selftests/bpf: fix bpf_testmod.ko recompilation logic

bpf_testmod.ko build rule declared dependency on VMLINUX_BTF, but the variable
itself was initialized after the rule was declared, which often caused
bpf_testmod.ko to not be re-compiled. Fix by moving VMLINUX_BTF determination
sooner.

Also enforce bpf_testmod.ko recompilation when we detect that vmlinux image
changed by removing bpf_testmod/bpf_testmod.ko. This is necessary to generate
correct module's split BTF. Without it, Kbuild's module build logic might
determine that nothing changed on the kernel side and thus bpf_testmod.ko
shouldn't be rebuilt, so won't re-generate module BTF, which often leads to
module's BTF with wrong string offsets against vmlinux BTF. Removing .ko file
forces Kbuild to re-build the module.

Reported-by: Alexei Starovoitov <ast@kernel.org>
Fixes: 9f7fa225894c ("selftests/bpf: Add bpf_testmod kernel module for testing")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20201211015946.4062098-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 50b3495..8b515a1 100644 (file)
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -116,6 +116,13 @@ INCLUDE_DIR := $(SCRATCH_DIR)/include
 BPFOBJ := $(BUILD_DIR)/libbpf/libbpf.a
 RESOLVE_BTFIDS := $(BUILD_DIR)/resolve_btfids/resolve_btfids
 
+VMLINUX_BTF_PATHS ?= $(if $(O),$(O)/vmlinux)                           \
+                    $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux)    \
+                    ../../../../vmlinux                                \
+                    /sys/kernel/btf/vmlinux                            \
+                    /boot/vmlinux-$(shell uname -r)
+VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
+
 # Define simple and short `make test_progs`, `make test_sysctl`, etc targets
 # to build individual tests.
 # NOTE: Semicolon at the end is critical to override lib.mk's default static
@@ -140,6 +147,7 @@ $(OUTPUT)/urandom_read: urandom_read.c
 
 $(OUTPUT)/bpf_testmod.ko: $(VMLINUX_BTF) $(wildcard bpf_testmod/Makefile bpf_testmod/*.[ch])
        $(call msg,MOD,,$@)
+       $(Q)$(RM) bpf_testmod/bpf_testmod.ko # force re-compilation
        $(Q)$(MAKE) $(submake_extras) -C bpf_testmod
        $(Q)cp bpf_testmod/bpf_testmod.ko $@
 
@@ -147,13 +155,6 @@ $(OUTPUT)/test_stub.o: test_stub.c $(BPFOBJ)
        $(call msg,CC,,$@)
        $(Q)$(CC) -c $(CFLAGS) -o $@ $<
 
-VMLINUX_BTF_PATHS ?= $(if $(O),$(O)/vmlinux)                           \
-                    $(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT)/vmlinux)    \
-                    ../../../../vmlinux                                \
-                    /sys/kernel/btf/vmlinux                            \
-                    /boot/vmlinux-$(shell uname -r)
-VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
-
 DEFAULT_BPFTOOL := $(SCRATCH_DIR)/sbin/bpftool
 
 $(OUTPUT)/runqslower: $(BPFOBJ) | $(DEFAULT_BPFTOOL)