[SRU][G, H][PATCH v2 1/3] UBUNTU: SAUCE: selftests/bpf: Clarify build error if no vmlinux

[Kamal Mostafa]

BugLink: https://bugs.launchpad.net/bugs/1908144

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 at canonical.com>
Signed-off-by: Andrii Nakryiko <andrii at kernel.org>
Link: https://lore.kernel.org/bpf/20201215182011.15755-1-kamal@canonical.com
Signed-off-by: Kamal Mostafa <kamal at 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 50965cc7bf09..ed907f0cba74 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -144,6 +144,9 @@ 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
 
 DEFAULT_BPFTOOL := $(SCRATCH_DIR)/sbin/bpftool
 
-- 
2.17.1