selftests: net: extract BPF building logic from the Makefile

The BPF sample building code looks a little bit spaghetti-ish
so move it out to its own Makefile snippet. Similar in the spirit
to how we include lib.mk. libynl will soon get a similar snippet.

There is a small change hiding in the move, the relative
paths (../../.., ../.. etc) are replaced with variables
from lib.mk such as top_srcdir and selfdir.

Link: https://lore.kernel.org/r/20240423183542.3807234-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/Makefile

@@ -108,55 +108,4 @@ $(OUTPUT)/tcp_inq: LDLIBS += -lpthread
 $(OUTPUT)/bind_bhash: LDLIBS += -lpthread
 $(OUTPUT)/io_uring_zerocopy_tx: CFLAGS += -I../../../include/
 
-# Rules to generate bpf objs
-CLANG ?= clang
-SCRATCH_DIR := $(OUTPUT)/tools
-BUILD_DIR := $(SCRATCH_DIR)/build
-BPFDIR := $(abspath ../../../lib/bpf)
-APIDIR := $(abspath ../../../include/uapi)
-
-CCINCLUDE += -I../bpf
-CCINCLUDE += -I../../../../usr/include/
-CCINCLUDE += -I$(SCRATCH_DIR)/include
-
-BPFOBJ := $(BUILD_DIR)/libbpf/libbpf.a
-
-MAKE_DIRS := $(BUILD_DIR)/libbpf
-$(MAKE_DIRS):
-	$(call msg,MKDIR,,$@)
-	$(Q)mkdir -p $@
-
-# Get Clang's default includes on this system, as opposed to those seen by
-# '--target=bpf'. This fixes "missing" files on some architectures/distros,
-# such as asm/byteorder.h, asm/socket.h, asm/sockios.h, sys/cdefs.h etc.
-#
-# Use '-idirafter': Don't interfere with include mechanics except where the
-# build would have failed anyways.
-define get_sys_includes
-$(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))
-
-BPF_PROG_OBJS := $(patsubst %.c,$(OUTPUT)/%.o,$(wildcard *.bpf.c))
-
-$(BPF_PROG_OBJS): $(OUTPUT)/%.o : %.c $(BPFOBJ) | $(MAKE_DIRS)
-	$(call msg,BPF_PROG,,$@)
-	$(Q)$(CLANG) -O2 -g --target=bpf $(CCINCLUDE) $(CLANG_SYS_INCLUDES) \
-	-c $< -o $@
-
-$(BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)		       \
-	   $(APIDIR)/linux/bpf.h					       \
-	   | $(BUILD_DIR)/libbpf
-	$(call msg,MAKE,,$@)
-	$(Q)$(MAKE) $(submake_extras) -C $(BPFDIR) OUTPUT=$(BUILD_DIR)/libbpf/ \
-		    EXTRA_CFLAGS='-g -O0'				       \
-		    DESTDIR=$(SCRATCH_DIR) prefix= all install_headers
-
-EXTRA_CLEAN := $(SCRATCH_DIR)
+include bpf.mk

tools/testing/selftests/net/bpf.mk

@@ -0,0 +1,53 @@
+# SPDX-License-Identifier: GPL-2.0
+# Rules to generate bpf objs
+CLANG ?= clang
+SCRATCH_DIR := $(OUTPUT)/tools
+BUILD_DIR := $(SCRATCH_DIR)/build
+BPFDIR := $(top_srcdir)/tools/lib/bpf
+APIDIR := $(top_srcdir)/tools/include/uapi
+
+CCINCLUDE += -I$(selfdir)/bpf
+CCINCLUDE += -I$(top_srcdir)/usr/include/
+CCINCLUDE += -I$(SCRATCH_DIR)/include
+
+BPFOBJ := $(BUILD_DIR)/libbpf/libbpf.a
+
+MAKE_DIRS := $(BUILD_DIR)/libbpf
+$(MAKE_DIRS):
+	$(call msg,MKDIR,,$@)
+	$(Q)mkdir -p $@
+
+# Get Clang's default includes on this system, as opposed to those seen by
+# '--target=bpf'. This fixes "missing" files on some architectures/distros,
+# such as asm/byteorder.h, asm/socket.h, asm/sockios.h, sys/cdefs.h etc.
+#
+# Use '-idirafter': Don't interfere with include mechanics except where the
+# build would have failed anyways.
+define get_sys_includes
+$(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))
+
+BPF_PROG_OBJS := $(patsubst %.c,$(OUTPUT)/%.o,$(wildcard *.bpf.c))
+
+$(BPF_PROG_OBJS): $(OUTPUT)/%.o : %.c $(BPFOBJ) | $(MAKE_DIRS)
+	$(call msg,BPF_PROG,,$@)
+	$(Q)$(CLANG) -O2 -g --target=bpf $(CCINCLUDE) $(CLANG_SYS_INCLUDES) \
+	-c $< -o $@
+
+$(BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)		       \
+	   $(APIDIR)/linux/bpf.h					       \
+	   | $(BUILD_DIR)/libbpf
+	$(call msg,MAKE,,$@)
+	$(Q)$(MAKE) $(submake_extras) -C $(BPFDIR) OUTPUT=$(BUILD_DIR)/libbpf/ \
+		    EXTRA_CFLAGS='-g -O0'				       \
+		    DESTDIR=$(SCRATCH_DIR) prefix= all install_headers
+
+EXTRA_CLEAN += $(SCRATCH_DIR)