[PATCH v2] kbuild: do not export LDFLAGS_vmlinux to decompressor Makefiles

[Masahiro Yamada <masahiroy@kernel.org>]

Nathan Chancellor reports an error message from $(NM) if GNU Make 4.4
is used to build the ARM decompressor.

  $ 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 it
works fine.

GNU Make 4.4 expands KBSS_SZ in the parse stage, where the directory
arm/arm/boot/compressed does not exit yet. When compiled O= option,
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>
---

Changes in v2:
  - Postpone the change for arch/arm/boot/compressed/Makefile
  - Update the commit description

 Makefile | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index dfba294ae790..658fc0e602c9 100644
--- a/Makefile
+++ b/Makefile
@@ -549,7 +549,7 @@ LDFLAGS_MODULE  =
 CFLAGS_KERNEL	=
 RUSTFLAGS_KERNEL =
 AFLAGS_KERNEL	=
-export LDFLAGS_vmlinux =
+LDFLAGS_vmlinux =
 
 # Use USERINCLUDE when you must reference the UAPI directories only.
 USERINCLUDE    := \
@@ -1248,6 +1248,11 @@ vmlinux.o modules.builtin.modinfo modules.builtin: vmlinux_o
 	@:
 
 PHONY += vmlinux
+# LDFLAGS_vmlinux flags from the top Makefile and arch/*/Makefile should be
+# exported for building the top vmlinux, but not for building decompressors.
+# They happen to have the same base name (arch/*/boot/compressed/vmlinux).
+# The decompressor Makefiles should not inherit LDFLAGS_vmlinux.
+vmlinux: export LDFLAGS_vmlinux := $(LDFLAGS_vmlinux)
 vmlinux: vmlinux.o $(KBUILD_LDS) modpost
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.vmlinux
 
-- 
2.34.1