Index: Makefile.inc1
===================================================================
--- Makefile.inc1
+++ Makefile.inc1
@@ -1608,6 +1608,12 @@
 KRNLOBJDIR=	${OBJTOP}${KERNSRCDIR:C,^${.CURDIR},,}
 KERNCONFDIR?=	${KRNLCONFDIR}
 
+.if ${MK_REPRODUCIBLE_BUILD} != "no"
+REPRODUCIBLE_KRNLOBJDIR=	${REPRODUCIBLE_OBJTOP}${KERNSRCDIR:C,^${.CURDIR},,}
+.else
+REPRODUCIBLE_KRNLOBJDIR=	${KRNLOBJDIR}
+.endif
+
 BUILDKERNELS=
 INSTALLKERNEL=
 .if defined(NO_INSTALLKERNEL)
@@ -1690,7 +1696,8 @@
 	@echo "--------------------------------------------------------------"
 	@echo ">>> stage 2.2: rebuilding the object tree"
 	@echo "--------------------------------------------------------------"
-	${_+_}cd ${KRNLOBJDIR}/${_kernel}; ${KMAKE} obj
+	${_+_}cd ${KRNLOBJDIR}/${_kernel}; ${KMAKE} obj \
+		REPRODUCIBLE_KOBJDIR=${REPRODUCIBLE_KRNLOBJDIR}/${_kernel}
 .endif
 	@echo
 	@echo "--------------------------------------------------------------"
@@ -1701,7 +1708,8 @@
 	@echo "--------------------------------------------------------------"
 	@echo ">>> stage 3.1: building everything"
 	@echo "--------------------------------------------------------------"
-	${_+_}cd ${KRNLOBJDIR}/${_kernel}; ${KMAKE} all -DNO_MODULES_OBJ
+	${_+_}cd ${KRNLOBJDIR}/${_kernel}; ${KMAKE} all -DNO_MODULES_OBJ \
+		REPRODUCIBLE_KOBJDIR=${REPRODUCIBLE_KRNLOBJDIR}/${_kernel}
 	@echo "--------------------------------------------------------------"
 	@echo ">>> Kernel build for ${_kernel} completed on `LC_ALL=C date`"
 	@echo "--------------------------------------------------------------"
Index: sys/conf/kern.post.mk
===================================================================
--- sys/conf/kern.post.mk
+++ sys/conf/kern.post.mk
@@ -346,6 +346,18 @@
 .depend: .PRECIOUS ${SRCS}
 
 _MAP_DEBUG_PREFIX= yes
+.if ${MK_REPRODUCIBLE_BUILD} != "no"
+_sysroot=${CC:M--sysroot=*:[-1]:S,--sysroot=,,}
+_map_sysdir=${REPRODUCIBLE_SYSDIR}
+CFLAGS+= -ffile-prefix-map=${_sysroot}/=/
+CFLAGS+= -ffile-prefix-map=${SYSDIR}=${_map_sysdir}
+.if defined(REPRODUCIBLE_KOBJDIR) && \
+    "${REPRODUCIBLE_KOBJDIR}" != "${.OBJDIR}"
+CFLAGS+= -ffile-prefix-map=${.OBJDIR}=${REPRODUCIBLE_KOBJDIR}
+.endif
+.else
+_map_sysdir=${SYSDIR}
+.endif
 
 _ILINKS= machine
 .if ${MACHINE} != ${MACHINE_CPUARCH} && ${MACHINE} != "arm64"
@@ -363,9 +375,9 @@
 .endif
 .if defined(_MAP_DEBUG_PREFIX)
 .if ${_link} == "machine"
-CFLAGS+= -fdebug-prefix-map=./machine=${SYSDIR}/${MACHINE}/include
+CFLAGS+= -ffile-prefix-map=./machine=${_map_sysdir}/${MACHINE}/include
 .else
-CFLAGS+= -fdebug-prefix-map=./${_link}=${SYSDIR}/${_link}/include
+CFLAGS+= -ffile-prefix-map=./${_link}=${_map_sysdir}/${_link}/include
 .endif
 .endif
 .endfor
Index: sys/conf/kern.pre.mk
===================================================================
--- sys/conf/kern.pre.mk
+++ sys/conf/kern.pre.mk
@@ -26,6 +26,8 @@
 
 # The kernel build always expects .OBJDIR=.CURDIR.
 .OBJDIR: ${.CURDIR}
+# This gets fed by Makefile.inc1 iff we're doing a reproducible build.
+REPRODUCIBLE_KOBJDIR?=	${.CURDIR}
 
 .if defined(NO_OBJWALK) || ${MK_AUTO_OBJ} == "yes"
 NO_OBJWALK=		t
@@ -345,6 +347,9 @@
 # them.
 
 MKMODULESENV+=	MAKEOBJDIRPREFIX=${.OBJDIR}/modules KMODDIR=${KODIR}
+.if ${MK_REPRODUCIBLE_BUILD} != "no"
+MKMODULESENV+=	REPRODUCIBLE_MODOBJDIR=${REPRODUCIBLE_KOBJDIR}/modules
+.endif
 MKMODULESENV+=	MACHINE_CPUARCH=${MACHINE_CPUARCH}
 MKMODULESENV+=	MACHINE=${MACHINE} MACHINE_ARCH=${MACHINE_ARCH}
 MKMODULESENV+=	MODULES_EXTRA="${MODULES_EXTRA}" WITHOUT_MODULES="${WITHOUT_MODULES}"
Index: sys/conf/kmod.mk
===================================================================
--- sys/conf/kmod.mk
+++ sys/conf/kmod.mk
@@ -272,6 +272,18 @@
 .endif
 
 _MAP_DEBUG_PREFIX= yes
+.if ${MK_REPRODUCIBLE_BUILD} != "no"
+_sysroot=${CC:M--sysroot=*:[-1]:S,--sysroot=,,}
+_map_sysdir=${REPRODUCIBLE_SYSDIR}
+CFLAGS+= -ffile-prefix-map=${_sysroot}/=/
+CFLAGS+= -ffile-prefix-map=${SYSDIR}=${_map_sysdir}
+.if defined(REPRODUCIBLE_MODOBJDIR) && \
+    "${REPRODUCIBLE_MODOBJDIR}${REPRODUCIBLE_SYSDIR}" != "${MAKEOBJDIRPREFIX}${SYSDIR}"
+CFLAGS+= -ffile-prefix-map=${MAKEOBJDIRPREFIX}${SYSDIR}=${REPRODUCIBLE_MODOBJDIR}${REPRODUCIBLE_SYSDIR}
+.endif
+.else
+_map_sysdir=${SYSDIR}
+.endif
 
 _ILINKS=machine
 .if ${MACHINE_CPUARCH} == "i386" || ${MACHINE_CPUARCH} == "amd64"
@@ -293,9 +305,9 @@
 .endif
 .if defined(_MAP_DEBUG_PREFIX)
 .if ${_link} == "machine"
-CFLAGS+= -fdebug-prefix-map=./machine=${SYSDIR}/${MACHINE}/include
+CFLAGS+= -ffile-prefix-map=./machine=${_map_sysdir}/${MACHINE}/include
 .else
-CFLAGS+= -fdebug-prefix-map=./${_link}=${SYSDIR}/${_link}/include
+CFLAGS+= -ffile-prefix-map=./${_link}=${_map_sysdir}/${_link}/include
 .endif
 .endif
 .endfor