kpatch-build: add cross-compilation support

This patch introduces ability to pick a CROSS_COMPILE
environment variable which can be used for
specifying cross-complier prefix.
It allows to build live patches not only on
target system, but also on hosts for a target other
than the one on which the compiler is running

Author: rbilovol

kpatch-build/kpatch-build

@@ -57,6 +57,7 @@ declare -a PATCH_LIST
 APPLIED_PATCHES=0
 OOT_MODULE=
 KLP_REPLACE=1
+KPATCH_CROSS_COMPILE="$CROSS_COMPILE"
 
 warn() {
 	echo "ERROR: $1" >&2
@@ -278,7 +279,7 @@ gcc_version_check() {
 	# gcc --version varies between distributions therefore extract version
 	# by compiling a test file and compare it to vmlinux's version.
 	echo 'void main(void) {}' > "$c"
-	out="$(gcc -c -pg -ffunction-sections -o "$o" "$c" 2>&1)"
+	out="$("${KPATCH_CROSS_COMPILE}"gcc -c -pg -ffunction-sections -o "$o" "$c" 2>&1)"
 	gccver="$(gcc_version_from_file "$o")"
 	kgccver="$(gcc_version_from_file "$target")"
 
@@ -288,7 +289,7 @@ gcc_version_check() {
 		return 1
 	fi
 
-	out="$(gcc -c -gz=none -o "$o" "$c" 2>&1)"
+	out="$("${KPATCH_CROSS_COMPILE}"gcc -c -gz=none -o "$o" "$c" 2>&1)"
 	if [[ -z "$out" ]]; then
 		DEBUG_KCFLAGS="-gz=none"
 	fi
@@ -946,14 +947,14 @@ if [ "$CONFIG_CC_IS_CLANG" -eq 1 ]; then
 	MAKEVARS+=("CC=${KPATCH_CC_PREFIX}clang")
 	MAKEVARS+=("HOSTCC=clang")
 else
-	MAKEVARS+=("CC=${KPATCH_CC_PREFIX}gcc")
+	MAKEVARS+=("CC=${KPATCH_CC_PREFIX}${KPATCH_CROSS_COMPILE}gcc")
 fi
 
 if [ "$CONFIG_LD_IS_LLD" -eq 1 ]; then
 	MAKEVARS+=("LD=${KPATCH_CC_PREFIX}ld.lld")
 	MAKEVARS+=("HOSTLD=ld.lld")
 else
-	MAKEVARS+=("LD=${KPATCH_CC_PREFIX}ld")
+	MAKEVARS+=("LD=${KPATCH_CC_PREFIX}${KPATCH_CROSS_COMPILE}ld")
 fi
 
 
@@ -1144,7 +1145,7 @@ fi
 cd "$TEMPDIR/output" || die
 # $KPATCH_LDFLAGS and result of find used as list, no quotes.
 # shellcheck disable=SC2086,SC2046
-ld -r $KPATCH_LDFLAGS -o ../patch/tmp_output.o $(find . -name "*.o") 2>&1 | logger || die
+"$KPATCH_CROSS_COMPILE"ld -r $KPATCH_LDFLAGS -o ../patch/tmp_output.o $(find . -name "*.o") 2>&1 | logger || die
 
 if [[ "$USE_KLP" -eq 1 ]]; then
 	cp -f "$TEMPDIR"/patch/tmp_output.o "$TEMPDIR"/patch/output.o || die
@@ -1153,7 +1154,7 @@ if [[ "$USE_KLP" -eq 1 ]]; then
 else
 	# Add .kpatch.checksum for kpatch script
 	md5sum ../patch/tmp_output.o | awk '{printf "%s\0", $1}' > checksum.tmp || die
-	objcopy --add-section .kpatch.checksum=checksum.tmp --set-section-flags .kpatch.checksum=alloc,load,contents,readonly  ../patch/tmp_output.o || die
+	"$KPATCH_CROSS_COMPILE"objcopy --add-section .kpatch.checksum=checksum.tmp --set-section-flags .kpatch.checksum=alloc,load,contents,readonly  ../patch/tmp_output.o || die
 	rm -f checksum.tmp
 	"$TOOLSDIR"/create-kpatch-module "$TEMPDIR"/patch/tmp_output.o "$TEMPDIR"/patch/output.o 2>&1 | logger 1
 	check_pipe_status create-kpatch-module
@@ -1174,7 +1175,8 @@ done
 
 export KPATCH_BUILD="$KPATCH_BUILD" KPATCH_NAME="$MODNAME" \
 KBUILD_EXTRA_SYMBOLS="$KBUILD_EXTRA_SYMBOLS" \
-KPATCH_LDFLAGS="$KPATCH_LDFLAGS"
+KPATCH_LDFLAGS="$KPATCH_LDFLAGS" \
+CROSS_COMPILE="$KPATCH_CROSS_COMPILE"
 save_env
 
 make "${MAKEVARS[@]}" 2>&1 | logger || die