Re: [PATCH] kbuild: Remove debug info from kallsyms linking

Kees Cook
2020-03-03T04:49:01Z

On Mon, Feb 24, 2020 at 09:16:17PM -0800, Kees Cook wrote:
> 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.
>
> 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>

Ping. Masahiro what do you think of this? It saves me a fair bit of time
on the link stage... I bet the BPF folks would be interested too. :)

-Kees

> ---
> scripts/link-vmlinux.sh | 28 +++++++++++++++++++---------
> 1 file changed, 19 insertions(+), 9 deletions(-)
>
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index dd484e92752e..ac569e197bfa 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/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
> + btf_vmlinux_bin_o=.btf.vmlinux.bin.o
> + 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
> --
> 2.20.1
>
>
> --
> Kees Cook

--
Kees Cook