linux-hardening - [PATCH] um: clang: Strip out -mno-global-merge from USER

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Message-Id: <20220303090643.241747-1-davidgow@google.com>
Date:   Thu,  3 Mar 2022 17:06:42 +0800
From:   David Gow <davidgow@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     David Gow <davidgow@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        linux-um@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: [PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

The things built with USER_CFLAGS don't seem to recognise it as a
compiler option, and print a warning:
clang: warning: argument unused during compilation: '-mno-global-merge' [-Wunused-command-line-argument]

Fixes: 744814d2fa ("um: Allow builds with Clang")
Signed-off-by: David Gow <davidgow@...gle.com>
---

This warning shows up after merging:
https://lore.kernel.org/lkml/20220227184517.504931-6-keescook@chromium.org/

I'm not 100% sure why this is necessary, but it does seem to work. All
the attempts to get rid of -mno-global-merge entirely have been met with
skepticism, but I'm guessing that it's not a problem for just the UML
"user" files, as they shouldn't(?) interact too much with modules.

 arch/um/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/um/Makefile b/arch/um/Makefile
index f2fe63bfd819..320b09cd513c 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -75,6 +75,10 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
 		-D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
 		-idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__
 
+ifdef CONFIG_CC_IS_CLANG
+USER_CFLAGS := $(patsubst -mno-global-merge,,$(USER_CFLAGS))
+endif
+
 #This will adjust *FLAGS accordingly to the platform.
 include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
 
-- 
2.35.1.616.g0bdcbb4464-goog