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From:   Kamal Mostafa <kamal@canonical.com>
To:     andrii.nakryiko@gmail.com
Cc:     Kamal Mostafa <kamal@canonical.com>, stable@vger.kernel.org,
        Shuah Khan <shuah@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, linux-kselftest@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH v2] selftests/bpf: clarify build error if no vmlinux
Date:   Tue, 15 Dec 2020 10:20:10 -0800
Message-Id: <20201215182011.15755-1-kamal@canonical.com>
In-Reply-To: <CAEf4BzYBvz4TDayTE=Bc_bjqvOGaavmmw1sJhOCKhq9DwUpd4A@mail.gmail.com>
References: <CAEf4BzYBvz4TDayTE=Bc_bjqvOGaavmmw1sJhOCKhq9DwUpd4A@mail.gmail.com>
Precedence: bulk

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>
---

[v2] moves the check to right after the VMLINUX_BTF definition.

 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..7ba631f495f7 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -146,6 +146,9 @@ 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