kbuild: check the minimum assembler version in Kconfig

commit ba64beb174 upstream.

Documentation/process/changes.rst defines the minimum assembler version
(binutils version), but we have never checked it in the build time.

Kbuild never invokes 'as' directly because all assembly files in the
kernel tree are *.S, hence must be preprocessed. I do not expect
raw assembly source files (*.s) would be added to the kernel tree.

Therefore, we always use $(CC) as the assembler driver, and commit
aa824e0c96 ("kbuild: remove AS variable") removed 'AS'. However,
we are still interested in the version of the assembler acting behind.

As usual, the --version option prints the version string.

  $ as --version | head -n 1
  GNU assembler (GNU Binutils for Ubuntu) 2.35.1

But, we do not have $(AS). So, we can add the -Wa prefix so that
$(CC) passes --version down to the backing assembler.

  $ gcc -Wa,--version | head -n 1
  gcc: fatal error: no input files
  compilation terminated.

OK, we need to input something to satisfy gcc.

  $ gcc -Wa,--version -c -x assembler /dev/null -o /dev/null | head -n 1
  GNU assembler (GNU Binutils for Ubuntu) 2.35.1

The combination of Clang and GNU assembler works in the same way:

  $ clang -no-integrated-as -Wa,--version -c -x assembler /dev/null -o /dev/null | head -n 1
  GNU assembler (GNU Binutils for Ubuntu) 2.35.1

Clang with the integrated assembler fails like this:

  $ clang -integrated-as -Wa,--version -c -x assembler /dev/null -o /dev/null | head -n 1
  clang: error: unsupported argument '--version' to option 'Wa,'

For the last case, checking the error message is fragile. If the
proposal for -Wa,--version support [1] is accepted, this may not be
even an error in the future.

One easy way is to check if -integrated-as is present in the passed
arguments. We did not pass -integrated-as to CLANG_FLAGS before, but
we can make it explicit.

Nathan pointed out -integrated-as is the default for all of the
architectures/targets that the kernel cares about, but it goes
along with "explicit is better than implicit" policy. [2]

With all this in my mind, I implemented scripts/as-version.sh to
check the assembler version in Kconfig time.

  $ scripts/as-version.sh gcc
  GNU 23501
  $ scripts/as-version.sh clang -no-integrated-as
  GNU 23501
  $ scripts/as-version.sh clang -integrated-as
  LLVM 0

[1]: https://github.com/ClangBuiltLinux/linux/issues/1320
[2]: https://lore.kernel.org/linux-kbuild/20210307044253.v3h47ucq6ng25iay@archlinux-ax161/

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
[nathan: Backport to 5.10. Drop minimum version checking, as it is not
         required in 5.10]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Makefile

@@ -580,7 +580,9 @@ endif
 ifneq ($(GCC_TOOLCHAIN),)
 CLANG_FLAGS	+= --gcc-toolchain=$(GCC_TOOLCHAIN)
 endif
-ifneq ($(LLVM_IAS),1)
+ifeq ($(LLVM_IAS),1)
+CLANG_FLAGS	+= -integrated-as
+else
 CLANG_FLAGS	+= -no-integrated-as
 endif
 CLANG_FLAGS	+= -Werror=unknown-warning-option

init/Kconfig

@@ -47,6 +47,18 @@ config CLANG_VERSION
 	int
 	default $(shell,$(srctree)/scripts/clang-version.sh $(CC))
 
+config AS_IS_GNU
+	def_bool $(success,test "$(as-name)" = GNU)
+
+config AS_IS_LLVM
+	def_bool $(success,test "$(as-name)" = LLVM)
+
+config AS_VERSION
+	int
+	# Use clang version if this is the integrated assembler
+	default CLANG_VERSION if AS_IS_LLVM
+	default $(as-version)
+
 config LLD_VERSION
 	int
 	default $(shell,$(srctree)/scripts/lld-version.sh $(LD))

scripts/Kconfig.include

@@ -42,6 +42,12 @@ $(error-if,$(failure,command -v $(LD)),linker '$(LD)' not found)
 # Fail if the linker is gold as it's not capable of linking the kernel proper
 $(error-if,$(success, $(LD) -v | grep -q gold), gold linker '$(LD)' not supported)
 
+# Get the assembler name, version, and error out if it is not supported.
+as-info := $(shell,$(srctree)/scripts/as-version.sh $(CC) $(CLANG_FLAGS))
+$(error-if,$(success,test -z "$(as-info)"),Sorry$(comma) this assembler is not supported.)
+as-name := $(shell,set -- $(as-info) && echo $1)
+as-version := $(shell,set -- $(as-info) && echo $2)
+
 # machine bit flags
 #  $(m32-flag): -m32 if the compiler supports it, or an empty string otherwise.
 #  $(m64-flag): -m64 if the compiler supports it, or an empty string otherwise.

scripts/as-version.sh

@@ -0,0 +1,69 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Print the assembler name and its version in a 5 or 6-digit form.
+# Also, perform the minimum version check.
+# (If it is the integrated assembler, return 0 as the version, and
+# skip the version check.)
+
+set -e
+
+# Convert the version string x.y.z to a canonical 5 or 6-digit form.
+get_canonical_version()
+{
+	IFS=.
+	set -- $1
+
+	# If the 2nd or 3rd field is missing, fill it with a zero.
+	#
+	# The 4th field, if present, is ignored.
+	# This occurs in development snapshots as in 2.35.1.20201116
+	echo $((10000 * $1 + 100 * ${2:-0} + ${3:-0}))
+}
+
+# Clang fails to handle -Wa,--version unless -no-integrated-as is given.
+# We check -(f)integrated-as, expecting it is explicitly passed in for the
+# integrated assembler case.
+check_integrated_as()
+{
+	while [ $# -gt 0 ]; do
+		if [ "$1" = -integrated-as -o "$1" = -fintegrated-as ]; then
+			# For the intergrated assembler, we do not check the
+			# version here. It is the same as the clang version, and
+			# it has been already checked by scripts/cc-version.sh.
+			echo LLVM 0
+			exit 0
+		fi
+		shift
+	done
+}
+
+check_integrated_as "$@"
+
+orig_args="$@"
+
+# Get the first line of the --version output.
+IFS='
+'
+set -- $(LC_ALL=C "$@" -Wa,--version -c -x assembler /dev/null -o /dev/null 2>/dev/null)
+
+# Split the line on spaces.
+IFS=' '
+set -- $1
+
+if [ "$1" = GNU -a "$2" = assembler ]; then
+	shift $(($# - 1))
+	version=$1
+	name=GNU
+else
+	echo "$orig_args: unknown assembler invoked" >&2
+	exit 1
+fi
+
+# Some distributions append a package release number, as in 2.34-4.fc32
+# Trim the hyphen and any characters that follow.
+version=${version%-*}
+
+cversion=$(get_canonical_version $version)
+
+echo $name $cversion

scripts/dummy-tools/gcc

@@ -67,6 +67,12 @@ if arg_contain -E "$@"; then
 	fi
 fi
 
+# To set CONFIG_AS_IS_GNU
+if arg_contain -Wa,--version "$@"; then
+	echo "GNU assembler (scripts/dummy-tools) 2.50"
+	exit 0
+fi
+
 if arg_contain -S "$@"; then
 	# For scripts/gcc-x86-*-has-stack-protector.sh
 	if arg_contain -fstack-protector "$@"; then