Index: UPDATING
===================================================================
--- UPDATING
+++ UPDATING
@@ -51,6 +51,14 @@
 
 ****************************** SPECIAL WARNING: ******************************
 
+20171028:
+	Building in a FreeBSD src checkout will automatically created object
+	directories now rather than store files in the current directory if
+	'make obj' was not ran.  Calling 'make obj' is no longer necesarry.
+	This feature can be disabled by setting WITHOUT_AUTO_OBJ=yes in
+	/etc/src-env.conf (not /etc/src.conf), or passing the option in the
+	environment.
+
 20171028:
 	The default MAKEOBJDIR has changed from /usr/obj/<srcdir> for native
 	builds, and /usr/obj/<arch>/<srcdir> for cross-builds, to a unified
Index: share/mk/src.sys.mk
===================================================================
--- share/mk/src.sys.mk
+++ share/mk/src.sys.mk
@@ -14,7 +14,7 @@
 
 # Validate that the user didn't try setting an env-only variable in
 # their src.conf. This benefits from already including bsd.mkopt.mk.
-.for var in ${__ENV_ONLY_OPTIONS}
+.for var in ${__ENV_ONLY_OPTIONS:O:u}
 __presrcconf_${var}:=	${MK_${var}:U-}${WITHOUT_${var}:Uno:Dyes}${WITH_${var}:Uno:Dyes}
 .endfor
 
@@ -22,7 +22,7 @@
 _srcconf_included_:	.NOTMAIN
 
 # Validate the env-only variables.
-.for var in ${__ENV_ONLY_OPTIONS}
+.for var in ${__ENV_ONLY_OPTIONS:O:u}
 __postrcconf_${var}:=	${MK_${var}:U-}${WITHOUT_${var}:Uno:Dyes}${WITH_${var}:Uno:Dyes}
 .if ${__presrcconf_${var}} != ${__postrcconf_${var}}
 .error Option ${var} may only be defined in ${SRC_ENV_CONF}, environment, or make argument, not ${SRCCONF}.
Index: share/mk/src.sys.obj.mk
===================================================================
--- share/mk/src.sys.obj.mk
+++ share/mk/src.sys.obj.mk
@@ -93,7 +93,68 @@
 OBJROOT:=	${OBJTOP}/
 .endif
 
-# Assign this directory as .OBJDIR if possible
+# Try to enable MK_AUTO_OBJ by default if we can write to the OBJROOT.  Only
+# do this if AUTO_OBJ is not disabled by the user, not cleaning, and this
+# is the first make ran.
+.if ${.MAKE.LEVEL} == 0 && \
+    ${MK_AUTO_OBJ} == "no" && empty(.MAKEOVERRIDES:MMK_AUTO_OBJ) && \
+    !defined(WITHOUT_AUTO_OBJ) && !make(showconfig) && !make(print-dir) && \
+    !defined(NO_OBJ) && \
+    (${.TARGETS} == "" || ${.TARGETS:Nclean*:N*clean:Ndestroy*} != "")
+# Find the last existing directory component and check if we can write to it.
+# If the last component is a symlink then recurse on the new path.
+CheckAutoObj= \
+DirIsCreatable() { \
+	[ -w "$${1}" ] && return 0; \
+	d="$${1}"; \
+	IFS=/; \
+	set -- $${d}; \
+	unset dir; \
+	while [ $$\# -gt 0 ]; do \
+		d="$${1}"; \
+		shift; \
+		if ! [ -d "$${dir}$${d}/" ]; then \
+			if [ -L "$${dir}$${d}" ]; then \
+				dir="$$(readlink "$${dir}$${d}")/"; \
+				for d in "$${@}"; do \
+					dir="$${dir}$${d}/"; \
+				done; \
+				ret=0; \
+				DirIsCreatable "$${dir%/}" || ret=$$?; \
+				return $${ret}; \
+			else \
+				break; \
+			fi; \
+		fi; \
+		dir="$${dir}$${d}/"; \
+	done; \
+	[ -w "$${dir}" ]; \
+}; \
+CheckAutoObj() { \
+	if DirIsCreatable "$${1}"; then \
+		echo yes; \
+	else \
+		echo no; \
+	fi; \
+}
+.if !empty(MAKEOBJDIRPREFIX)
+WANTED_OBJDIR=	${MAKEOBJDIRPREFIX}${.CURDIR}
+.else
+WANTED_OBJDIR=	${MAKEOBJDIR}
+.endif
+OBJDIR_WRITABLE!= \
+	${CheckAutoObj}; CheckAutoObj "${WANTED_OBJDIR}" || echo no
+# Export the decision to sub-makes.
+MK_AUTO_OBJ:=	${OBJDIR_WRITABLE}
+.export MK_AUTO_OBJ
+.elif make(showconfig)
+# Need to export for showconfig internally running make -dg1.  It is enabled
+# in sys.mk by default.
+.export MK_AUTO_OBJ
+.endif	# ${MK_AUTO_OBJ} == "no" && ...
+
+# Assign this directory as .OBJDIR if possible after determining if AUTO_OBJ
+# can be enabled by default.
 .if ${MK_AUTO_OBJ} == "no"
 # The expected OBJDIR already exists, set it as .OBJDIR.
 .if !empty(MAKEOBJDIRPREFIX) && exists(${MAKEOBJDIRPREFIX}${.CURDIR})
Index: share/mk/sys.mk
===================================================================
--- share/mk/sys.mk
+++ share/mk/sys.mk
@@ -20,6 +20,12 @@
 __DEFAULT_YES_OPTIONS+= \
 	UNIFIED_OBJDIR
 
+# src.sys.obj.mk enables AUTO_OBJ by default if possible but it is otherwise
+# disabled.  Ensure src.conf.5 shows it as default on.
+.if make(showconfig)
+__DEFAULT_YES_OPTIONS+= AUTO_OBJ
+.endif
+
 # Some options we need now
 __DEFAULT_NO_OPTIONS= \
 	DIRDEPS_BUILD \
Index: tools/build/options/WITHOUT_AUTO_OBJ
===================================================================
--- /dev/null
+++ tools/build/options/WITHOUT_AUTO_OBJ
@@ -0,0 +1,3 @@
+.\" $FreeBSD$
+Disable automatic creation of objdirs.
+This is enabled by default if the wanted OBJDIR is writable by the current user.