linux-kernel - [PATCH] um: Fix filtering '-mno-global-merge'

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Date:   Tue, 22 Mar 2022 10:35:47 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        David Gow <davidgow@...gle.com>, linux-um@...ts.infradead.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH] um: Fix filtering '-mno-global-merge'

When booting a clang compiled UML kernel, the kernel panics when trying
to run init:

  wait_stub_done : failed to wait for SIGTRAP, pid = 651294, n = 651294, errno = 0, status = 0xb7f
  Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

After the commit in Fixes, many flags from KBUILD_CFLAGS do not appear
in USER_CFLAGS, likely due to USER_CFLAGS initially being a recursive
variable ("VAR =") then being switched to a simple ("VAR :=") variable.
For example, diffing arch/x86/um/.ptrace_user.o.cmd shows flags such as
'-Os' and '-fno-delete-null-pointer-checks' getting dropped, which both
impact code generation.

Rework the filtering to use filter-out instead of patsubst, which allows
all the patterns that USER_CFLAGS cares about to be excluded in one
command and ensures all flags from KBUILD_CFLAGS are transferred over to
USER_CFLAGS properly, which resolves the boot issue noted above.

Fixes: 6580c5c18fb3 ("um: clang: Strip out -mno-global-merge from USER_CFLAGS")
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 arch/um/Makefile | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/arch/um/Makefile b/arch/um/Makefile
index 320b09cd513c..d202f501e9e1 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -70,15 +70,11 @@ KBUILD_CFLAGS += $(CFLAGS) $(CFLAGS-y) -D__arch_um__ \
 
 KBUILD_AFLAGS += $(ARCH_INCLUDE)
 
-USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
+USER_CFLAGS = $(filter-out $(KERNEL_DEFINES) -I% -mno-global-merge,$(KBUILD_CFLAGS)) \
 		$(ARCH_INCLUDE) $(MODE_INCLUDE) $(filter -I%,$(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)
 

base-commit: 82017457957a550d7d00dde419435dd74a890887
-- 
2.35.1