[PATCH v2 2/2] kbuild: link lib-y objects to vmlinux forcibly when CONFIG_MODULES=y

Masahiro Yamada
2020-03-11T22:38:40Z

Kbuild supports not only obj-y but also lib-y to list objects linked to
vmlinux.

The difference between them is that all the objects from obj-y are
forcibly linked to vmlinux by using --whole-archive, whereas the objects
from lib-y are linked as needed; if there is no user of a lib-y object,
it is not linked.

lib-y is intended to list utility functions that may be called from all
over the place (and may be unused at all), but it is a problem for
EXPORT_SYMBOL(). Even if there is no call-site in the vmlinux, we need
to keep exported symbols for the use from loadable modules.

Commit 7f2084fa55e6 ("[kbuild] handle exports in lib-y objects reliably")
worked around it by linking a dummy object, lib-ksyms.o, which contains
references to all the symbols exported from lib.a in that directory.
It uses the linker script command, EXTERN. Unfortunately, the meaning of
EXTERN of ld.lld is different from that of ld.bfd. Therefore, this does
not work with LD=ld.lld (CBL issue #515).

Anyway, the build rule of lib-ksyms.o is somewhat tricky. So, I want to
get rid of it.

At first, I was thinking of accumulating lib-y objects into obj-y
(or even replacing lib-y with obj-y entirely), but the lib-y syntax
is used beyond the ordinary use in lib/ and arch/*/lib/.

Examples:

- drivers/firmware/efi/libstub/Makefile builds lib.a, which is linked
into vmlinux in the own way (arm64), or linked to the decompressor
(arm, x86).

- arch/alpha/lib/Makefile builds lib.a which is linked not only to
vmlinux, but also to bootloaders in arch/alpha/boot/Makefile.

- arch/xtensa/boot/lib/Makefile builds lib.a for use from
arch/xtensa/boot/boot-redboot/Makefile.

One more thing, adding everything to obj-y would increase the vmlinux
size of allnoconfig (or tinyconfig).

For less impact, I tweaked the destination of lib.a at the top Makefile;
when CONFIG_MODULES=y, lib.a goes to KBUILD_VMLINUX_OBJS, which is
forcibly linked to vmlinux, otherwise lib.a goes to KBUILD_VMLINUX_LIBS
as before.

The size impact for normal usecases is quite small since at lease one
symbol in every lib-y object is eventually called by someone. In case
you are intrested, here are the figures.

x86_64_defconfig:

text data bss dec hex filename
19566602 5422072 1589328 26578002 1958c52 vmlinux.before
19566932 5422104 1589328 26578364 1958dbc vmlinux.after

The case with the biggest impact is allnoconfig + CONFIG_MODULES=y.

ARCH=x86 allnoconfig + CONFIG_MODULES=y:

text data bss dec hex filename
1175162 254740 1220608 2650510 28718e vmlinux.before
1177974 254836 1220608 2653418 287cea vmlinux.after

Hopefully this is still not a big deal. The per-file trimming with the
static library is not so effective after all.

If fine-grained optimization is desired, some architectures support
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION, which trims dead code per-symbol
basis. When LTO is supported in mainline, even better optimization will
be possible.

Link: https://github.com/ClangBuiltLinux/linux/issues/515
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reported-by: kbuild test robot <lkp@intel.com>
---

Changes in v2: None

Makefile | 7 ++++++-
scripts/Makefile.build | 17 -----------------
2 files changed, 6 insertions(+), 18 deletions(-)

diff --git a/Makefile b/Makefile
index 6a538b79e61e..2cdbd4b3e36d 100644
--- a/Makefile
+++ b/Makefile
@@ -1034,8 +1034,13 @@ init-y := $(patsubst %/, %/built-in.a, $(init-y))
core-y := $(patsubst %/, %/built-in.a, $(core-y))
drivers-y := $(patsubst %/, %/built-in.a, $(drivers-y))
net-y := $(patsubst %/, %/built-in.a, $(net-y))
+libs-y2 := $(patsubst %/, %/built-in.a, $(filter %/, $(libs-y)))
+ifdef CONFIG_MODULES
+libs-y1 := $(filter-out %/, $(libs-y))
+libs-y2 += $(patsubst %/, %/lib.a, $(filter %/, $(libs-y)))
+else
libs-y1 := $(patsubst %/, %/lib.a, $(libs-y))
-libs-y2 := $(patsubst %/, %/built-in.a, $(filter-out %.a, $(libs-y)))
+endif
virt-y := $(patsubst %/, %/built-in.a, $(virt-y))

# Externally visible symbols (used by link-vmlinux.sh)
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index a1730d42e5f3..356601994f3a 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -65,7 +65,6 @@ endif

ifneq ($(strip $(lib-y) $(lib-m) $(lib-)),)
lib-target := $(obj)/lib.a
-real-obj-y += $(obj)/lib-ksyms.o
endif

ifdef need-builtin
@@ -410,22 +409,6 @@ $(lib-target): $(lib-y) FORCE

targets += $(lib-target)

-dummy-object = $(obj)/.lib_exports.o
-ksyms-lds = $(dot-target).lds
-
-quiet_cmd_export_list = EXPORTS $@
-cmd_export_list = $(OBJDUMP) -h $< | \
- sed -ne '/___ksymtab/s/.*+\([^ ]*\).*/EXTERN(\1)/p' >$(ksyms-lds);\
- rm -f $(dummy-object);\
- echo | $(CC) $(a_flags) -c -o $(dummy-object) -x assembler -;\
- $(LD) $(ld_flags) -r -o $@ -T $(ksyms-lds) $(dummy-object);\
- rm $(dummy-object) $(ksyms-lds)
-
-$(obj)/lib-ksyms.o: $(lib-target) FORCE
- $(call if_changed,export_list)
-
-targets += $(obj)/lib-ksyms.o
-
endif

# NOTE:
--
2.17.1