bpf: preload: Install libbpf headers when building

API headers from libbpf should not be accessed directly from the
library's source directory. Instead, they should be exported with "make
install_headers". Let's make sure that bpf/preload/Makefile installs the
headers properly when building.

Note that we declare an additional dependency for iterators/iterators.o:
having $(LIBBPF_A) as a dependency to "$(obj)/bpf_preload_umd" is not
sufficient, as it makes it required only at the linking step. But we
need libbpf to be compiled, and in particular its headers to be
exported, before we attempt to compile iterators.o. The issue would not
occur before this commit, because libbpf's headers were not exported and
were always available under tools/lib/bpf.
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211007194438.34443-7-quentin@isovalent.com

kernel/bpf/preload/Makefile

 # SPDX-License-Identifier: GPL-2.0
 
 LIBBPF_SRCS = $(srctree)/tools/lib/bpf/
-LIBBPF_A = $(obj)/libbpf.a
-LIBBPF_OUT = $(abspath $(obj))
+LIBBPF_OUT = $(abspath $(obj))/libbpf
+LIBBPF_A = $(LIBBPF_OUT)/libbpf.a
+LIBBPF_DESTDIR = $(LIBBPF_OUT)
+LIBBPF_INCLUDE = $(LIBBPF_DESTDIR)/include
 
 # Although not in use by libbpf's Makefile, set $(O) so that the "dummy" test
 # in tools/scripts/Makefile.include always succeeds when building the kernel
 # with $(O) pointing to a relative path, as in "make O=build bindeb-pkg".
-$(LIBBPF_A):
-	$(Q)$(MAKE) -C $(LIBBPF_SRCS) O=$(LIBBPF_OUT)/ OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a
+$(LIBBPF_A): | $(LIBBPF_OUT)
+	$(Q)$(MAKE) -C $(LIBBPF_SRCS) O=$(LIBBPF_OUT)/ OUTPUT=$(LIBBPF_OUT)/   \
+		DESTDIR=$(LIBBPF_DESTDIR) prefix=			       \
+		$(LIBBPF_OUT)/libbpf.a install_headers
+
+libbpf_hdrs: $(LIBBPF_A)
+
+.PHONY: libbpf_hdrs
+
+$(LIBBPF_OUT):
+	$(call msg,MKDIR,$@)
+	$(Q)mkdir -p $@
 
 userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi \
-	-I $(srctree)/tools/lib/ -Wno-unused-result
+	-I $(LIBBPF_INCLUDE) -Wno-unused-result
 
 userprogs := bpf_preload_umd
 
 clean-files := $(userprogs) bpf_helper_defs.h FEATURE-DUMP.libbpf staticobjs/ feature/
+clean-files += $(LIBBPF_OUT) $(LIBBPF_DESTDIR)
+
+$(obj)/iterators/iterators.o: | libbpf_hdrs
 
 bpf_preload_umd-objs := iterators/iterators.o
 bpf_preload_umd-userldlibs := $(LIBBPF_A) -lelf -lz