resolve_btfids: Support for BTF modifications

MAINTAINERS

@@ -4672,6 +4672,7 @@ F:	net/sched/act_bpf.c
 F:	net/sched/cls_bpf.c
 F:	samples/bpf/
 F:	scripts/bpf_doc.py
+F:	scripts/gen-btf.sh
 F:	scripts/Makefile.btf
 F:	scripts/pahole-version.sh
 F:	tools/bpf/

scripts/Makefile.modfinal

@@ -38,9 +38,8 @@ quiet_cmd_btf_ko = BTF [M] $@
       cmd_btf_ko = 							\
 	if [ ! -f $(objtree)/vmlinux ]; then				\
 		printf "Skipping BTF generation for %s due to unavailability of vmlinux\n" $@ 1>&2; \
-	else								\
-		LLVM_OBJCOPY="$(OBJCOPY)" $(PAHOLE) -J $(PAHOLE_FLAGS) $(MODULE_PAHOLE_FLAGS) --btf_base $(objtree)/vmlinux $@; \
-		$(RESOLVE_BTFIDS) -b $(objtree)/vmlinux $@;		\
+	else	\
+		$(objtree)/scripts/gen-btf.sh --btf_base $(objtree)/vmlinux $@; \
 	fi;
 
 # Same as newer-prereqs, but allows to exclude specified extra dependencies

scripts/gen-btf.sh

@@ -0,0 +1,166 @@
+#!/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" ${objtree}/include/config/auto.conf
+}
+
+info()
+{
+	printf "  %-7s %s\n" "${1}" "${2}"
+}
+
+case "${KBUILD_VERBOSE}" in
+*1*)
+	set -x
+	;;
+esac
+
+if ! is_enabled CONFIG_DEBUG_INFO_BTF; then
+	exit 0
+fi
+
+gen_btf_data()
+{
+	info BTF "${ELF_FILE}"
+	btf1="${ELF_FILE}.btf.1"
+	${PAHOLE} -J ${PAHOLE_FLAGS}			\
+		${BTF_BASE:+--btf_base ${BTF_BASE}}	\
+		--btf_encode_detached=${btf1}		\
+		"${ELF_FILE}"
+
+	info BTFIDS "${ELF_FILE}"
+	RESOLVE_BTFIDS_OPTS=""
+	if is_enabled CONFIG_WERROR; then
+		RESOLVE_BTFIDS_OPTS+=" --fatal_warnings "
+	fi
+	if [ -n "${KBUILD_VERBOSE}" ]; then
+		RESOLVE_BTFIDS_OPTS+=" -v "
+	fi
+	${RESOLVE_BTFIDS} ${RESOLVE_BTFIDS_OPTS}	\
+		${BTF_BASE:+--btf_base ${BTF_BASE}}	\
+		--btf ${btf1} "${ELF_FILE}"
+}
+
+gen_btf_o()
+{
+	local btf_data=${ELF_FILE}.btf.o
+
+	# Create ${btf_data} which contains just .BTF section but no symbols. Add
+	# SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all
+	# deletes all symbols including __start_BTF and __stop_BTF, which will
+	# be redefined in the linker script.
+	info OBJCOPY "${btf_data}"
+	echo "" | ${CC} -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="${btf_data}" conv=notrunc bs=1 seek=16 status=none
+}
+
+embed_btf_data()
+{
+	info OBJCOPY "${ELF_FILE}"
+
+	${OBJCOPY} \
+		--add-section .BTF=${ELF_FILE}.btf			\
+		--add-section .BTF.base=${ELF_FILE}.distilled_base.btf	\
+		${ELF_FILE}
+
+	# a module might not have a .BTF_ids section
+	if [ -f "${ELF_FILE}.btf_ids" ]; then
+		${OBJCOPY} --update-section .BTF_ids=${ELF_FILE}.btf_ids ${ELF_FILE}
+	fi
+}
+
+cleanup()
+{
+	rm -f "${ELF_FILE}.btf.1"
+	rm -f "${ELF_FILE}.btf"
+	if [ "${BTFGEN_MODE}" == "module" ]; then
+		rm -f "${ELF_FILE}.distilled_base.btf"
+		rm -f "${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

scripts/link-vmlinux.sh

@@ -105,34 +105,6 @@ vmlinux_link()
 		${kallsymso} ${btf_vmlinux_bin_o} ${arch_vmlinux_o} ${ldlibs}
 }
 
-# generate .BTF typeinfo from DWARF debuginfo
-# ${1} - vmlinux image
-gen_btf()
-{
-	local btf_data=${1}.btf.o
-
-	info BTF "${btf_data}"
-	LLVM_OBJCOPY="${OBJCOPY}" ${PAHOLE} -J ${PAHOLE_FLAGS} ${1}
-
-	# 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. Add 2>/dev/null to suppress GNU
-	# objcopy warnings: "empty loadable segment detected at ..."
-	${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \
-		--strip-all ${1} "${btf_data}" 2>/dev/null
-	# 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
-
-	btf_vmlinux_bin_o=${btf_data}
-}
-
 # Create ${2}.o file with all symbols from the ${1} object file
 kallsyms()
 {
@@ -204,6 +176,7 @@ if is_enabled CONFIG_ARCH_WANTS_PRE_LINK_VMLINUX; then
 fi
 
 btf_vmlinux_bin_o=
+btfids_vmlinux=
 kallsymso=
 strip_debug=
 generate_map=
@@ -224,11 +197,13 @@ if is_enabled CONFIG_KALLSYMS || is_enabled CONFIG_DEBUG_INFO_BTF; then
 fi
 
 if is_enabled CONFIG_DEBUG_INFO_BTF; then
-	if ! gen_btf .tmp_vmlinux1; then
+	if ! scripts/gen-btf.sh .tmp_vmlinux1; then
 		echo >&2 "Failed to generate BTF for vmlinux"
 		echo >&2 "Try to disable CONFIG_DEBUG_INFO_BTF"
 		exit 1
 	fi
+	btf_vmlinux_bin_o=.tmp_vmlinux1.btf.o
+	btfids_vmlinux=.tmp_vmlinux1.btf_ids
 fi
 
 if is_enabled CONFIG_KALLSYMS; then
@@ -281,14 +256,9 @@ fi
 
 vmlinux_link "${VMLINUX}"
 
-# fill in BTF IDs
 if is_enabled CONFIG_DEBUG_INFO_BTF; then
-	info BTFIDS "${VMLINUX}"
-	RESOLVE_BTFIDS_ARGS=""
-	if is_enabled CONFIG_WERROR; then
-		RESOLVE_BTFIDS_ARGS=" --fatal_warnings "
-	fi
-	${RESOLVE_BTFIDS} ${RESOLVE_BTFIDS_ARGS} "${VMLINUX}"
+	info OBJCOPY ${btfids_vmlinux}
+	${OBJCOPY} --update-section .BTF_ids=${btfids_vmlinux} ${VMLINUX}
 fi
 
 mksysmap "${VMLINUX}" System.map