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libbpf: Stop using feature-detection Makefiles 

Libbpf's Makefile relies on Linux tools infrastructure's feature detection
framework, but libbpf's needs are very modest: it detects the presence of
libelf and libz, both of which are mandatory. So it doesn't benefit much from
the framework, but pays significant costs in terms of maintainability and
debugging experience, when something goes wrong. The other feature detector,
testing for the presernce of minimal BPF API in system headers is long
obsolete as well, providing no value.

So stop using feature detection and just assume the presence of libelf and
libz during build time. Worst case, user will get a clear and actionable
linker error, e.g.:

  /usr/bin/ld: cannot find -lelf

On the other hand, we completely bypass recurring issues various users
reported over time with false negatives of feature detection (libelf or libz
not being detected, while they are actually present in the system).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/bpf/20210203203445.3356114-1-andrii@kernel.org
This commit is contained in:
Andrii Nakryiko 2021-02-03 12:34:45 -08:00 committed by Daniel Borkmann
parent 65e6dcf733
commit 5f10c1aac8
2 changed files with 4 additions and 44 deletions
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1
tools/lib/bpf/.gitignore vendored
View File

@ -1,7 +1,6 @@
# SPDX-License-Identifier: GPL-2.0-only
libbpf_version.h
libbpf.pc
FEATURE-DUMP.libbpf
libbpf.so.*
TAGS
tags

47
tools/lib/bpf/Makefile
View File

@ -58,28 +58,7 @@ ifndef VERBOSE
VERBOSE = 0
endif
FEATURE_USER = .libbpf
FEATURE_TESTS = libelf zlib bpf
FEATURE_DISPLAY = libelf zlib bpf
INCLUDES = -I. -I$(srctree)/tools/include -I$(srctree)/tools/include/uapi
FEATURE_CHECK_CFLAGS-bpf = $(INCLUDES)
check_feat := 1
NON_CHECK_FEAT_TARGETS := clean TAGS tags cscope help
ifdef MAKECMDGOALS
ifeq ($(filter-out $(NON_CHECK_FEAT_TARGETS),$(MAKECMDGOALS)),)
check_feat := 0
endif
endif
ifeq ($(check_feat),1)
ifeq ($(FEATURES_DUMP),)
include $(srctree)/tools/build/Makefile.feature
else
include $(FEATURES_DUMP)
endif
endif
export prefix libdir src obj
@ -157,7 +136,7 @@ all: fixdep
all_cmd: $(CMD_TARGETS) check
$(BPF_IN_SHARED): force elfdep zdep bpfdep $(BPF_HELPER_DEFS)
$(BPF_IN_SHARED): force $(BPF_HELPER_DEFS)
@(test -f ../../include/uapi/linux/bpf.h -a -f ../../../include/uapi/linux/bpf.h && ( \
(diff -B ../../include/uapi/linux/bpf.h ../../../include/uapi/linux/bpf.h >/dev/null) || \
echo "Warning: Kernel ABI header at 'tools/include/uapi/linux/bpf.h' differs from latest version at 'include/uapi/linux/bpf.h'" >&2 )) || true
@ -175,7 +154,7 @@ $(BPF_IN_SHARED): force elfdep zdep bpfdep $(BPF_HELPER_DEFS)
echo "Warning: Kernel ABI header at 'tools/include/uapi/linux/if_xdp.h' differs from latest version at 'include/uapi/linux/if_xdp.h'" >&2 )) || true
$(Q)$(MAKE) $(build)=libbpf OUTPUT=$(SHARED_OBJDIR) CFLAGS="$(CFLAGS) $(SHLIB_FLAGS)"
$(BPF_IN_STATIC): force elfdep zdep bpfdep $(BPF_HELPER_DEFS)
$(BPF_IN_STATIC): force $(BPF_HELPER_DEFS)
$(Q)$(MAKE) $(build)=libbpf OUTPUT=$(STATIC_OBJDIR)
$(BPF_HELPER_DEFS): $(srctree)/tools/include/uapi/linux/bpf.h
@ -264,34 +243,16 @@ install_pkgconfig: $(PC_FILE)
install: install_lib install_pkgconfig install_headers
### Cleaning rules
config-clean:
$(call QUIET_CLEAN, feature-detect)
$(Q)$(MAKE) -C $(srctree)/tools/build/feature/ clean >/dev/null
clean: config-clean
clean:
$(call QUIET_CLEAN, libbpf) $(RM) -rf $(CMD_TARGETS) \
*~ .*.d .*.cmd LIBBPF-CFLAGS $(BPF_HELPER_DEFS) \
$(SHARED_OBJDIR) $(STATIC_OBJDIR) \
$(addprefix $(OUTPUT), \
*.o *.a *.so *.so.$(LIBBPF_MAJOR_VERSION) *.pc)
$(call QUIET_CLEAN, core-gen) $(RM) $(OUTPUT)FEATURE-DUMP.libbpf
PHONY += force elfdep zdep bpfdep cscope tags
PHONY += force cscope tags
force:
elfdep:
@if [ "$(feature-libelf)" != "1" ]; then echo "No libelf found"; exit 1 ; fi
zdep:
@if [ "$(feature-zlib)" != "1" ]; then echo "No zlib found"; exit 1 ; fi
bpfdep:
@if [ "$(feature-bpf)" != "1" ]; then echo "BPF API too old"; exit 1 ; fi
cscope:
ls *.c *.h > cscope.files
cscope -b -q -I $(srctree)/include -f cscope.out