selftests/bpf: Clarify build error if no vmlinux

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")
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201215182011.15755-1-kamal@canonical.com

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 8c33e999319a9aa6549f4d91e6e622cd97d945fd..c51df6b91befe50b6740e6f25c05d503885b7c1f 100644 (file)
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -121,6 +121,9 @@ VMLINUX_BTF_PATHS ?= $(if $(O),$(O)/vmlinux)                                \
                     /sys/kernel/btf/vmlinux                            \
                     /boot/vmlinux-$(shell uname -r)
 VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
+ifeq ($(VMLINUX_BTF),)
+$(error Cannot find a vmlinux for VMLINUX_BTF at any of "$(VMLINUX_BTF_PATHS)")
+endif
 
 # Define simple and short `make test_progs`, `make test_sysctl`, etc targets
 # to build individual tests.