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|
#!/bin/sh
# SPDX-License-Identifier: GPL-2.0
# Copyright (c) 2025 Meta Platforms, Inc. and affiliates.
#
# This script generates BTF data for the provided ELF file.
#
# Kernel BTF generation involves these conceptual steps:
# 1. pahole generates BTF from DWARF data
# 2. resolve_btfids applies kernel-specific btf2btf
# transformations and computes data for .BTF_ids section
# 3. the result gets linked/objcopied into the target binary
#
# How step (3) should be done differs between vmlinux, and
# kernel modules, which is the primary reason for the existence
# of this script.
#
# For modules the script expects vmlinux passed in as --btf_base.
# Generated .BTF, .BTF.base and .BTF_ids sections become embedded
# into the input ELF file with objcopy.
#
# For vmlinux the input file remains unchanged and two files are produced:
# - ${1}.btf.o ready for linking into vmlinux
# - ${1}.BTF_ids with .BTF_ids data blob
# This output is consumed by scripts/link-vmlinux.sh
set -e
usage()
{
echo "Usage: $0 [--btf_base <file>] <target ELF file>"
exit 1
}
BTF_BASE=""
while [ $# -gt 0 ]; do
case "$1" in
--btf_base)
BTF_BASE="$2"
shift 2
;;
-*)
echo "Unknown option: $1" >&2
usage
;;
*)
break
;;
esac
done
if [ $# -ne 1 ]; then
usage
fi
ELF_FILE="$1"
shift
is_enabled() {
grep -q "^$1=y" ${objtree}/include/config/auto.conf
}
case "${KBUILD_VERBOSE}" in
*1*)
set -x
;;
esac
gen_btf_data()
{
btf1="${ELF_FILE}.BTF.1"
${PAHOLE} -J ${PAHOLE_FLAGS} \
${BTF_BASE:+--btf_base ${BTF_BASE}} \
--btf_encode_detached=${btf1} \
"${ELF_FILE}"
${RESOLVE_BTFIDS} ${RESOLVE_BTFIDS_FLAGS} \
${BTF_BASE:+--btf_base ${BTF_BASE}} \
--btf ${btf1} "${ELF_FILE}"
}
gen_btf_o()
{
btf_data=${ELF_FILE}.btf.o
# Create ${btf_data} which contains just .BTF section but no symbols. Add
# SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all
# deletes all symbols including __start_BTF and __stop_BTF, which will
# be redefined in the linker script.
echo "" | ${CC} ${CLANG_FLAGS} ${KBUILD_CPPFLAGS} ${KBUILD_CFLAGS} -fno-lto -c -x c -o ${btf_data} -
${OBJCOPY} --add-section .BTF=${ELF_FILE}.BTF \
--set-section-flags .BTF=alloc,readonly ${btf_data}
${OBJCOPY} --only-section=.BTF --strip-all ${btf_data}
# Change e_type to ET_REL so that it can be used to link final vmlinux.
# GNU ld 2.35+ and lld do not allow an ET_EXEC input.
if is_enabled CONFIG_CPU_BIG_ENDIAN; then
et_rel='\0\1'
else
et_rel='\1\0'
fi
printf "${et_rel}" | dd of="${btf_data}" conv=notrunc bs=1 seek=16 status=none
}
embed_btf_data()
{
${OBJCOPY} --add-section .BTF=${ELF_FILE}.BTF ${ELF_FILE}
# a module might not have a .BTF_ids or .BTF.base section
btf_base="${ELF_FILE}.BTF.base"
if [ -f "${btf_base}" ]; then
${OBJCOPY} --add-section .BTF.base=${btf_base} ${ELF_FILE}
fi
btf_ids="${ELF_FILE}.BTF_ids"
if [ -f "${btf_ids}" ]; then
${RESOLVE_BTFIDS} --patch_btfids ${btf_ids} ${ELF_FILE}
fi
}
cleanup()
{
rm -f "${ELF_FILE}.BTF.1"
rm -f "${ELF_FILE}.BTF"
if [ "${BTFGEN_MODE}" = "module" ]; then
rm -f "${ELF_FILE}.BTF.base"
rm -f "${ELF_FILE}.BTF_ids"
fi
}
trap cleanup EXIT
BTFGEN_MODE="vmlinux"
if [ -n "${BTF_BASE}" ]; then
BTFGEN_MODE="module"
fi
gen_btf_data
case "${BTFGEN_MODE}" in
vmlinux)
gen_btf_o
;;
module)
embed_btf_data
;;
esac
exit 0
|
