kbuild: export top-level LDFLAGS_vmlinux only to scripts/Makefile.vmlinux

Nathan Chancellor reports that $(NM) emits an error message when
GNU Make 4.4 is used to build the ARM zImage.

$ make-4.4 ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- O=build defconfig zImage
[snip]
LD vmlinux
NM System.map
SORTTAB vmlinux
OBJCOPY arch/arm/boot/Image
Kernel: arch/arm/boot/Image is ready
arm-linux-gnueabi-nm: 'arch/arm/boot/compressed/../../../../vmlinux': No such file
/bin/sh: 1: arithmetic expression: expecting primary: " "
LDS arch/arm/boot/compressed/vmlinux.lds
AS arch/arm/boot/compressed/head.o
GZIP arch/arm/boot/compressed/piggy_data
AS arch/arm/boot/compressed/piggy.o
CC arch/arm/boot/compressed/misc.o

This occurs since GNU Make commit 98da874c4303 ("[SV 10593] Export
variables to $(shell ...) commands"), and the O= option is needed to
reproduce it. The generated zImage is correct despite the error message.

As the commit description of 98da874c4303 [1] says, exported variables
are passed down to $(shell ) functions, which means exported recursive
variables might be expanded earlier than before, in the parse stage.

The following test code demonstrates the change for GNU Make 4.4.

[Test Makefile]

$(shell echo hello > foo)
export foo = $(shell cat bar/../foo)
$(shell mkdir bar)

all:
@echo $(foo)

[GNU Make 4.3]

$ rm -rf bar; make-4.3
hello

[GNU Make 4.4]

$ rm -rf bar; make-4.4
cat: bar/../foo: No such file or directory
hello

The 'foo' is a resursively expanded (i.e. lazily expanded) variable.

GNU Make 4.3 expands 'foo' just before running the recipe '@echo $(foo)',
at this point, the directory 'bar' exists.

GNU Make 4.4 expands 'foo' to evaluate $(shell mkdir bar) because it is
exported. At this point, the directory 'bar' does not exit yet. The cat
command cannot resolve the bar/../foo path, hence the error message.

Let's get back to the kernel Makefile.

In arch/arm/boot/compressed/Makefile, KBSS_SZ is referenced by
LDFLAGS_vmlinux, which is recursive and also exported by the top
Makefile.

GNU Make 4.3 expands KBSS_SZ just before running the recipes, so no
error message.

GNU Make 4.4 expands KBSS_SZ in the parse stage, where the directory
arm/arm/boot/compressed does not exit yet. When compiled with O=,
the output directory is created by $(shell mkdir -p $(obj-dirs))
in scripts/Makefile.build.

There are two ways to fix this particular issue:

- change "$(obj)/../../../../vmlinux" in KBSS_SZ to "vmlinux"
- unexport LDFLAGS_vmlinux

This commit takes the latter course because it is what I originally
intended.

Commit 3ec8a5b33dea ("kbuild: do not export LDFLAGS_vmlinux")
unexported LDFLAGS_vmlinux.

Commit 5d4aeffbf709 ("kbuild: rebuild .vmlinux.export.o when its
prerequisite is updated") accidentally exported it again.

We can clean up arch/arm/boot/compressed/Makefile later.

[1]: https://git.savannah.gnu.org/cgit/make.git/commit/?id=98da874c43035a490cdca81331724f233a3d0c9a

Link: https://lore.kernel.org/all/Y7i8+EjwdnhHtlrr@dev-arch.thelio-3990X/
Fixes: 5d4aeffbf709 ("kbuild: rebuild .vmlinux.export.o when its prerequisite is updated")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Nathan Chancellor <nathan@kernel.org>