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

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Date:   Sat, 26 Mar 2022 12:29:55 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        David Gow <davidgow@...gle.com>, linux-um@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] um: Fix filtering '-mno-global-merge'

On Wed, Mar 23, 2022 at 2:39 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> 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>



Can we remove -mno-global-merge entirely?


61163efae02040f66a95 was a very old commit,
without enough explanation.

Shall we remove  -mno-global-merge, and do compile-tests.
If we are hit by problems for arm/arm64, we can re-add it.









> ---
>  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
>


-- 
Best Regards
Masahiro Yamada