diff --git a/toolbox/Android.mk b/toolbox/Android.mk
index 52716e9..9ade759 100644
--- a/toolbox/Android.mk
+++ b/toolbox/Android.mk
@@ -80,10 +80,16 @@
 
 $(LOCAL_PATH)/getevent.c: $(intermediates)/input.h-labels.h
 
+UAPI_INPUT_EVENT_CODES_H := bionic/libc/kernel/uapi/linux/input-event-codes.h
 INPUT_H_LABELS_H := $(intermediates)/input.h-labels.h
 $(INPUT_H_LABELS_H): PRIVATE_LOCAL_PATH := $(LOCAL_PATH)
-$(INPUT_H_LABELS_H): PRIVATE_CUSTOM_TOOL = $(PRIVATE_LOCAL_PATH)/generate-input.h-labels.py > $@
-$(INPUT_H_LABELS_H): $(LOCAL_PATH)/Android.mk $(LOCAL_PATH)/generate-input.h-labels.py
+# The PRIVATE_CUSTOM_TOOL line uses = to evaluate the output path late.
+# We copy the input path so it can't be accidentally modified later.
+$(INPUT_H_LABELS_H): PRIVATE_UAPI_INPUT_EVENT_CODES_H := $(UAPI_INPUT_EVENT_CODES_H)
+$(INPUT_H_LABELS_H): PRIVATE_CUSTOM_TOOL = $(PRIVATE_LOCAL_PATH)/generate-input.h-labels.py $(PRIVATE_UAPI_INPUT_EVENT_CODES_H) > $@
+# The dependency line though gets evaluated now, so the PRIVATE_ copy doesn't exist yet,
+# and the original can't yet have been modified, so this is both sufficient and necessary.
+$(INPUT_H_LABELS_H): $(LOCAL_PATH)/Android.mk $(LOCAL_PATH)/generate-input.h-labels.py $(UAPI_INPUT_EVENT_CODES_H)
 $(INPUT_H_LABELS_H):
 	$(transform-generated-source)
 
diff --git a/toolbox/generate-input.h-labels.py b/toolbox/generate-input.h-labels.py
index ebb9588..30485a0 100755
--- a/toolbox/generate-input.h-labels.py
+++ b/toolbox/generate-input.h-labels.py
@@ -18,6 +18,7 @@
 
 import os
 import re
+import sys
 
 input_prop_list = []
 ev_list = []
@@ -36,7 +37,7 @@
 
 r = re.compile(r'#define\s+(\S+)\s+((?:0x)?\d+)')
 
-with open('bionic/libc/kernel/uapi/linux/input.h', 'r') as f:
+with open(sys.argv[1], 'r') as f:
   for line in f:
     m = r.match(line)
     if m: