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")
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(+)

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

>

On Tue, Dec 15, 2020 at 10:20 AM 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>
> ---

Applied to bpf tree, thanks. This conflicted with a67079b03165
("selftests/bpf: fix bpf_testmod.ko recompilation logic"), I resolved
the conflict and capitalized "Cannot" in the error message.

>
> [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
>