Commit 522397d0 authored by Ihor Solodrai's avatar Ihor Solodrai Committed by Andrii Nakryiko
Browse files

resolve_btfids: Change in-place update with raw binary output 

Currently resolve_btfids updates .BTF_ids section of an ELF file
in-place, based on the contents of provided BTF, usually within the
same input file, and optionally a BTF base.

Change resolve_btfids behavior to enable BTF transformations as part
of its main operation. To achieve this, in-place ELF write in
resolve_btfids is replaced with generation of the following binaries:
  * ${1}.BTF with .BTF section data
  * ${1}.BTF_ids with .BTF_ids section data if it existed in ${1}
  * ${1}.BTF.base with .BTF.base section data for out-of-tree modules

The execution of resolve_btfids and consumption of its output is
orchestrated by scripts/gen-btf.sh introduced in this patch.

The motivation for emitting binary data is that it allows simplifying
resolve_btfids implementation by delegating ELF update to the $OBJCOPY
tool [1], which is already widely used across the codebase.

There are two distinct paths for BTF generation and resolve_btfids
application in the kernel build: for vmlinux and for kernel modules.

For the vmlinux binary a .BTF section is added in a roundabout way to
ensure correct linking. The patch doesn't change this approach, only
the implementation is a little different.

Before this patch it worked as follows:

  * pahole consumed .tmp_vmlinux1 [2] and added .BTF section with
    llvm-objcopy [3] to it
  * then everything except the .BTF section was stripped from .tmp_vmlinux1
    into a .tmp_vmlinux1.bpf.o object [2], later linked into vmlinux
  * resolve_btfids was executed later on vmlinux.unstripped [4],
    updating it in-place

After this patch gen-btf.sh implements the following:

  * pahole consumes .tmp_vmlinux1 and produces a *detached* file with
    raw BTF data
  * resolve_btfids consumes .tmp_vmlinux1 and detached BTF to produce
    (potentially modified) .BTF, and .BTF_ids sections data
  * a .tmp_vmlinux1.bpf.o object is then produced with objcopy copying
    BTF output of resolve_btfids
  * .BTF_ids data gets embedded into vmlinux.unstripped in
    link-vmlinux.sh by objcopy --update-section

For kernel modules, creating a special .bpf.o file is not necessary,
and so embedding of sections data produced by resolve_btfids is
straightforward with objcopy.

With this patch an ELF file becomes effectively read-only within
resolve_btfids, which allows deleting elf_update() call and satellite
code (like compressed_section_fix [5]).

Endianness handling of .BTF_ids data is also changed. Previously the
"flags" part of the section was bswapped in sets_patch() [6], and then
Elf_Type was modified before elf_update() to signal to libelf that
bswap may be necessary. With this patch we explicitly bswap entire
data buffer on load and on dump.

[1] https://lore.kernel.org/bpf/131b4190-9c49-4f79-a99d-c00fac97fa44@linux.dev/
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/link-vmlinux.sh?h=v6.18#n110
[3] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/tree/btf_encoder.c?h=v1.31#n1803
[4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/link-vmlinux.sh?h=v6.18#n284
[5] https://lore.kernel.org/bpf/20200819092342.259004-1-jolsa@kernel.org/
[6] https://lore.kernel.org/bpf/cover.1707223196.git.vmalik@redhat.com/



Signed-off-by: default avatarIhor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Tested-by: default avatarAlan Maguire <alan.maguire@oracle.com>
Acked-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20251219181825.1289460-3-ihor.solodrai@linux.dev
parent 014e1cdb

MAINTAINERS

+1 −0
Original line number Diff line number Diff line
@@ -4766,6 +4766,7 @@ 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.btf

+7 −5
Original line number Diff line number Diff line
@@ -18,13 +18,15 @@ pahole-flags-$(call test-ge, $(pahole-ver), 126) = -j$(JOBS) --btf_features=enc 


pahole-flags-$(call test-ge, $(pahole-ver), 130) += --btf_features=attributes



ifneq ($(KBUILD_EXTMOD),)

module-pahole-flags-$(call test-ge, $(pahole-ver), 128) += --btf_features=distilled_base

endif



endif



pahole-flags-$(CONFIG_PAHOLE_HAS_LANG_EXCLUDE)		+= --lang_exclude=rust



export PAHOLE_FLAGS := $(pahole-flags-y)

export MODULE_PAHOLE_FLAGS := $(module-pahole-flags-y)
 


resolve-btfids-flags-y :=

resolve-btfids-flags-$(CONFIG_WERROR) += --fatal_warnings

resolve-btfids-flags-$(if $(KBUILD_EXTMOD),y) += --distill_base

resolve-btfids-flags-$(if $(KBUILD_VERBOSE),y) += --verbose



export RESOLVE_BTFIDS_FLAGS := $(resolve-btfids-flags-y)

scripts/Makefile.modfinal

+2 −3
Original line number Diff line number Diff line
@@ -43,8 +43,7 @@ quiet_cmd_btf_ko = BTF [M] $@ 
	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 $@;		\

		$(srctree)/scripts/gen-btf.sh --btf_base $(objtree)/vmlinux $@; \

	fi;



# Same as newer-prereqs, but allows to exclude specified extra dependencies

scripts/Makefile.vmlinux

+1 −1
Original line number Diff line number Diff line
@@ -71,7 +71,7 @@ targets += vmlinux.unstripped .vmlinux.export.o 
vmlinux.unstripped: scripts/link-vmlinux.sh vmlinux.o .vmlinux.export.o $(KBUILD_LDS) FORCE

	+$(call if_changed_dep,link_vmlinux)

ifdef CONFIG_DEBUG_INFO_BTF

vmlinux.unstripped: $(RESOLVE_BTFIDS)

vmlinux.unstripped: $(RESOLVE_BTFIDS) $(srctree)/scripts/gen-btf.sh

endif



ifdef CONFIG_BUILDTIME_TABLE_SORT

scripts/gen-btf.sh

0 → 100755
+157 −0
Original line number Diff line number Diff line 
#!/bin/bash

# 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





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} ${RESOLVE_BTFIDS_FLAGS}	\

		${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} ${CLANG_FLAGS} -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}.BTF"

	${OBJCOPY} --add-section .BTF=${ELF_FILE}.BTF ${ELF_FILE}



	# a module might not have a .BTF_ids or .BTF.base section

	local btf_base="${ELF_FILE}.BTF.base"

	if [ -f "${btf_base}" ]; then

		${OBJCOPY} --add-section .BTF.base=${btf_base} ${ELF_FILE}

	fi

	local btf_ids="${ELF_FILE}.BTF_ids"

	if [ -f "${btf_ids}" ]; then

		${OBJCOPY} --update-section .BTF_ids=${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