makefile: support libbpf symbol versioning in shared library mode

Similarly to Linux's 1bd63524593b ("libbpf: handle symbol versioning properly
for libbpf.a"), add necessary changes to build static and shared object
files separately with extra shared library flags. This allows to
properly handle symbol versioning in shared library mode, while still
having statically linkable library.

Cc: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>

src/.gitignore

@@ -2,3 +2,5 @@
 *.a
 /libbpf.pc
 /libbpf.so*
+/staticobjs
+/sharedobjs

src/Makefile

@@ -15,9 +15,7 @@ ifneq ($(FEATURE_REALLOCARRAY),)
 	ALL_CFLAGS += -DCOMPAT_NEED_REALLOCARRAY
 endif
 
-ifndef BUILD_STATIC_ONLY
+SHARED_CFLAGS += -fPIC -fvisibility=hidden -DSHARED
-	ALL_CFLAGS += -fPIC -fvisibility=hidden
-endif
 
 CFLAGS ?= -g -O2 -Werror -Wall
 ALL_CFLAGS += $(CFLAGS)
@@ -31,22 +29,25 @@ else
 endif
 
 OBJDIR ?= .
-
+SHARED_OBJDIR := $(OBJDIR)/sharedobjs
-OBJS := $(addprefix $(OBJDIR)/,bpf.o btf.o libbpf.o libbpf_errno.o netlink.o \
+STATIC_OBJDIR := $(OBJDIR)/staticobjs
+OBJS := bpf.o btf.o libbpf.o libbpf_errno.o netlink.o \
 	nlattr.o str_error.o libbpf_probes.o bpf_prog_linfo.o xsk.o \
-	btf_dump.o hashmap.o)
+	btf_dump.o hashmap.o
+SHARED_OBJS := $(addprefix $(SHARED_OBJDIR)/,$(OBJS))
+STATIC_OBJS := $(addprefix $(STATIC_OBJDIR)/,$(OBJS))
 
-LIBS := $(OBJDIR)/libbpf.a
+STATIC_LIBS := $(OBJDIR)/libbpf.a
 ifndef BUILD_STATIC_ONLY
-	LIBS += $(OBJDIR)/libbpf.so \
+	SHARED_LIBS := $(OBJDIR)/libbpf.so \
-		$(OBJDIR)/libbpf.so.$(LIBBPF_MAJOR_VERSION) \
+		       $(OBJDIR)/libbpf.so.$(LIBBPF_MAJOR_VERSION) \
-		$(OBJDIR)/libbpf.so.$(LIBBPF_VERSION)
+		       $(OBJDIR)/libbpf.so.$(LIBBPF_VERSION)
 	VERSION_SCRIPT := libbpf.map
 endif
 
 HEADERS := bpf.h libbpf.h btf.h xsk.h libbpf_util.h
-UAPI_HEADERS := $(addprefix $(TOPDIR)/include/uapi/linux/,bpf.h bpf_common.h \
+UAPI_HEADERS := $(addprefix $(TOPDIR)/include/uapi/linux/,\
-	btf.h)
+			    bpf.h bpf_common.h btf.h)
 
 PC_FILE := $(OBJDIR)/libbpf.pc
 
@@ -65,9 +66,9 @@ LIBDIR ?= $(PREFIX)/$(LIBSUBDIR)
 INCLUDEDIR ?= $(PREFIX)/include
 UAPIDIR ?= $(PREFIX)/include
 
-all: $(LIBS) $(PC_FILE)
+all: $(STATIC_LIBS) $(SHARED_LIBS) $(PC_FILE)
 
-$(OBJDIR)/libbpf.a: $(OBJS)
+$(OBJDIR)/libbpf.a: $(STATIC_OBJS)
 	$(AR) rcs $@ $^
 
 $(OBJDIR)/libbpf.so: $(OBJDIR)/libbpf.so.$(LIBBPF_MAJOR_VERSION)
@@ -76,7 +77,7 @@ $(OBJDIR)/libbpf.so: $(OBJDIR)/libbpf.so.$(LIBBPF_MAJOR_VERSION)
 $(OBJDIR)/libbpf.so.$(LIBBPF_MAJOR_VERSION): $(OBJDIR)/libbpf.so.$(LIBBPF_VERSION)
 	ln -sf $(^F) $@
 
-$(OBJDIR)/libbpf.so.$(LIBBPF_VERSION): $(OBJS)
+$(OBJDIR)/libbpf.so.$(LIBBPF_VERSION): $(SHARED_OBJS)
 	$(CC) -shared -Wl,--version-script=$(VERSION_SCRIPT) \
 		      -Wl,-soname,libbpf.so.$(LIBBPF_MAJOR_VERSION) \
 		      $^ $(ALL_LDFLAGS) -o $@
@@ -87,9 +88,18 @@ $(OBJDIR)/libbpf.pc:
 		-e "s|@VERSION@|$(LIBBPF_VERSION)|" \
 		< libbpf.pc.template > $@
 
-$(OBJDIR)/%.o: %.c
+$(STATIC_OBJDIR):
+	mkdir -p $(STATIC_OBJDIR)
+
+$(SHARED_OBJDIR):
+	mkdir -p $(SHARED_OBJDIR)
+
+$(STATIC_OBJDIR)/%.o: %.c | $(STATIC_OBJDIR)
 	$(CC) $(ALL_CFLAGS) $(CPPFLAGS) -c $< -o $@
 
+$(SHARED_OBJDIR)/%.o: %.c | $(SHARED_OBJDIR)
+	$(CC) $(ALL_CFLAGS) $(SHARED_CFLAGS) $(CPPFLAGS) -c $< -o $@
+
 define do_install
 	if [ ! -d '$(DESTDIR)$2' ]; then		\
 		$(INSTALL) -d -m 755 '$(DESTDIR)$2';	\
@@ -120,4 +130,4 @@ install_pkgconfig: $(PC_FILE)
 	$(call do_install,$(PC_FILE),$(LIBDIR)/pkgconfig,644)
 
 clean:
-	rm -f *.o *.a *.so *.so.* *.pc
+	rm -rf *.o *.a *.so *.so.* *.pc $(SHARED_OBJDIR) $(STATIC_OBJDIR)