On Mon, Dec 14, 2020 at 12:21 PM Kamal Mostafa kamal@canonical.com wrote:

If Makefile cannot find any of the vmlinux's in its VMLINUX_BTF_PATHS list, it tries to run btftool incorrectly, with VMLINUX_BTF unset:

bpftool btf dump file $(VMLINUX_BTF) format c

Such that the keyword 'format' is misinterpreted as the path to vmlinux. The resulting build error message is fairly cryptic:

  GEN      vmlinux.h
Error: failed to load BTF from format: No such file or directory

This patch makes the failure reason clearer by yielding this instead:

Makefile:...: *** cannot find a vmlinux for VMLINUX_BTF at any of
"{paths}".  Stop.

Fixes: acbd06206bbb ("selftests/bpf: Add vmlinux.h selftest exercising tracing of syscalls") Cc: stable@vger.kernel.org # 5.7+ Signed-off-by: Kamal Mostafa kamal@canonical.com

---

tools/testing/selftests/bpf/Makefile | 3 +++ 1 file changed, 3 insertions(+)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile index 542768f5195b..93ed34ef6e3f 100644 --- a/tools/testing/selftests/bpf/Makefile +++ b/tools/testing/selftests/bpf/Makefile @@ -196,6 +196,9 @@ $(BUILD_DIR)/libbpf $(BUILD_DIR)/bpftool $(BUILD_DIR)/resolve_btfids $(INCLUDE_D $(INCLUDE_DIR)/vmlinux.h: $(VMLINUX_BTF) | $(BPFTOOL) $(INCLUDE_DIR) ifeq ($(VMLINUX_H),) $(call msg,GEN,,$@) +ifeq ($(VMLINUX_BTF),) +$(error cannot find a vmlinux for VMLINUX_BTF at any of "$(VMLINUX_BTF_PATHS)") +endif $(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@ else $(call msg,CP,,$@)

This handles only one possible case where $(VMLINUX_BTF) is expected.

Given $(VMLINUX_BTF) is now almost mandatory to have (bpf_testmod.ko is dependent on it, for instance), I think it's acceptable to just add such a check right after VMLINUX_BTF definition, outside of any specific rule. The downside is that some unrelated targets (e.g., non-test_progs tests) will fail if VMLINUX_BTF is not found, even if they don't need it, but the most common use case us to build test_progs, as well as bpf_testmod.ko and runqslower, so I think it's an OK trade off. Worst case, one can work around such with VMLINUX_BTF=whatever command-line overload.

-- 2.17.1