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

[Nicolas Schier]

On Sun, Jan 08, 2023 at 06:53:42PM +0900 Masahiro Yamada wrote:
> Date: Sun,  8 Jan 2023 18:53:42 +0900
> From: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> To: linux-kbuild@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx, Nathan Chancellor <nathan@xxxxxxxxxx>,
>  Masahiro Yamada <masahiroy@xxxxxxxxxx>, Nick Desaulniers
>  <ndesaulniers@xxxxxxxxxx>, Nicolas Schier <nicolas@xxxxxxxxx>
> Subject: [PATCH v2] kbuild: do not export LDFLAGS_vmlinux to decompressor
>  Makefiles
> Message-Id: <20230108095342.2766757-1-masahiroy@xxxxxxxxxx>
> X-Mailer: git-send-email 2.34.1
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> ---
> 
> 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(-)

Once again thanks for the interesting details!

Reviewed-by: Nicolas Schier <nicolas@xxxxxxxxx>

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