[PATCH 2/3] um: make stub compilation more robust

[Johannes Berg]

From: Johannes Berg <johannes.berg at intel.com>

With clang, stub compilation could result in getting stack
frames where none are allowed, or ways of making it be not
relocatable. Set -ftrivial-auto-var-init=uninitialized for
more code, and -fomit-frame-pointer as well to avoid these
issues.

Signed-off-by: Johannes Berg <johannes.berg at intel.com>
---
 arch/um/kernel/skas/Makefile | 10 ++++++++--
 arch/x86/um/Makefile         |  3 +++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/um/kernel/skas/Makefile b/arch/um/kernel/skas/Makefile
index 3384be42691f..0df1e49300f6 100644
--- a/arch/um/kernel/skas/Makefile
+++ b/arch/um/kernel/skas/Makefile
@@ -40,8 +40,14 @@ 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
-- 
2.55.0