On 7/21/26 19:48, Ziyang Men wrote:
> The libbpf + bpftool + vmlinux.h + BPF-object + skeleton build tool-chain
> is currently duplicated across tools/testing/selftests/{bpf,sched_ext,
> hid}/, each carrying ~100-140 lines of near-identical Makefile. As more
> subsystems grow BPF-based selftests, the duplication scales poorly.
>
> Add tools/testing/selftests/lib.bpf.mk, a single includable fragment that
> provides the whole chain end-to-end. It builds the in-tree libbpf.a and a
> host bpftool, generates vmlinux.h from the kernel's BTF, compiles *.bpf.c
> into BPF objects (clang --target=bpf) and generates their skeletons. It
> also provides what the user-space test binary needs to build: the header
> search paths (so #includes resolve), a bpf_link macro that wraps the link
> command, and BPF_LDLIBS, so the test can be statically linked against
> libbpf.a.
>
> To use: set BPF_SRCS and OVERRIDE_TARGETS := 1 before including ../lib.mk
> (so lib.mk's default link rule is suppressed), then include ../lib.bpf.mk
> and list $(BPF_SKELS) as prerequisites of the test binary, e.g.,:
>
> BPF_SRCS := progs/foo.bpf.c
> OVERRIDE_TARGETS := 1
> include ../lib.mk
> include ../lib.bpf.mk
> $(OUTPUT)/foo_test: foo_test.c $(BPF_SKELS)
> $(call bpf_link,$@,$<)
>
> This saves much work for configuring selftests in other folder, such the
> cgroup.
>
> net/bpf.mk (which only builds *.bpf.o, without skeleton or vmlinux.h
> generation) is left unchanged; replacing the existing duplication
> mentioned above is the next step.
>
> Suggested-by: Shakeel Butt <[email protected]>
> Suggested-by: Eduard Zingerman <[email protected]>
> Suggested-by: Mykola Lysenko <[email protected]>
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Ziyang Men <[email protected]>
> ---
> tools/testing/selftests/lib.bpf.mk | 247 +++++++++++++++++++++++++++++
> 1 file changed, 247 insertions(+)
> create mode 100644 tools/testing/selftests/lib.bpf.mk
>
> diff --git a/tools/testing/selftests/lib.bpf.mk
> b/tools/testing/selftests/lib.bpf.mk
> new file mode 100644
> index 000000000000..6f175c6568e9
> --- /dev/null
> +++ b/tools/testing/selftests/lib.bpf.mk
> @@ -0,0 +1,247 @@
> +# SPDX-License-Identifier: GPL-2.0
> +#
> +# Shared fragment for selftests that compile *.bpf.c into BPF objects +
> +# skeletons and link them into userspace test binaries, without each
> +# subsystem's Makefile re-implementing the libbpf/bpftool/vmlinux.h
> machinery.
> +#
> +# Caller contract (per-test Makefile):
> +#
> +# BPF_SRCS := foo.bpf.c bar.bpf.c
> +# TEST_GEN_PROGS := foo_test
> +# OVERRIDE_TARGETS := 1 # MUST be set before lib.mk
> +# include ../lib.mk # defines OUTPUT, CC, Q, msg,
> +# include ../lib.bpf.mk # selfdir, top_srcdir; honours
> OVERRIDE
> +#
> +# $(OUTPUT)/foo_test: foo_test.c $(BPF_SKELS)
> +# $(call bpf_link,$@,$<)
> +#
> +# Optional knobs (set before including lib.bpf.mk):
> +# BPF_PROG_EXT - source suffix, default .bpf.c; set to .c for the
> legacy
> +# "progs/foo.c -> foo.bpf.o" layout.
> +# BPF_EXTRA_HDRS - extra prerequisites (headers) for the BPF objects.
> +# BPF_EXTRA_CFLAGS - appended to BPF_CFLAGS for the BPF compile.
> +# BPF_SKEL_EXT - skeleton suffix, default .skel.h (e.g. .bpf.skel.h).
> +# BPF_GEN_SUBSKEL - if set, also emit a subskeleton next to each skeleton.
> +# BPF_OBJ_DIR - dir for generated *.bpf.o (default $(OUTPUT)).
> +# BPF_SKEL_DIR - dir for generated skeletons (default $(OUTPUT)).
> +# A caller with unusual compile needs may override BPF_CFLAGS wholesale after
> +# the include (the object recipe expands it lazily).
> +# BPF_SRCS entries may live in a subdirectory (e.g. progs/foo.bpf.c); the
> +# objects and skeletons are always emitted flat under $(OUTPUT), keyed by the
> +# source basename (foo.bpf.o / foo.skel.h).
> +#
> +# lib.mk MUST be included first: this fragment consumes the vars it defines
> +# (OUTPUT, top_srcdir, CC, CLANG, Q, msg) and needs OVERRIDE_TARGETS to have
> +# already suppressed lib.mk's default link rule.
> +
> +include $(top_srcdir)/tools/scripts/Makefile.arch # ARCH / SRCARCH
> +# Pull in the shared toolchain definitions (HOSTCC/HOSTLD/CLANG) so this
> +# fragment picks the *same* host compiler the libbpf and bpftool sub-makes
> +# will: in particular HOSTCC becomes clang under LLVM=1 (gcc otherwise).
> +# Without this the host bpftool and its bootstrap libbpf can end up built
> with
> +# a mix of gcc and clang, which trips clang on gcc-only flags
> (-Wstrict-aliasing=3).
> +#
> +# Makefile.include's allow-override resets CC to a bare "clang", which would
> +# drop the --target=<arch> flag lib.mk set for LLVM=1 cross builds (make
> LLVM=1
> +# ARCH=arm64). lib.mk ran first and configured CC for the target, so save it
> +# across the include and restore it afterwards.
> +lib_bpf_mk_saved_cc := $(CC)
> +include $(top_srcdir)/tools/scripts/Makefile.include
> +CC := $(lib_bpf_mk_saved_cc)
> +
> +CLANG ?= clang
> +HOSTCC ?= gcc
> +HOSTLD ?= ld
> +ifneq ($(V),1)
> +submake_extras := feature_display=0
> +endif
> +
> +# ---- paths & tools --------------------------------------------------------
> +TOOLSDIR := $(top_srcdir)/tools
> +LIBDIR := $(TOOLSDIR)/lib
> +BPFDIR := $(LIBDIR)/bpf
> +TOOLSINCDIR := $(TOOLSDIR)/include
> +BPFTOOLDIR := $(TOOLSDIR)/bpf/bpftool
> +APIDIR := $(TOOLSINCDIR)/uapi
> +
> +# Everything generated lives under $(OUTPUT) so O= and in-tree both work and
> +# per-test builds (distinct $(OUTPUT)) never collide.
> +SCRATCH_DIR := $(OUTPUT)/tools
> +BUILD_DIR := $(SCRATCH_DIR)/build
> +INCLUDE_DIR := $(SCRATCH_DIR)/include
> +BPFOBJ := $(BUILD_DIR)/libbpf/libbpf.a
> +
> +# bpftool must run on the *host*; split the host toolchain out when
> cross-building.
> +ifneq ($(CROSS_COMPILE),)
> +HOST_BUILD_DIR := $(BUILD_DIR)/host
> +HOST_SCRATCH_DIR := $(OUTPUT)/host-tools
> +else
> +HOST_BUILD_DIR := $(BUILD_DIR)
> +HOST_SCRATCH_DIR := $(SCRATCH_DIR)
> +endif
> +HOST_BPFOBJ := $(HOST_BUILD_DIR)/libbpf/libbpf.a
> +DEFAULT_BPFTOOL := $(HOST_SCRATCH_DIR)/sbin/bpftool
> +BPFTOOL ?= $(DEFAULT_BPFTOOL)
> +
> +# ---- vmlinux BTF discovery -----------------------------------------------
> +VMLINUX_BTF_PATHS ?= $(if $(O),$(O)/vmlinux) \
> + $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux) \
> + $(top_srcdir)/vmlinux \
> + /sys/kernel/btf/vmlinux \
> + /boot/vmlinux-$(shell uname -r)
> +VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
> +ifeq ($(VMLINUX_BTF),)
> +$(error Cannot find a vmlinux for VMLINUX_BTF at any of
> "$(VMLINUX_BTF_PATHS)")
> +endif
> +
> +# ---- clang flags ----------------------------------------------------------
> +# Clang's default system includes (not the ones seen under --target=bpf);
> fixes
> +# "missing" asm/byteorder.h etc. '-idirafter' so we never shadow real
> includes.
> +define get_sys_includes
> +$(shell $(1) $(2) -v -E - </dev/null 2>&1 \
> + | sed -n '/<...> search starts here:/,/End of search list./{ s|
> \(/.*\)|-idirafter \1|p }') \
> +$(shell $(1) $(2) -dM -E - </dev/null | grep '__riscv_xlen ' | awk
> '{printf("-D__riscv_xlen=%d -D__BITS_PER_LONG=%d", $$3, $$3)}')
> +endef
> +ifneq ($(CROSS_COMPILE),)
> +CLANG_TARGET_ARCH = --target=$(notdir $(CROSS_COMPILE:%-=%))
> +endif
> +CLANG_SYS_INCLUDES = $(call get_sys_includes,$(CLANG),$(CLANG_TARGET_ARCH))
> +
> +IS_LITTLE_ENDIAN = $(shell $(CC) -dM -E - </dev/null | \
> + grep 'define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__')
> +MENDIAN = $(if $(IS_LITTLE_ENDIAN),-mlittle-endian,-mbig-endian)
> +
> +# Prefer -mcpu=v3; fall back to v2 on clang too old to know it (probed once).
> +CLANG_BPF_CPU := $(shell $(CLANG) --target=bpf -mcpu=help 2>&1 | grep -q
> 'v3' \
> + && echo v3 || echo v2)
> +
> +# -fms-extensions + -Wno-microsoft-anon-tag: required so clang accepts the
> +# anonymous nested struct/union members bpftool emits into vmlinux.h.
> +BPF_CFLAGS = -g -Wall -Werror -D__TARGET_ARCH_$(SRCARCH) $(MENDIAN) \
> + -I$(INCLUDE_DIR) -I$(APIDIR) -I$(TOOLSINCDIR) \
> + -std=gnu11 \
> + -fno-strict-aliasing \
> + -fms-extensions -Wno-microsoft-anon-tag \
> + -Wno-compare-distinct-pointer-types \
> + $(CLANG_SYS_INCLUDES) $(BPF_EXTRA_CFLAGS)
> +
> +# $1 = src .bpf.c, $2 = dst .bpf.o
> +define BPF_BUILD_RULE
> + $(call msg,CLNG-BPF,,$2)
> + $(Q)$(CLANG) $(BPF_CFLAGS) -O2 --target=bpf -mcpu=$(CLANG_BPF_CPU) -c
> $1 -o $2
> +endef
> +
> +# ---- output dirs for generated objects/skeletons --------------------------
> +# Default: flat under $(OUTPUT) (what bpf/, hid/, cgroup/ do). A caller may
> +# segregate the generated files into subdirs, e.g. BPF_SKEL_DIR :=
> $(OUTPUT)/...
> +BPF_OBJ_DIR ?= $(OUTPUT)
> +BPF_SKEL_DIR ?= $(OUTPUT)
> +
> +# ---- scratch dirs ---------------------------------------------------------
> +MAKE_DIRS := $(sort $(BUILD_DIR)/libbpf $(HOST_BUILD_DIR)/libbpf
> \
> + $(HOST_BUILD_DIR)/bpftool $(INCLUDE_DIR) \
> + $(filter-out $(OUTPUT),$(BPF_OBJ_DIR) $(BPF_SKEL_DIR)))
> +$(MAKE_DIRS):
> + $(call msg,MKDIR,,$@)
> + $(Q)mkdir -p $@
> +
> +# ---- libbpf (target) ------------------------------------------------------
> +# Pass ARCH/CROSS_COMPILE/CC through: lib.mk's CC is file-origin and is not
> +# exported, so without this the libbpf sub-make would rebuild for the host
> under
> +# a pure-LLVM cross build (make LLVM=1 ARCH=<arch>). -fPIC keeps the static
> +# libbpf linkable into position-independent (PIE) test binaries.
> +$(BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile) \
> + $(APIDIR)/linux/bpf.h | $(BUILD_DIR)/libbpf
> + $(Q)$(MAKE) $(submake_extras) -C $(BPFDIR) OUTPUT=$(BUILD_DIR)/libbpf/ \
> + ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE) CC="$(CC)" \
> + EXTRA_CFLAGS='-g -O0 -fPIC' \
Looking at the same line in tools/testing/selftests/bpf/Makefile:
EXTRA_CFLAGS='-g $(OPT_FLAGS) $(SAN_CFLAGS) $(EXTRA_CFLAGS)' \
is there a reason why this is not respected and -O0 and -fPIC are
hard-coded? What if someone wants to build selftests (including libbpf)
with -O2 and some additional flags? Is that possible with the new
lib.bpf.mk? It would be nice if it used at least EXTRA_CFLAGS or
USERCFLAGS defined in lib.mk.
Same questions would apply to other rules for libbpf and bpftool below.
Thanks!
Viktor
> + DESTDIR=$(SCRATCH_DIR) prefix= all install_headers
> +
> +# ---- libbpf (host) -- a distinct rule only when cross-compiling -----------
> +ifneq ($(BPFOBJ),$(HOST_BPFOBJ))
> +$(HOST_BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile) \
> + | $(HOST_BUILD_DIR)/libbpf
> + $(Q)$(MAKE) $(submake_extras) -C $(BPFDIR) ARCH= CROSS_COMPILE= \
> + OUTPUT=$(HOST_BUILD_DIR)/libbpf/ CC=$(HOSTCC) LD=$(HOSTLD) \
> + EXTRA_CFLAGS='-g -O0' \
> + DESTDIR=$(HOST_SCRATCH_DIR) prefix= all install_headers
> +endif
> +
> +# ---- bpftool (host) -------------------------------------------------------
> +$(DEFAULT_BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile) \
> + $(HOST_BPFOBJ) | $(HOST_BUILD_DIR)/bpftool
> + $(Q)$(MAKE) $(submake_extras) -C $(BPFTOOLDIR) \
> + ARCH= CROSS_COMPILE= CC=$(HOSTCC) LD=$(HOSTLD) \
> + EXTRA_CFLAGS='-g -O0' \
> + OUTPUT=$(HOST_BUILD_DIR)/bpftool/ \
> + LIBBPF_OUTPUT=$(HOST_BUILD_DIR)/libbpf/ \
> + LIBBPF_DESTDIR=$(HOST_SCRATCH_DIR)/ \
> + prefix= DESTDIR=$(HOST_SCRATCH_DIR)/ install-bin
> +
> +# ---- vmlinux.h ------------------------------------------------------------
> +$(INCLUDE_DIR)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL) | $(INCLUDE_DIR)
> +ifeq ($(VMLINUX_H),)
> + $(call msg,GEN,,$@)
> + $(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@
> +else
> + $(call msg,CP,,$@)
> + $(Q)cp "$(VMLINUX_H)" $@
> +endif
> +
> +# ---- BPF objects + skeletons ---------------------------------------------
> +# Sources may sit in a subdir and use the modern *.bpf.c or the legacy *.c
> +# suffix (BPF_PROG_EXT); objects go in $(BPF_OBJ_DIR) and skeletons in
> +# $(BPF_SKEL_DIR) (both default to $(OUTPUT)), keyed by the source basename.
> +BPF_PROG_EXT ?= .bpf.c
> +bpf_stems := $(patsubst %$(BPF_PROG_EXT),%,$(notdir $(BPF_SRCS)))
> +# The output namespace is flat, so two sources with the same basename would
> +# collapse into one object/skeleton; fail loudly instead of silently
> dropping.
> +ifneq ($(words $(bpf_stems)),$(words $(sort $(bpf_stems))))
> +$(error lib.bpf.mk: BPF_SRCS has colliding basenames: $(BPF_SRCS))
> +endif
> +# BPF_SKEL_EXT lets a caller pick the skeleton suffix (default .skel.h; e.g.
> +# .bpf.skel.h). With BPF_GEN_SUBSKEL set, a matching subskeleton is emitted
> +# alongside (foo.subskel.h / foo.bpf.subskel.h).
> +BPF_SKEL_EXT ?= .skel.h
> +BPF_SUBSKEL_EXT := $(patsubst %skel.h,%subskel.h,$(BPF_SKEL_EXT))
> +BPF_OBJS := $(addprefix $(BPF_OBJ_DIR)/,$(addsuffix .bpf.o,$(bpf_stems)))
> +BPF_SKELS := $(addprefix $(BPF_SKEL_DIR)/,$(addsuffix
> $(BPF_SKEL_EXT),$(bpf_stems)))
> +
> +# Locate the sources wherever the caller keeps them (e.g. progs/).
> +vpath %$(BPF_PROG_EXT) $(sort $(dir $(BPF_SRCS)))
> +
> +$(BPF_OBJS): $(BPF_OBJ_DIR)/%.bpf.o: %$(BPF_PROG_EXT) $(BPF_EXTRA_HDRS) \
> + $(wildcard *.bpf.h) $(INCLUDE_DIR)/vmlinux.h | $(BPF_OBJ_DIR)
> $(BPFOBJ)
> + $(call BPF_BUILD_RULE,$<,$@)
> +
> +$(BPF_SKELS): $(BPF_SKEL_DIR)/%$(BPF_SKEL_EXT): $(BPF_OBJ_DIR)/%.bpf.o
> $(BPFTOOL) | $(BPF_SKEL_DIR)
> + $(call msg,GEN-SKEL,,$@)
> + $(Q)$(BPFTOOL) gen object $(<:.o=.linked.o) $<
> + $(Q)$(BPFTOOL) gen skeleton $(<:.o=.linked.o) name $(notdir
> $(<:.bpf.o=)) > $@
> +ifneq ($(BPF_GEN_SUBSKEL),)
> + $(Q)$(BPFTOOL) gen subskeleton $(<:.o=.linked.o) name $(notdir
> $(<:.bpf.o=)) > $(@:$(BPF_SKEL_EXT)=$(BPF_SUBSKEL_EXT))
> +endif
> +
> +# ---- exports consumed by the caller --------------------------------------
> +# -I$(OUTPUT)/-I$(BPF_SKEL_DIR): so the test .c can #include "foo.skel.h".
> +# -I$(INCLUDE_DIR): so userspace can pull in the generated vmlinux.h if
> needed.
> +CFLAGS += -I$(OUTPUT) -I$(BPF_SKEL_DIR) -I$(INCLUDE_DIR)
> +
> +# Static libbpf.a first, then its deps. libbpf may pull in zstd (BTF
> decompress)
> +# only when built against it; link -lzstd only if libzstd is present.
> +BPF_LDLIBS := $(BPFOBJ) -lelf -lz
> +ifneq ($(shell pkg-config --exists libzstd 2>/dev/null && echo y),)
> +BPF_LDLIBS += -lzstd
> +endif
> +
> +TEST_GEN_FILES += $(BPF_OBJS)
> +
> +# Link helper: $1 = output binary, $2 = test .c (skels are the target's
> deps).
> +define bpf_link
> + $(call msg,BINARY,,$1)
> + $(Q)$(CC) $(CFLAGS) $2 $(BPF_LDLIBS) $(LDLIBS) -o $1
> +endef
> +
> +EXTRA_CLEAN += $(SCRATCH_DIR) $(HOST_SCRATCH_DIR) \
> + $(addprefix $(BPF_OBJ_DIR)/,*.bpf.o *.linked.o) \
> + $(addprefix $(BPF_SKEL_DIR)/,*$(BPF_SKEL_EXT)
> *$(BPF_SUBSKEL_EXT))
