selftests/bpf: Pass all BPF .o's through BPF static linker

Pass all individual BPF object files (generated from progs/*.c) through
`bpftool gen object` command to validate that BPF static linker doesn't
corrupt them.

As an additional sanity checks, validate that passing resulting object files
through linker again results in identical ELF files. Exact same ELF contents
can be guaranteed only after two passes, as after the first pass ELF sections
order changes, and thus .BTF.ext data sections order changes. That, in turn,
means that strings are added into the final BTF string sections in different
order, so .BTF strings data might not be exactly the same. But doing another
round of linking afterwards should result in the identical ELF file, which is
checked with additional `diff` command.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210318194036.3521577-12-andrii@kernel.org

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index dbca39f4538245f7977ccb7a436b6f9e32c4e5ea..3a7c02add70c3a9b096997b74d3697a732131649 100644 (file)
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -355,12 +355,13 @@ $(TRUNNER_BPF_OBJS): $(TRUNNER_OUTPUT)/%.o:                               \
        $$(call $(TRUNNER_BPF_BUILD_RULE),$$<,$$@,                      \
                                          $(TRUNNER_BPF_CFLAGS))
 
-$(TRUNNER_BPF_SKELS): $(TRUNNER_OUTPUT)/%.skel.h:                      \
-                     $(TRUNNER_OUTPUT)/%.o                             \
-                     $(BPFTOOL)                                        \
-                     | $(TRUNNER_OUTPUT)
+$(TRUNNER_BPF_SKELS): %.skel.h: %.o $(BPFTOOL) | $(TRUNNER_OUTPUT)
        $$(call msg,GEN-SKEL,$(TRUNNER_BINARY),$$@)
-       $(Q)$$(BPFTOOL) gen skeleton $$< > $$@
+       $(Q)$$(BPFTOOL) gen object $$(<:.o=.linked.o) $$<
+       $(Q)$$(BPFTOOL) gen object $$(<:.o=.linked2.o) $$(<:.o=.linked.o)
+       $(Q)$$(BPFTOOL) gen object $$(<:.o=.linked3.o) $$(<:.o=.linked2.o)
+       $(Q)diff $$(<:.o=.linked2.o) $$(<:.o=.linked3.o)
+       $(Q)$$(BPFTOOL) gen skeleton $$(<:.o=.linked.o) name $$(notdir $$(<:.o=)) > $$@
 endif
 
 # ensure we set up tests.h header generation rule just once