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Date:   Tue, 7 Apr 2020 13:11:52 -0400
From:   Jeremy Cline <jcline@redhat.com>
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     linux-kbuild@vger.kernel.org,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Philipp Rudo <prudo@linux.ibm.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] kbuild: add dummy toolchains to enable all cc-option
 etc. in Kconfig
Message-ID: <20200407171152.GC196945@dev.jcline.org>
References: <20200407155352.16617-1-masahiroy@kernel.org>
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On Wed, Apr 08, 2020 at 12:53:52AM +0900, Masahiro Yamada wrote:
> Staring v4.18, Kconfig evaluates compiler capabilities, and hides CONFIG
> options your compiler does not support. This works well if you configure
> and build the kernel on the same host machine.
> 
> It is inconvenient if you prepare the .config that is carried to a
> different build environment (typically this happens when you package
> the kernel for distros) because using a different compiler potentially
> produces different CONFIG options than the real build environment.
> So, you probably want to make as many options visible as possible.
> In other words, you need to create a super-set of CONFIG options that
> cover any build environment. If some of the CONFIG options turned out
> to be unsupported on the build machine, they are automatically disabled
> by the nature of Kconfig.
> 
> However, it is not feasible to get a full-featured compiler for every
> arch.
> 
> This issue was discussed here:
> 
>   https://lkml.org/lkml/2019/12/9/620
> 
> Other than distros, savedefconfig is also a problem. Some arch sub-systems
> periodically resync defconfig files. If you use a less-capable compiler
> for savedefconfig, options that do not meet 'depends on $(cc-option,...)'
> will be forcibly disabled. So, 'make defconfig && make savedefconfig'
> may silently change the behavior.
> 
> This commit adds a set of dummy toolchains that pretend to support any
> feature.
> 
> Most of compiler features are tested by cc-option, which simply checks
> the exit code of $(CC). The dummy tools are shell scripts that always
> exit with 0. So, $(cc-option, ...) is evaluated as 'y'.
> 
> There are more complicated checks such as:
> 
>   scripts/gcc-x86_{32,64}-has-stack-protector.sh
>   scripts/gcc-plugin.sh
>   scripts/tools-support-relr.sh
> 
> scripts/dummy-tools/gcc passes all checks.
> 
> From the top directory of the source tree, you can do:
> 
>    $ make CROSS_COMPILE=scripts/dummy-tools/ oldconfig
> 
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> Reviewed-by: Philipp Rudo <prudo@linux.ibm.com>
> ---
> 
> Changes in v2:
>   - support --version and -v for ld, objdump, nm

Great, "make ARCH=powerpc CROSS_COMPILE=scripts/dummy-tools/ help" no
longer spits out "/bin/sh: line 0: [: -ge: unary operator expected" and
everything looks to work as expected.

Tested-by: Jeremy Cline <jcline@redhat.com>

> 
>  scripts/dummy-tools/gcc     | 91 +++++++++++++++++++++++++++++++++++++
>  scripts/dummy-tools/ld      | 30 ++++++++++++
>  scripts/dummy-tools/nm      |  1 +
>  scripts/dummy-tools/objcopy |  1 +
>  4 files changed, 123 insertions(+)
>  create mode 100755 scripts/dummy-tools/gcc
>  create mode 100755 scripts/dummy-tools/ld
>  create mode 120000 scripts/dummy-tools/nm
>  create mode 120000 scripts/dummy-tools/objcopy
> 
> diff --git a/scripts/dummy-tools/gcc b/scripts/dummy-tools/gcc
> new file mode 100755
> index 000000000000..33487e99d83e
> --- /dev/null
> +++ b/scripts/dummy-tools/gcc
> @@ -0,0 +1,91 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0-only
> +#
> +# Staring v4.18, Kconfig evaluates compiler capabilities, and hides CONFIG
> +# options your compiler does not support. This works well if you configure and
> +# build the kernel on the same host machine.
> +#
> +# It is inconvenient if you prepare the .config that is carried to a different
> +# build environment (typically this happens when you package the kernel for
> +# distros) because using a different compiler potentially produces different
> +# CONFIG options than the real build environment. So, you probably want to make
> +# as many options visible as possible. In other words, you need to create a
> +# super-set of CONFIG options that cover any build environment. If some of the
> +# CONFIG options turned out to be unsupported on the build machine, they are
> +# automatically disabled by the nature of Kconfig.
> +#
> +# However, it is not feasible to get a full-featured compiler for every arch.
> +# Hence these dummy toolchains to make all compiler tests pass.
> +#
> +# Usage:
> +#
> +# From the top directory of the source tree, run
> +#
> +#   $ make CROSS_COMPILE=scripts/dummy-tools/ oldconfig
> +#
> +# Most of compiler features are tested by cc-option, which simply checks the
> +# exit code of $(CC). This script does nothing and just exits with 0 in most
> +# cases. So, $(cc-option, ...) is evaluated as 'y'.
> +#
> +# This scripts caters to more checks; handle --version and pre-process __GNUC__
> +# etc. to pretend to be GCC, and also do right things to satisfy some scripts.
> +
> +# Check if the first parameter appears in the rest. Succeeds if found.
> +# This helper is useful if a particular option was passed to this script.
> +# Typically used like this:
> +#   arg_contain <word-you-are-searching-for> "$@"
> +arg_contain ()
> +{
> +	search="$1"
> +	shift
> +
> +	while [ $# -gt 0 ]
> +	do
> +		if [ "$search" = "$1" ]; then
> +			return 0
> +		fi
> +		shift
> +	done
> +
> +	return 1
> +}
> +
> +# To set CONFIG_CC_IS_GCC=y
> +if arg_contain --version "$@"; then
> +	echo "gcc (scripts/dummy-tools/gcc)"
> +	exit 0
> +fi
> +
> +if arg_contain -E "$@"; then
> +	# For scripts/gcc-version.sh; This emulates GCC 20.0.0
> +	if arg_contain - "$@"; then
> +		sed 's/^__GNUC__$/20/; s/^__GNUC_MINOR__$/0/; s/^__GNUC_PATCHLEVEL__$/0/'
> +		exit 0
> +	else
> +		echo "no input files" >&2
> +		exit 1
> +	fi
> +fi
> +
> +if arg_contain -S "$@"; then
> +	# For scripts/gcc-x86-*-has-stack-protector.sh
> +	if arg_contain -fstack-protector "$@"; then
> +		echo "%gs"
> +		exit 0
> +	fi
> +fi
> +
> +# For scripts/gcc-plugin.sh
> +if arg_contain -print-file-name=plugin "$@"; then
> +	plugin_dir=$(mktemp -d)
> +
> +	sed -n 's/.*#include "\(.*\)"/\1/p' $(dirname $0)/../gcc-plugins/gcc-common.h |
> +	while read header
> +	do
> +		mkdir -p $plugin_dir/include/$(dirname $header)
> +		touch $plugin_dir/include/$header
> +	done
> +
> +	echo $plugin_dir
> +	exit 0
> +fi
> diff --git a/scripts/dummy-tools/ld b/scripts/dummy-tools/ld
> new file mode 100755
> index 000000000000..f68233050405
> --- /dev/null
> +++ b/scripts/dummy-tools/ld
> @@ -0,0 +1,30 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0-only
> +
> +# Dummy script that always succeeds.
> +
> +# Check if the first parameter appears in the rest. Succeeds if found.
> +# This helper is useful if a particular option was passed to this script.
> +# Typically used like this:
> +#   arg_contain <word-you-are-searching-for> "$@"
> +arg_contain ()
> +{
> +	search="$1"
> +	shift
> +
> +	while [ $# -gt 0 ]
> +	do
> +		if [ "$search" = "$1" ]; then
> +			return 0
> +		fi
> +		shift
> +	done
> +
> +	return 1
> +}
> +
> +if arg_contain --version "$@" || arg_contain -v "$@"; then
> +	progname=$(basename $0)
> +	echo "GNU $progname (scripts/dummy-tools/$progname) 2.50"
> +	exit 0
> +fi
> diff --git a/scripts/dummy-tools/nm b/scripts/dummy-tools/nm
> new file mode 120000
> index 000000000000..c0648b38dd42
> --- /dev/null
> +++ b/scripts/dummy-tools/nm
> @@ -0,0 +1 @@
> +ld
> \ No newline at end of file
> diff --git a/scripts/dummy-tools/objcopy b/scripts/dummy-tools/objcopy
> new file mode 120000
> index 000000000000..c0648b38dd42
> --- /dev/null
> +++ b/scripts/dummy-tools/objcopy
> @@ -0,0 +1 @@
> +ld
> \ No newline at end of file
> -- 
> 2.17.1
>