[SRU][G, H][PATCH 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")
Cc: stable at vger.kernel.org # 5.7+
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
Reference: https://lore.kernel.org/bpf/20201214202049.7205-1-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 50965cc7bf09..0b924c168965 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -188,6 +188,9 @@ $(BUILD_DIR)/libbpf $(BUILD_DIR)/bpftool $(INCLUDE_DIR):
 
 $(INCLUDE_DIR)/vmlinux.h: $(VMLINUX_BTF) | $(BPFTOOL) $(INCLUDE_DIR)
 	$(call msg,GEN,,$@)
+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 > $@
 
 # Get Clang's default includes on this system, as opposed to those seen by
-- 
2.17.1