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Date:   Sun, 8 Jan 2023 14:22:37 +0100
From:   Nicolas Schier <nicolas@fjasle.eu>
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH v2] kbuild: do not export LDFLAGS_vmlinux to decompressor
 Makefiles
Message-ID: <Y7rDnda0PzCw2Eao@fjasle.eu>
References: <20230108095342.2766757-1-masahiroy@kernel.org>
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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@kernel.org>
> To: linux-kbuild@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org, Nathan Chancellor <nathan@kernel.org>,
>  Masahiro Yamada <masahiroy@kernel.org>, Nick Desaulniers
>  <ndesaulniers@google.com>, Nicolas Schier <nicolas@fjasle.eu>
> Subject: [PATCH v2] kbuild: do not export LDFLAGS_vmlinux to decompressor
>  Makefiles
> Message-Id: <20230108095342.2766757-1-masahiroy@kernel.org>
> X-Mailer: git-send-email 2.34.1
>=20
> Nathan Chancellor reports an error message from $(NM) if GNU Make 4.4
> is used to build the ARM decompressor.
>=20
>   $ make-4.4 ARCH=3Darm CROSS_COMPILE=3Darm-linux-gnueabi- O=3Dbuild defc=
onfig 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': N=
o 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
>=20
> This occurs since GNU Make commit 98da874c4303 ("[SV 10593] Export
> variables to $(shell ...) commands"), and the O=3D option is needed to
> reproduce it. The generated zImage is correct despite the error message.
>=20
> 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.
>=20
> The following test code demonstrates the change for GNU Make 4.4.
>=20
> [Test Makefile]
>=20
>   $(shell echo hello > foo)
>   export foo =3D $(shell cat bar/../foo)
>   $(shell mkdir bar)
>=20
>   all:
>           @echo $(foo)
>=20
> [GNU Make 4.3]
>=20
>   $ rm -rf bar; make-4.3
>   hello
>=20
> [GNU Make 4.4]
>=20
>   $ rm -rf bar; make-4.4
>   cat: bar/../foo: No such file or directory
>   hello
>=20
> The 'foo' is a resursively expanded (i.e. lazily expanded) variable.
>=20
> GNU Make 4.3 expands 'foo' just before running the recipe '@echo $(foo)',
> at this point, the directory 'bar' exists.
>=20
> 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.
>=20
> Let's get back to the kernel Makefile.
>=20
> In arch/arm/boot/compressed/Makefile, KBSS_SZ is referenced by
> LDFLAGS_vmlinux, which is recursive and also exported by the top
> Makefile.
>=20
> GNU Make 4.3 expands KBSS_SZ just before running the recipes, so it
> works fine.
>=20
> 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=3D option,
> the output directory is created by $(shell mkdir -p $(obj-dirs))
> in scripts/Makefile.build.
>=20
> There are two ways to fix this particular issue:
>=20
>  - change "$(obj)/../../../../vmlinux" in KBSS_SZ to "vmlinux"
>  - unexport LDFLAGS_vmlinux
>=20
> This commit takes the latter course because it is what I originally
> intended.
>=20
> Commit 3ec8a5b33dea ("kbuild: do not export LDFLAGS_vmlinux")
> unexported LDFLAGS_vmlinux.
>=20
> Commit 5d4aeffbf709 ("kbuild: rebuild .vmlinux.export.o when its
> prerequisite is updated") accidentally exported it again.
>=20
> We can clean up arch/arm/boot/compressed/Makefile later.
>=20
> [1]: https://git.savannah.gnu.org/cgit/make.git/commit/?id=3D98da874c4303=
5a490cdca81331724f233a3d0c9a
>=20
> Link: https://lore.kernel.org/all/Y7i8+EjwdnhHtlrr@dev-arch.thelio-3990X/
> Fixes: 5d4aeffbf709 ("kbuild: rebuild .vmlinux.export.o when its prerequi=
site is updated")
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> ---
>=20
> Changes in v2:
>   - Postpone the change for arch/arm/boot/compressed/Makefile
>   - Update the commit description
>=20
>  Makefile | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)

Once again thanks for the interesting details!

Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

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