diff --git a/Modules/_ctypes/libffi/src/dlmalloc.c b/Modules/_ctypes/libffi/src/dlmalloc.c
index 5c9f9c2..2773953 100644
--- a/Modules/_ctypes/libffi/src/dlmalloc.c
+++ b/Modules/_ctypes/libffi/src/dlmalloc.c
@@ -457,6 +457,11 @@
 #define LACKS_ERRNO_H
 #define MALLOC_FAILURE_ACTION
 #define MMAP_CLEARS 0 /* WINCE and some others apparently don't clear */
+#elif !defined _GNU_SOURCE
+/* mremap() on Linux requires this via sys/mman.h
+ * See roundup issue 10309
+ */
+#define _GNU_SOURCE 1
 #endif  /* WIN32 */
 
 #ifdef __OS2__