/Makefile.sys.inc b/Makefile.sys.inc
index c21ed19bdc62..6bb597de2fd7 100644
--- a/Makefile.sys.inc
+++ b/Makefile.sys.inc
@@ -33,6 +33,11 @@ _CAN_USE_AUTO_OBJ?= yes
 .if !defined(_CAN_USE_AUTO_OBJ)
 _MAKEARGS+=		MK_AUTO_OBJ=no
 MK_AUTO_OBJ=		no
+# This will prevent src.sys.obj.mk from opportunistically enabling AUTO_OBJ
+# in this make execution and for sub-makes.  For all of these targets we
+# just want to read any existing OBJDIR but we don't care if we can create
+# or write to them.
+.MAKEOVERRIDES+=	MK_AUTO_OBJ
 .endif
 .endif	# empty(.MAKEOVERRIDES:MMK_AUTO_OBJ)
 
diff --git a/share/mk/src.sys.obj.mk b/share/mk/src.sys.obj.mk
index fe15717d4f05..3671f974742a 100644
--- a/share/mk/src.sys.obj.mk
+++ b/share/mk/src.sys.obj.mk
@@ -19,6 +19,12 @@
 #  TARGET.TARGET_ARCH added in as it assumes that MAKEOBJDIRPREFIX is
 #  nested in the existing OBJTOP with TARGET.TARGET_ARCH in it.
 #
+#  The expected OBJDIR is stored in __objdir for auto.obj.mk to use.
+#
+#  AUTO_OBJ is opportunistically enabled if the computed .OBJDIR is writable
+#  by the current user.  Some top-level targets disable this behavior in
+#  Makefile.sys.inc.
+#
 
 _default_makeobjdirprefix?=	/usr/obj
 _default_makeobjdir=	$${.CURDIR:S,^$${SRCTOP},$${OBJTOP},}