kbuild: Remove debug info from kallsyms linking

When CONFIG_DEBUG_INFO is enabled, the two kallsyms linking steps spend
time collecting and writing the dwarf sections to the temporary output
files. kallsyms does not need this information, and leaving it off
halves their linking time. This is especially noticeable without
CONFIG_DEBUG_INFO_REDUCED. The BTF linking stage, however, does still
need those details.

Refactor the BTF and kallsyms generation stages slightly for more
regularized temporary names. Skip debug during kallsyms links.
Additionally move "info BTF" to the correct place since commit
8959e392 ("kbuild: Parameterize kallsyms generation and correct
reporting"), which added "info LD ..." to vmlinux_link calls.

For a full debug info build with BTF, my link time goes from 1m06s to
0m54s, saving about 12 seconds, or 18%.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/202003031814.4AEA3351@keescook

scripts/link-vmlinux.sh

@@ -63,12 +63,18 @@ vmlinux_link()
 	local lds="${objtree}/${KBUILD_LDS}"
 	local output=${1}
 	local objects
+	local strip_debug
 
 	info LD ${output}
 
 	# skip output file argument
 	shift
 
+	# The kallsyms linking does not need debug symbols included.
+	if [ "$output" != "${output#.tmp_vmlinux.kallsyms}" ] ; then
+		strip_debug=-Wl,--strip-debug
+	fi
+
 	if [ "${SRCARCH}" != "um" ]; then
 		objects="--whole-archive			\
 			${KBUILD_VMLINUX_OBJS}			\
@@ -79,6 +85,7 @@ vmlinux_link()
 			${@}"
 
 		${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux}	\
+			${strip_debug#-Wl,}			\
 			-o ${output}				\
 			-T ${lds} ${objects}
 	else
@@ -91,6 +98,7 @@ vmlinux_link()
 			${@}"
 
 		${CC} ${CFLAGS_vmlinux}				\
+			${strip_debug}				\
 			-o ${output}				\
 			-Wl,-T,${lds}				\
 			${objects}				\
@@ -106,6 +114,8 @@ gen_btf()
 {
 	local pahole_ver
 	local bin_arch
+	local bin_format
+	local bin_file
 
 	if ! [ -x "$(command -v ${PAHOLE})" ]; then
 		echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"
@@ -118,8 +128,9 @@ gen_btf()
 		return 1
 	fi
 
-	info "BTF" ${2}
 	vmlinux_link ${1}
+
+	info "BTF" ${2}
 	LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
 
 	# dump .BTF section into raw binary file to link with final vmlinux
@@ -127,11 +138,12 @@ gen_btf()
 		cut -d, -f1 | cut -d' ' -f2)
 	bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
 		awk '{print $4}')
+	bin_file=.btf.vmlinux.bin
 	${OBJCOPY} --change-section-address .BTF=0 \
 		--set-section-flags .BTF=alloc -O binary \
-		--only-section=.BTF ${1} .btf.vmlinux.bin
+		--only-section=.BTF ${1} $bin_file
 	${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \
-		--rename-section .data=.BTF .btf.vmlinux.bin ${2}
+		--rename-section .data=.BTF $bin_file ${2}
 }
 
 # Create ${2} .o file with all symbols from the ${1} object file
@@ -166,8 +178,8 @@ kallsyms()
 kallsyms_step()
 {
 	kallsymso_prev=${kallsymso}
-	kallsymso=.tmp_kallsyms${1}.o
-	kallsyms_vmlinux=.tmp_vmlinux${1}
+	kallsyms_vmlinux=.tmp_vmlinux.kallsyms${1}
+	kallsymso=${kallsyms_vmlinux}.o
 
 	vmlinux_link ${kallsyms_vmlinux} "${kallsymso_prev}" ${btf_vmlinux_bin_o}
 	kallsyms ${kallsyms_vmlinux} ${kallsymso}
@@ -190,7 +202,6 @@ cleanup()
 {
 	rm -f .btf.*
 	rm -f .tmp_System.map
-	rm -f .tmp_kallsyms*
 	rm -f .tmp_vmlinux*
 	rm -f System.map
 	rm -f vmlinux
@@ -257,9 +268,8 @@ tr '\0' '\n' < modules.builtin.modinfo | sed -n 's/^[[:alnum:]:_]*\.file=//p' |
 
 btf_vmlinux_bin_o=""
 if [ -n "${CONFIG_DEBUG_INFO_BTF}" ]; then
-	if gen_btf .tmp_vmlinux.btf .btf.vmlinux.bin.o ; then
 	btf_vmlinux_bin_o=.btf.vmlinux.bin.o
-	else
+	if ! gen_btf .tmp_vmlinux.btf $btf_vmlinux_bin_o ; then
 		echo >&2 "Failed to generate BTF for vmlinux"
 		echo >&2 "Try to disable CONFIG_DEBUG_INFO_BTF"
 		exit 1