diff --git a/arch/um/kernel/skas/Makefile b/arch/um/kernel/skas/Makefile
index c098d087c3f2..2cf82eba52f8 100644
--- a/arch/um/kernel/skas/Makefile
+++ b/arch/um/kernel/skas/Makefile
@@ -44,8 +44,14 @@ targets += stub_exe.dbg stub_exe $(stub_exe_objs-y)
 CFLAGS_stub.o := $(CFLAGS_NO_HARDENING)
 CFLAGS_stub_exe.o := $(CFLAGS_NO_HARDENING)
 
-# Clang will call memset() from __builtin_alloca() when stack variable
-# initialization is enabled, which is used in stub_exe.c.
+# stub_signal_restorer() must not touch stack pointer, clang doesn't
+# always omit the frame pointer automatically
+CFLAGS_stub.o += -fomit-frame-pointer
+CFLAGS_stub_exe.o += -fomit-frame-pointer
+
+# Clang's stack var initialisation can use memset() or similar and that
+# cannot be done for this code since we embed/move the stub
+CFLAGS_stub.o += $(call cc-option, -ftrivial-auto-var-init=uninitialized)
 CFLAGS_stub_exe.o += $(call cc-option, -ftrivial-auto-var-init=uninitialized)
 
 UNPROFILE_OBJS := stub.o stub_exe.o
diff --git a/arch/x86/um/Makefile b/arch/x86/um/Makefile
index f9ea75bf43ac..100b68f601f9 100644
--- a/arch/x86/um/Makefile
+++ b/arch/x86/um/Makefile
@@ -47,4 +47,7 @@ include/generated/user_constants.h: $(obj)/user-offsets.s FORCE
 UNPROFILE_OBJS := stub_segv.o
 CFLAGS_stub_segv.o := $(CFLAGS_NO_HARDENING)
 
+# see arch/um/kernel/skas/Makefile - this goes into the stub
+CFLAGS_stub_segv.o += $(call cc-option, -ftrivial-auto-var-init=uninitialized)
+
 include $(srctree)/arch/um/scripts/Makefile.rules