Re: [PATCH v3] Makefile: lld: tell clang to use lld

From: Masahiro Yamada
Date: Fri Apr 05 2019 - 06:17:41 EST

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On Tue, Apr 2, 2019 at 4:09 PM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>
> This is needed because clang doesn't select which linker to use based on
> $LD but rather -fuse-ld={bfd,gold,lld,<absolute path to linker>}. This
> is problematic especially for cc-ldoption, which checks for linker flag
> support via invoking the compiler, rather than the linker.
>
> Select the linker via absolute path from $PATH via `which`. This allows
> you to build with:
>
> $ make LD=ld.lld
> $ make LD=ld.lld-8
> $ make LD=/path/to/ld.lld
>
> Add -Qunused-arguments to KBUILD_CPPFLAGS when using LLD, as otherwise
> Clang likes to complain about -fuse-lld= being unused when compiling but
> not linking (-c) such as when cc-option is used.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/342
> Link: https://github.com/ClangBuiltLinux/linux/issues/366
> Link: https://github.com/ClangBuiltLinux/linux/issues/357
> Suggested-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> Suggested-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> Changes V2->V3:
> * Use absolute path based on `which $LD` as per Masahiro.

This is not what I suggested. I wanted to say:
"You cannot do this for GCC, so do not do it at all".

I want to propose alternative solution.
Please check the attached patches.

To apply 0002, you need the following as a prerequisite:

https://lore.kernel.org/patchwork/patch/1057685/

--
Best Regards
Masahiro Yamada
From 7fcb7806991d9b644173879c3c1fcbbded532435 Mon Sep 17 00:00:00 2001
From: Masahiro Yamada <yamada.masahiro@socionext.com>
Date: Fri, 5 Apr 2019 17:46:14 +0900
Subject: [PATCH 1/2] ARM: vdso: use $(LD) instead of $(CC) to link VDSO

We use $(LD) to link vmlinux, modules, decompressors, etc.

VDSO is the only exceptional case where $(CC) is used as the frontend
of linking, but I do not know why. VDSO uses a special linker script,
and does not link standard libraries at all.

I changed the Makefile to use $(LD) rahter than $(CC). I confirmed
the same vdso.so.raw was still produced.

Users will be able to use their favorite linker (e.g. lld instead of
of bfd) by passing LD= from the command line.

My plan is to rewrite all VDSO Makefiles to use $(LD), then delete
cc-ldoption.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
---
arch/arm/vdso/Makefile | 21 ++++++++-------------
1 file changed, 8 insertions(+), 13 deletions(-)

diff --git a/arch/arm/vdso/Makefile b/arch/arm/vdso/Makefile
index f4efff9d3afb..fadf554d9391 100644
--- a/arch/arm/vdso/Makefile
+++ b/arch/arm/vdso/Makefile
@@ -10,12 +10,12 @@ obj-vdso := $(addprefix $(obj)/, $(obj-vdso))
ccflags-y := -fPIC -fno-common -fno-builtin -fno-stack-protector
ccflags-y += -DDISABLE_BRANCH_PROFILING

-VDSO_LDFLAGS := -Wl,-Bsymbolic -Wl,--no-undefined -Wl,-soname=linux-vdso.so.1
-VDSO_LDFLAGS += -Wl,-z,max-page-size=4096 -Wl,-z,common-page-size=4096
-VDSO_LDFLAGS += -nostdlib -shared
-VDSO_LDFLAGS += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv)
-VDSO_LDFLAGS += $(call cc-ldoption, -Wl$(comma)--build-id)
-VDSO_LDFLAGS += $(call cc-ldoption, -fuse-ld=bfd)
+ldflags-y = -Bsymbolic --no-undefined -soname=linux-vdso.so.1 \
+ -z max-page-size=4096 -z common-page-size=4096 \
+ -nostdlib -shared \
+ $(call ld-option, --hash-style=sysv) \
+ $(call ld-option, --build-id) \
+ -T

obj-$(CONFIG_VDSO) += vdso.o
extra-$(CONFIG_VDSO) += vdso.lds
@@ -37,8 +37,8 @@ KCOV_INSTRUMENT := n
$(obj)/vdso.o : $(obj)/vdso.so

# Link rule for the .so file
-$(obj)/vdso.so.raw: $(src)/vdso.lds $(obj-vdso) FORCE
- $(call if_changed,vdsold)
+$(obj)/vdso.so.raw: $(obj)/vdso.lds $(obj-vdso) FORCE
+ $(call if_changed,ld)

$(obj)/vdso.so.dbg: $(obj)/vdso.so.raw $(obj)/vdsomunge FORCE
$(call if_changed,vdsomunge)
@@ -48,11 +48,6 @@ $(obj)/%.so: OBJCOPYFLAGS := -S
$(obj)/%.so: $(obj)/%.so.dbg FORCE
$(call if_changed,objcopy)

-# Actual build commands
-quiet_cmd_vdsold = VDSO $@
- cmd_vdsold = $(CC) $(c_flags) $(VDSO_LDFLAGS) \
- -Wl,-T $(filter %.lds,$^) $(filter %.o,$^) -o $@
-
quiet_cmd_vdsomunge = MUNGE $@
cmd_vdsomunge = $(objtree)/$(obj)/vdsomunge $< $@

--
2.17.1

From db93aba5086ebc0558c193101d56c2e5a5878dda Mon Sep 17 00:00:00 2001
From: Masahiro Yamada <yamada.masahiro@socionext.com>
Date: Thu, 4 Apr 2019 23:40:18 +0900
Subject: [PATCH 2/2] arm64: vdso: use $(LD) instead of $(CC) to link VDSO

We use $(LD) to link vmlinux, modules, decompressors, etc.

VDSO is the only exceptional case where $(CC) is used as the frontend
of linking, but I do not know why. VDSO uses a special linker script,
and does not link standard libraries at all.

I changed the Makefile to use $(LD) rahter than $(CC). I tested, and
VDSO still worked.

Users will be able to use their favorite linker (e.g. lld instead of
of bfd) by passing LD= from the command line.

My plan is to rewrite all VDSO Makefiles to use $(LD), then delete
cc-ldoption.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
---
arch/arm64/kernel/vdso/Makefile | 13 +++----------
1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
index a0af6bf6c11b..744b9dbaba03 100644
--- a/arch/arm64/kernel/vdso/Makefile
+++ b/arch/arm64/kernel/vdso/Makefile
@@ -12,17 +12,12 @@ obj-vdso := gettimeofday.o note.o sigreturn.o
targets := $(obj-vdso) vdso.so vdso.so.dbg
obj-vdso := $(addprefix $(obj)/, $(obj-vdso))

-ccflags-y := -shared -fno-common -fno-builtin
-ccflags-y += -nostdlib -Wl,-soname=linux-vdso.so.1 \
- $(call cc-ldoption, -Wl$(comma)--hash-style=sysv)
+ldflags-y := -shared -nostdlib -soname=linux-vdso.so.1 \
+ $(call ld-option, --hash-style=sysv) -n -T

# Disable gcov profiling for VDSO code
GCOV_PROFILE := n

-# Workaround for bare-metal (ELF) toolchains that neglect to pass -shared
-# down to collect2, resulting in silent corruption of the vDSO image.
-ccflags-y += -Wl,-shared
-
obj-y += vdso.o
extra-y += vdso.lds
CPPFLAGS_vdso.lds += -P -C -U$(ARCH)
@@ -32,7 +27,7 @@ $(obj)/vdso.o : $(obj)/vdso.so

# Link rule for the .so file, .lds has to be first
$(obj)/vdso.so.dbg: $(obj)/vdso.lds $(obj-vdso) FORCE
- $(call if_changed,vdsold)
+ $(call if_changed,ld)

# Strip rule for the .so file
$(obj)/%.so: OBJCOPYFLAGS := -S
@@ -52,8 +47,6 @@ $(obj-vdso): %.o: %.S FORCE
$(call if_changed_dep,vdsoas)

# Actual build commands
-quiet_cmd_vdsold = VDSOL $@
- cmd_vdsold = $(CC) $(c_flags) -Wl,-n -Wl,-T $(real-prereqs) -o $@
quiet_cmd_vdsoas = VDSOA $@
cmd_vdsoas = $(CC) $(a_flags) -c -o $@ $<

--
2.17.1

Next message: Thomas-Mich Richter: "Re: WARN_ON_ONCE() hit at kernel/events/core.c:330"
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In reply to: Nathan Chancellor: "Re: [PATCH v3] Makefile: lld: tell clang to use lld"
Next in thread: Kees Cook: "Re: [PATCH v3] Makefile: lld: tell clang to use lld"
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