Commit af73d78b authored by Kees Cook's avatar Kees Cook Committed by Daniel Borkmann
Browse files

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: default avatarKees Cook <keescook@chromium.org>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarAndrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/202003031814.4AEA3351@keescook
parent 0641cada

scripts/link-vmlinux.sh

+19 −9
Original line number Diff line number Diff line
@@ -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

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