selftests/bpf: Cross-compile bpftool

When the BPF selftests are cross-compiled, only the a host version of
bpftool is built. This version of bpftool is used on the host-side to
generate various intermediates, e.g., skeletons.

The test runners are also using bpftool, so the Makefile will symlink
bpftool from the selftest/bpf root, where the test runners will look
the tool:

  | $(Q)ln -sf $(if $2,..,.)/tools/build/bpftool/bootstrap/bpftool \
  |    $(OUTPUT)/$(if $2,$2/)bpftool

There are two problems for cross-compilation builds:

 1. There is no native (cross-compilation target) of bpftool
 2. The bootstrap/bpftool is never cross-compiled (by design)

Make sure that a native/cross-compiled version of bpftool is built,
and if CROSS_COMPILE is set, symlink the native/non-bootstrap version.

Acked-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20230214161253.183458-1-bjorn@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/Makefile

@@ -156,8 +156,9 @@ $(notdir $(TEST_GEN_PROGS)						\
 	 $(TEST_CUSTOM_PROGS)): %: $(OUTPUT)/% ;
 
 # sort removes libbpf duplicates when not cross-building
-MAKE_DIRS := $(sort $(BUILD_DIR)/libbpf $(HOST_BUILD_DIR)/libbpf	       \
-	       $(HOST_BUILD_DIR)/bpftool $(HOST_BUILD_DIR)/resolve_btfids      \
+MAKE_DIRS := $(sort $(BUILD_DIR)/libbpf $(HOST_BUILD_DIR)/libbpf	\
+	       $(BUILD_DIR)/bpftool $(HOST_BUILD_DIR)/bpftool		\
+	       $(HOST_BUILD_DIR)/resolve_btfids				\
 	       $(RUNQSLOWER_OUTPUT) $(INCLUDE_DIR))
 $(MAKE_DIRS):
 	$(call msg,MKDIR,,$@)
@@ -207,6 +208,14 @@ $(OUTPUT)/bpf_testmod.ko: $(VMLINUX_BTF) $(wildcard bpf_testmod/Makefile bpf_tes
 	$(Q)cp bpf_testmod/bpf_testmod.ko $@
 
 DEFAULT_BPFTOOL := $(HOST_SCRATCH_DIR)/sbin/bpftool
+ifneq ($(CROSS_COMPILE),)
+CROSS_BPFTOOL := $(SCRATCH_DIR)/sbin/bpftool
+TRUNNER_BPFTOOL := $(CROSS_BPFTOOL)
+USE_BOOTSTRAP := ""
+else
+TRUNNER_BPFTOOL := $(DEFAULT_BPFTOOL)
+USE_BOOTSTRAP := "bootstrap/"
+endif
 
 $(OUTPUT)/runqslower: $(BPFOBJ) | $(DEFAULT_BPFTOOL) $(RUNQSLOWER_OUTPUT)
 	$(Q)$(MAKE) $(submake_extras) -C $(TOOLSDIR)/bpf/runqslower	       \
@@ -218,7 +227,7 @@ $(OUTPUT)/runqslower: $(BPFOBJ) | $(DEFAULT_BPFTOOL) $(RUNQSLOWER_OUTPUT)
 		    EXTRA_LDFLAGS='$(SAN_LDFLAGS)' &&			       \
 		    cp $(RUNQSLOWER_OUTPUT)runqslower $@
 
-TEST_GEN_PROGS_EXTENDED += $(DEFAULT_BPFTOOL)
+TEST_GEN_PROGS_EXTENDED += $(TRUNNER_BPFTOOL)
 
 $(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED): $(BPFOBJ)
 
@@ -256,6 +265,18 @@ $(DEFAULT_BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile)    \
 		    LIBBPF_DESTDIR=$(HOST_SCRATCH_DIR)/			       \
 		    prefix= DESTDIR=$(HOST_SCRATCH_DIR)/ install-bin
 
+ifneq ($(CROSS_COMPILE),)
+$(CROSS_BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile)	\
+		    $(BPFOBJ) | $(BUILD_DIR)/bpftool
+	$(Q)$(MAKE) $(submake_extras)  -C $(BPFTOOLDIR)				\
+		    ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE)			\
+		    EXTRA_CFLAGS='-g -O0'					\
+		    OUTPUT=$(BUILD_DIR)/bpftool/				\
+		    LIBBPF_OUTPUT=$(BUILD_DIR)/libbpf/				\
+		    LIBBPF_DESTDIR=$(SCRATCH_DIR)/				\
+		    prefix= DESTDIR=$(SCRATCH_DIR)/ install-bin
+endif
+
 all: docs
 
 docs:
@@ -521,11 +542,12 @@ endif
 $(OUTPUT)/$(TRUNNER_BINARY): $(TRUNNER_TEST_OBJS)			\
 			     $(TRUNNER_EXTRA_OBJS) $$(BPFOBJ)		\
 			     $(RESOLVE_BTFIDS)				\
+			     $(TRUNNER_BPFTOOL)				\
 			     | $(TRUNNER_BINARY)-extras
 	$$(call msg,BINARY,,$$@)
 	$(Q)$$(CC) $$(CFLAGS) $$(filter %.a %.o,$$^) $$(LDLIBS) -o $$@
 	$(Q)$(RESOLVE_BTFIDS) --btf $(TRUNNER_OUTPUT)/btf_data.bpf.o $$@
-	$(Q)ln -sf $(if $2,..,.)/tools/build/bpftool/bootstrap/bpftool \
+	$(Q)ln -sf $(if $2,..,.)/tools/build/bpftool/$(USE_BOOTSTRAP)bpftool \
 		   $(OUTPUT)/$(if $2,$2/)bpftool
 
 endef