[PATCH v3 3/3] kbuild: merge temporary vmlinux for BTF and kallsyms

Masahiro Yamada
2024-06-10T11:27:09Z

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 <masahiroy@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
---

Changes in v3:
- Use ${strip_debug} for clean-up

Changes in v2:
- Fix the handling --strip-debug flag

scripts/link-vmlinux.sh | 41 ++++++++++++++++++++++++-----------------
1 file changed, 24 insertions(+), 17 deletions(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 83d605ba7241..c64158a04f82 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -105,11 +105,10 @@ vmlinux_link()

# 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"
@@ -122,8 +121,6 @@ gen_btf()
return 1
fi

- vmlinux_link ${1}
-
info BTF "${btf_data}"
LLVM_OBJCOPY="${OBJCOPY}" ${PAHOLE} -J ${PAHOLE_FLAGS} ${1}

@@ -177,15 +174,13 @@ kallsyms()
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}
@@ -228,11 +223,21 @@ kallsymso=
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
@@ -268,14 +273,16 @@ if is_enabled CONFIG_KALLSYMS; then
# 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

@@ -301,7 +308,7 @@ 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
--
2.43.0