Re: [PATCH bpf-next] selftests/bpf: Cross-compile bpftool

From: Björn Töpel <redacted>

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

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

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

There are two issues for cross-compilation builds:

 1. There is no native (cross-compilation target) build of bpftool
 2. The bootstrap variant of 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 to the native/non-bootstrap
version.

Signed-off-by: Björn Töpel <redacted>
---
 tools/testing/selftests/bpf/Makefile | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index b2eb3201b85a..b706750f71e2 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile

@@ -157,8 +157,9 @@ $(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):
 	$(call msg,MKDIR,,$@)

@@ -208,6 +209,14 @@ $(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	       \

@@ -255,6 +264,18 @@ $(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:

@@ -518,11 +539,12 @@ 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 \