kbuild: merge temporary vmlinux for BTF and kallsyms

CONFIG_DEBUG_INFO_BTF=y requires one additional link step.
(.tmp_vmlinux.btf)

CONFIG_KALLSYMS=y requires two additional link steps.
(.tmp_vmlinux.kallsyms1 and .tmp_vmlinux.kallsyms2)

Enabling both requires three additional link steps.

When CONFIG_DEBUG_INFO_BTF=y and CONFIG_KALLSYMS=y, the current build
process is as follows:

KSYMS .tmp_vmlinux.kallsyms0.S
AS .tmp_vmlinux.kallsyms0.o
LD .tmp_vmlinux.btf # temporary vmlinux for BTF
BTF .btf.vmlinux.bin.o
LD .tmp_vmlinux.kallsyms1 # temporary vmlinux for kallsyms step 1
NM .tmp_vmlinux.kallsyms1.syms
KSYMS .tmp_vmlinux.kallsyms1.S
AS .tmp_vmlinux.kallsyms1.o
LD .tmp_vmlinux.kallsyms2 # temporary vmlinux for kallsyms step 2
NM .tmp_vmlinux.kallsyms2.syms
KSYMS .tmp_vmlinux.kallsyms2.S
AS .tmp_vmlinux.kallsyms2.o
LD vmlinux # final vmlinux

This is redundant because the BTF generation and the kallsyms step 1 can
be performed against the same temporary vmlinux.

When both CONFIG_DEBUG_INFO_BTF and CONFIG_KALLSYMS are enabled, we can
reduce the number of link steps by one.

This commit changes the build process as follows:

KSYMS .tmp_vmlinux0.kallsyms.S
AS .tmp_vmlinux0.kallsyms.o
LD .tmp_vmlinux1 # temporary vmlinux for BTF and kallsyms step 1
BTF .tmp_vmlinux1.btf.o
NM .tmp_vmlinux1.syms
KSYMS .tmp_vmlinux1.kallsyms.S
AS .tmp_vmlinux1.kallsyms.o
LD .tmp_vmlinux2 # temporary vmlinux for kallsyms step 2
NM .tmp_vmlinux2.syms
KSYMS .tmp_vmlinux2.kallsyms.S
AS .tmp_vmlinux2.kallsyms.o
LD vmlinux # final vmlinux

Signed-off-by: Masahiro Yamada <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>

scripts/link-vmlinux.sh

···
 
 # generate .BTF typeinfo from DWARF debuginfo
 # ${1} - vmlinux image
-# ${2} - file to dump raw BTF data into
 gen_btf()
 {
 	local pahole_ver
-	local btf_data=${2}
+	local btf_data=${1}.btf.o
 
 	if ! [ -x "$(command -v ${PAHOLE})" ]; then
 		echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"
···
 		echo >&2 "BTF: ${1}: pahole version $(${PAHOLE} --version) is too old, need at least v1.16"
 		return 1
 	fi
-
-	vmlinux_link ${1}
 
 	info BTF "${btf_data}"
 	LLVM_OBJCOPY="${OBJCOPY}" ${PAHOLE} -J ${PAHOLE_FLAGS} ${1}
···
 	kallsymso=${2}.o
 }
 
-# Perform one step in kallsyms generation, including temporary linking of
-# vmlinux.
-kallsyms_step()
+# Perform kallsyms for the given temporary vmlinux.
+sysmap_and_kallsyms()
 {
-	kallsyms_vmlinux=.tmp_vmlinux.kallsyms${1}
+	mksysmap "${1}" "${1}.syms"
+	kallsyms "${1}.syms" "${1}.kallsyms"
 
-	vmlinux_link "${kallsyms_vmlinux}"
-	mksysmap "${kallsyms_vmlinux}" "${kallsyms_vmlinux}.syms"
-	kallsyms "${kallsyms_vmlinux}.syms" "${kallsyms_vmlinux}"
+	kallsyms_sysmap=${1}.syms
 }
 
 # Create map file with all symbols from ${1}
···
 strip_debug=
 
 if is_enabled CONFIG_KALLSYMS; then
-	kallsyms /dev/null .tmp_vmlinux.kallsyms0
+	kallsyms /dev/null .tmp_vmlinux0.kallsyms
+fi
+
+if is_enabled CONFIG_KALLSYMS || is_enabled CONFIG_DEBUG_INFO_BTF; then
+
+	# The kallsyms linking does not need debug symbols, but the BTF does.
+	if ! is_enabled CONFIG_DEBUG_INFO_BTF; then
+		strip_debug=1
+	fi
+
+	vmlinux_link .tmp_vmlinux1
 fi
 
 if is_enabled CONFIG_DEBUG_INFO_BTF; then
-	if ! gen_btf .tmp_vmlinux.btf .btf.vmlinux.bin.o ; then
+	if ! gen_btf .tmp_vmlinux1; then
 		echo >&2 "Failed to generate BTF for vmlinux"
 		echo >&2 "Try to disable CONFIG_DEBUG_INFO_BTF"
 		exit 1
···
 	# The kallsyms linking does not need debug symbols included.
 	strip_debug=1
 
-	kallsyms_step 1
+	sysmap_and_kallsyms .tmp_vmlinux1
 	size1=$(${CONFIG_SHELL} "${srctree}/scripts/file-size.sh" ${kallsymso})
 
-	kallsyms_step 2
+	vmlinux_link .tmp_vmlinux2
+	sysmap_and_kallsyms .tmp_vmlinux2
 	size2=$(${CONFIG_SHELL} "${srctree}/scripts/file-size.sh" ${kallsymso})
 
 	if [ $size1 -ne $size2 ] || [ -n "${KALLSYMS_EXTRA_PASS}" ]; then
-		kallsyms_step 3
+		vmlinux_link .tmp_vmlinux3
+		sysmap_and_kallsyms .tmp_vmlinux3
 	fi
 fi
 
···
 
 # step a (see comment above)
 if is_enabled CONFIG_KALLSYMS; then
-	if ! cmp -s System.map ${kallsyms_vmlinux}.syms; then
+	if ! cmp -s System.map "${kallsyms_sysmap}"; then
 		echo >&2 Inconsistent kallsyms data
 		echo >&2 'Try "make KALLSYMS_EXTRA_PASS=1" as a workaround'
 		exit 1