selftests: net: Add cross-compilation support for BPF programs

The selftests/net does not have proper cross-compilation support, and
does not properly state libbpf as a dependency. Mimic/copy the BPF
build from selftests/bpf, which has the nice side-effect that libbpf
is built as well.
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Anders Roxell <anders.roxell@linaro.org>
Link: https://lore.kernel.org/r/20221119171841.2014936-1-bjorn@kernel.orgSigned-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/bpf/Makefile

 # SPDX-License-Identifier: GPL-2.0
 
 CLANG ?= clang
+SCRATCH_DIR := $(OUTPUT)/tools
+BUILD_DIR := $(SCRATCH_DIR)/build
+BPFDIR := $(abspath ../../../lib/bpf)
+APIDIR := $(abspath ../../../include/uapi)
+
 CCINCLUDE += -I../../bpf
-CCINCLUDE += -I../../../../lib
 CCINCLUDE += -I../../../../../usr/include/
+CCINCLUDE += -I$(SCRATCH_DIR)/include
+
+BPFOBJ := $(BUILD_DIR)/libbpf/libbpf.a
+
+MAKE_DIRS := $(BUILD_DIR)/libbpf
+$(MAKE_DIRS):
+	mkdir -p $@
 
 TEST_CUSTOM_PROGS = $(OUTPUT)/bpf/nat6to4.o
 all: $(TEST_CUSTOM_PROGS)
 
-$(OUTPUT)/%.o: %.c
-	$(CLANG) -O2 -target bpf -c $< $(CCINCLUDE) -o $@
+# Get Clang's default includes on this system, as opposed to those seen by
+# '-target bpf'. This fixes "missing" files on some architectures/distros,
+# such as asm/byteorder.h, asm/socket.h, asm/sockios.h, sys/cdefs.h etc.
+#
+# Use '-idirafter': Don't interfere with include mechanics except where the
+# build would have failed anyways.
+define get_sys_includes
+$(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),$(CLANG_TARGET_ARCH))
+
+$(TEST_CUSTOM_PROGS): $(BPFOBJ)
+	$(CLANG) -O2 -target bpf -c $(@:.o=.c) $(CCINCLUDE) $(CLANG_SYS_INCLUDES) -o $@
+
+$(BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)		       \
+	   $(APIDIR)/linux/bpf.h					       \
+	   | $(BUILD_DIR)/libbpf
+	$(MAKE) $(submake_extras) -C $(BPFDIR) OUTPUT=$(BUILD_DIR)/libbpf/     \
+		    EXTRA_CFLAGS='-g -O0'				       \
+		    DESTDIR=$(SCRATCH_DIR) prefix= all install_headers
+
+EXTRA_CLEAN := $(TEST_CUSTOM_PROGS) $(SCRATCH_DIR)
 
-EXTRA_CLEAN := $(TEST_CUSTOM_PROGS)