diff --git a/core/Makefile b/core/Makefile
index d020335..f46ab11 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -4,15 +4,6 @@
 # intermedites-dir-for
 LOCAL_PATH := $(BUILD_SYSTEM)
 
-# Pick a reasonable string to use to identify files.
-ifneq (,$(filter eng.%,$(BUILD_NUMBER)))
-  # BUILD_NUMBER has a timestamp in it, which means that
-  # it will change every time.  Pick a stable value.
-  FILE_NAME_TAG := eng.$(USER)
-else
-  FILE_NAME_TAG := $(BUILD_NUMBER)
-endif
-
 # -----------------------------------------------------------------
 # Define rules to copy PRODUCT_COPY_FILES defined by the product.
 # PRODUCT_COPY_FILES contains words like <source file>:<dest file>[:<owner>].
@@ -238,28 +229,37 @@
 
 # The string used to uniquely identify the combined build and product; used by the OTA server.
 ifeq (,$(strip $(BUILD_FINGERPRINT)))
-  ifneq ($(filter eng.%,$(BUILD_NUMBER)),)
-    BF_BUILD_NUMBER := $(USER)$(shell $(DATE) +%m%d%H%M)
+  ifeq ($(strip $(HAS_BUILD_NUMBER)),false)
+    BF_BUILD_NUMBER := $(USER)$$($(DATE_FROM_FILE) +%m%d%H%M)
   else
-    BF_BUILD_NUMBER := $(BUILD_NUMBER)
+    BF_BUILD_NUMBER := $(file <$(BUILD_NUMBER_FILE))
   endif
   BUILD_FINGERPRINT := $(PRODUCT_BRAND)/$(TARGET_PRODUCT)/$(TARGET_DEVICE):$(PLATFORM_VERSION)/$(BUILD_ID)/$(BF_BUILD_NUMBER):$(TARGET_BUILD_VARIANT)/$(BUILD_VERSION_TAGS)
 endif
-ifneq ($(words $(BUILD_FINGERPRINT)),1)
-  $(error BUILD_FINGERPRINT cannot contain spaces: "$(BUILD_FINGERPRINT)")
-endif
+# unset it for safety.
+BF_BUILD_NUMBER :=
 
-$(shell mkdir -p $(PRODUCT_OUT) && echo $(BUILD_FINGERPRINT) > $(PRODUCT_OUT)/build_fingerprint.txt)
-BUILD_FINGERPRINT_FROM_FILE := $$(cat $(PRODUCT_OUT)/build_fingerprint.txt)
+BUILD_FINGERPRINT_FILE := $(PRODUCT_OUT)/build_fingerprint.txt
+ifneq (,$(shell mkdir -p $(PRODUCT_OUT) && echo $(BUILD_FINGERPRINT) >$(BUILD_FINGERPRINT_FILE) && grep " " $(BUILD_FINGERPRINT_FILE)))
+  $(error BUILD_FINGERPRINT cannot contain spaces: "$(file <$(BUILD_FINGERPRINT_FILE))")
+endif
+BUILD_FINGERPRINT_FROM_FILE := $$(cat $(BUILD_FINGERPRINT_FILE))
+# unset it for safety.
+BUILD_FINGERPRINT :=
 
 # The string used to uniquely identify the system build; used by the OTA server.
 # This purposefully excludes any product-specific variables.
 ifeq (,$(strip $(BUILD_THUMBPRINT)))
-  BUILD_THUMBPRINT := $(PLATFORM_VERSION)/$(BUILD_ID)/$(BUILD_NUMBER):$(TARGET_BUILD_VARIANT)/$(BUILD_VERSION_TAGS)
+  BUILD_THUMBPRINT := $(PLATFORM_VERSION)/$(BUILD_ID)/$(BUILD_NUMBER_FROM_FILE):$(TARGET_BUILD_VARIANT)/$(BUILD_VERSION_TAGS)
 endif
-ifneq ($(words $(BUILD_THUMBPRINT)),1)
-  $(error BUILD_THUMBPRINT cannot contain spaces: "$(BUILD_THUMBPRINT)")
+
+BUILD_THUMBPRINT_FILE := $(PRODUCT_OUT)/build_thumbprint.txt
+ifneq (,$(shell mkdir -p $(PRODUCT_OUT) && echo $(BUILD_THUMBPRINT) >$(BUILD_THUMBPRINT_FILE) && grep " " $(BUILD_THUMBPRINT_FILE)))
+  $(error BUILD_THUMBPRINT cannot contain spaces: "$(file <$(BUILD_THUMBPRINT_FILE))")
 endif
+BUILD_THUMBPRINT_FROM_FILE := $$(cat $(BUILD_THUMBPRINT_FILE))
+# unset it for safety.
+BUILD_THUMBPRINT :=
 
 KNOWN_OEM_THUMBPRINT_PROPERTIES := \
     ro.product.brand \
@@ -347,7 +347,7 @@
 			PLATFORM_VERSION_ALL_CODENAMES="$(PLATFORM_VERSION_ALL_CODENAMES)" \
 			BUILD_VERSION_TAGS="$(BUILD_VERSION_TAGS)" \
 			BUILD_FINGERPRINT="$(BUILD_FINGERPRINT_FROM_FILE)" \
-			$(if $(OEM_THUMBPRINT_PROPERTIES),BUILD_THUMBPRINT="$(BUILD_THUMBPRINT)") \
+			$(if $(OEM_THUMBPRINT_PROPERTIES),BUILD_THUMBPRINT="$(BUILD_THUMBPRINT_FROM_FILE)") \
 			TARGET_CPU_ABI_LIST="$(TARGET_CPU_ABI_LIST)" \
 			TARGET_CPU_ABI_LIST_32_BIT="$(TARGET_CPU_ABI_LIST_32_BIT)" \
 			TARGET_CPU_ABI_LIST_64_BIT="$(TARGET_CPU_ABI_LIST_64_BIT)" \
diff --git a/core/main.mk b/core/main.mk
index 0c165ca..ef55b4e 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -61,12 +61,23 @@
 # when using ninja.
 $(shell mkdir -p $(OUT_DIR) && \
     echo -n $(BUILD_NUMBER) > $(OUT_DIR)/build_number.txt)
+BUILD_NUMBER_FILE := $(OUT_DIR)/build_number.txt
+
 ifeq ($(HOST_OS),darwin)
 DATE_FROM_FILE := date -r $(BUILD_DATETIME_FROM_FILE)
 else
 DATE_FROM_FILE := date -d @$(BUILD_DATETIME_FROM_FILE)
 endif
 
+# Pick a reasonable string to use to identify files.
+ifeq ($(strip $(HAS_BUILD_NUMBER)),false)
+  # BUILD_NUMBER has a timestamp in it, which means that
+  # it will change every time.  Pick a stable value.
+  FILE_NAME_TAG := eng.$(USER)
+else
+  FILE_NAME_TAG := $(file <$(BUILD_NUMBER_FILE))
+endif
+
 # Make an empty directory, which can be used to make empty jars
 EMPTY_DIRECTORY := $(OUT_DIR)/empty
 $(shell mkdir -p $(EMPTY_DIRECTORY) && rm -rf $(EMPTY_DIRECTORY)/*)
diff --git a/core/tasks/tools/build_custom_image.mk b/core/tasks/tools/build_custom_image.mk
index 4d05237..a1151e9 100644
--- a/core/tasks/tools/build_custom_image.mk
+++ b/core/tasks/tools/build_custom_image.mk
@@ -151,7 +151,7 @@
 	    cat $(PRIVATE_DICT_FILE) >> $(PRIVATE_INTERMEDIATES)/image_info.txt)
 	# Generate the image.
 	$(if $(filter oem,$(PRIVATE_MOUNT_POINT)), \
-	  $(hide) echo "oem.buildnumber=$(BUILD_NUMBER)" >> $(PRIVATE_STAGING_DIR)/oem.prop)
+	  $(hide) echo "oem.buildnumber=$(BUILD_NUMBER_FROM_FILE)" >> $(PRIVATE_STAGING_DIR)/oem.prop)
 	$(hide) PATH=$(foreach p,$(INTERNAL_USERIMAGES_BINARY_PATHS),$(p):)$$PATH \
 	  build/make/tools/releasetools/build_image.py \
 	  $(PRIVATE_STAGING_DIR) $(PRIVATE_INTERMEDIATES)/image_info.txt $@ $(TARGET_OUT)
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index 858384b..e83d6fa 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -271,6 +271,7 @@
 # to soong_ui.
 BUILD_DATETIME :=
 
+HAS_BUILD_NUMBER := true
 ifndef BUILD_NUMBER
   # BUILD_NUMBER should be set to the source control value that
   # represents the current state of the source code.  E.g., a
@@ -282,4 +283,5 @@
   # from this date/time" value.  Make it start with a non-digit so that
   # anyone trying to parse it as an integer will probably get "0".
   BUILD_NUMBER := eng.$(shell echo $${USER:0:6}).$(shell $(DATE) +%Y%m%d.%H%M%S)
+  HAS_BUILD_NUMBER := false
 endif