diff --git a/compiler-rt/cmake/Modules/AddCompilerRT.cmake b/compiler-rt/cmake/Modules/AddCompilerRT.cmake
index fb2aee8e42ee2..d658b7009e859 100644
--- a/compiler-rt/cmake/Modules/AddCompilerRT.cmake
+++ b/compiler-rt/cmake/Modules/AddCompilerRT.cmake
@@ -162,7 +162,6 @@ endmacro()
 #                         OBJECT_LIBS <object libraries to use as sources>
 #                         PARENT_TARGET <convenience parent target>
 #                         ADDITIONAL_HEADERS <header files>
-#                         EXTENSIONS <boolean>
 #                         C_STANDARD <version>
 #                         CXX_STANDARD <version>)
 function(add_compiler_rt_runtime name type)
@@ -174,7 +173,7 @@ function(add_compiler_rt_runtime name type)
   cmake_parse_arguments(LIB
     ""
     "PARENT_TARGET;C_STANDARD;CXX_STANDARD"
-    "OS;ARCHS;SOURCES;CFLAGS;LINK_FLAGS;DEFS;DEPS;LINK_LIBS;OBJECT_LIBS;ADDITIONAL_HEADERS;EXTENSIONS"
+    "OS;ARCHS;SOURCES;CFLAGS;LINK_FLAGS;DEFS;DEPS;LINK_LIBS;OBJECT_LIBS;ADDITIONAL_HEADERS"
     ${ARGN})
   set(libnames)
   # Until we support this some other way, build compiler-rt runtime without LTO
@@ -445,10 +444,6 @@ function(add_compiler_rt_runtime name type)
     if(type STREQUAL "SHARED")
       rt_externalize_debuginfo(${libname})
     endif()
-
-    if(DEFINED LIB_EXTENSIONS)
-      set_target_properties(${libname} PROPERTIES C_EXTENSIONS ${LIB_EXTENSIONS})
-    endif()
   endforeach()
   if(LIB_PARENT_TARGET)
     add_dependencies(${LIB_PARENT_TARGET} ${libnames})
diff --git a/compiler-rt/lib/profile/CMakeLists.txt b/compiler-rt/lib/profile/CMakeLists.txt
index ac1451c8ceed1..a6402f80b890a 100644
--- a/compiler-rt/lib/profile/CMakeLists.txt
+++ b/compiler-rt/lib/profile/CMakeLists.txt
@@ -162,8 +162,7 @@ if(APPLE)
     CFLAGS ${EXTRA_FLAGS}
     SOURCES ${PROFILE_SOURCES}
     ADDITIONAL_HEADERS ${PROFILE_HEADERS}
-    PARENT_TARGET profile
-    EXTENSIONS ON)
+    PARENT_TARGET profile)
 else()
   add_compiler_rt_runtime(clang_rt.profile
     STATIC
@@ -171,6 +170,5 @@ else()
     CFLAGS ${EXTRA_FLAGS}
     SOURCES ${PROFILE_SOURCES}
     ADDITIONAL_HEADERS ${PROFILE_HEADERS}
-    PARENT_TARGET profile
-    EXTENSIONS ON)
+    PARENT_TARGET profile)
 endif()
diff --git a/compiler-rt/lib/profile/GCDAProfiling.c b/compiler-rt/lib/profile/GCDAProfiling.c
index ac01805e70adc..523ade5eafc15 100644
--- a/compiler-rt/lib/profile/GCDAProfiling.c
+++ b/compiler-rt/lib/profile/GCDAProfiling.c
@@ -21,6 +21,11 @@
 
 #if !defined(__Fuchsia__)
 
+#if defined(__linux__)
+// For fdopen()
+#define _DEFAULT_SOURCE
+#endif
+
 #include <errno.h>
 #include <fcntl.h>
 #include <stdint.h>
diff --git a/compiler-rt/lib/profile/InstrProfilingFile.c b/compiler-rt/lib/profile/InstrProfilingFile.c
index 354f21b786151..71127b05aafb8 100644
--- a/compiler-rt/lib/profile/InstrProfilingFile.c
+++ b/compiler-rt/lib/profile/InstrProfilingFile.c
@@ -8,6 +8,11 @@
 
 #if !defined(__Fuchsia__)
 
+#if defined(__linux__)
+// For fileno(), ftruncate(), getpagesize(), setenv()
+#define _DEFAULT_SOURCE
+#endif
+
 #include <assert.h>
 #include <errno.h>
 #include <stdio.h>
diff --git a/compiler-rt/lib/profile/InstrProfilingUtil.c b/compiler-rt/lib/profile/InstrProfilingUtil.c
index 0fae91cfb8950..a9d9df813764b 100644
--- a/compiler-rt/lib/profile/InstrProfilingUtil.c
+++ b/compiler-rt/lib/profile/InstrProfilingUtil.c
@@ -12,6 +12,11 @@
 #include <windows.h>
 #include "WindowsMMap.h"
 #else
+#if defined(__linux__)
+// For fdopen(), fileno(), getpagesize(), madvise()
+#define _DEFAULT_SOURCE
+#endif
+
 #include <errno.h>
 #include <fcntl.h>
 #include <sys/file.h>