bpf: iterators: Split iterators.lskel.h into little- and big- endian versions

iterators.lskel.h is little-endian, therefore bpf iterator is currently
broken on big-endian systems. Introduce a big-endian version and add
instructions regarding its generation. Unfortunately bpftool's
cross-endianness capabilities are limited to BTF right now, so the
procedure requires access to a big-endian machine or a configured
emulator.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-25-iii@linux.ibm.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/preload/bpf_preload_kern.c

@@ -3,7 +3,11 @@
 #include <linux/init.h>
 #include <linux/module.h>
 #include "bpf_preload.h"
-#include "iterators/iterators.lskel.h"
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#include "iterators/iterators.lskel-little-endian.h"
+#else
+#include "iterators/iterators.lskel-big-endian.h"
+#endif
 
 static struct bpf_link *maps_link, *progs_link;
 static struct iterators_bpf *skel;

kernel/bpf/preload/iterators/Makefile

@@ -35,20 +35,22 @@ endif
 
 .PHONY: all clean
 
-all: iterators.lskel.h
+all: iterators.lskel-little-endian.h
+
+big: iterators.lskel-big-endian.h
 
 clean:
 	$(call msg,CLEAN)
 	$(Q)rm -rf $(OUTPUT) iterators
 
-iterators.lskel.h: $(OUTPUT)/iterators.bpf.o | $(BPFTOOL)
+iterators.lskel-%.h: $(OUTPUT)/%/iterators.bpf.o | $(BPFTOOL)
 	$(call msg,GEN-SKEL,$@)
 	$(Q)$(BPFTOOL) gen skeleton -L $< > $@
 
-
-$(OUTPUT)/iterators.bpf.o: iterators.bpf.c $(BPFOBJ) | $(OUTPUT)
+$(OUTPUT)/%/iterators.bpf.o: iterators.bpf.c $(BPFOBJ) | $(OUTPUT)
 	$(call msg,BPF,$@)
-	$(Q)$(CLANG) -g -O2 -target bpf $(INCLUDES)			      \
+	$(Q)mkdir -p $(@D)
+	$(Q)$(CLANG) -g -O2 -target bpf -m$* $(INCLUDES)		      \
 		 -c $(filter %.c,$^) -o $@ &&				      \
 	$(LLVM_STRIP) -g $@
 

kernel/bpf/preload/iterators/README

 WARNING:
-If you change "iterators.bpf.c" do "make -j" in this directory to rebuild "iterators.skel.h".
+If you change "iterators.bpf.c" do "make -j" in this directory to
+rebuild "iterators.lskel-little-endian.h". Then, on a big-endian
+machine, do "make -j big" in this directory to rebuild
+"iterators.lskel-big-endian.h". Commit both resulting headers.
 Make sure to have clang 10 installed.
 See Documentation/bpf/bpf_devel_QA.rst