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

tools/lib/bpf/.gitignore

 # SPDX-License-Identifier: GPL-2.0-only
 libbpf_version.h
 libbpf.pc
-FEATURE-DUMP.libbpf
 libbpf.so.*
 TAGS
 tags

tools/lib/bpf/Makefile

@@ -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