Re: [PATCH bpf-next v3 02/11] tools/bpf/runqslower: Fix override option for VMLINUX_BTF

[Andrii Nakryiko]

On Thu, Jan 16, 2020 at 5:23 AM Toke HÃiland-JÃrgensen <toke@xxxxxxxxxx> wrote:
>
> From: Toke HÃiland-JÃrgensen <toke@xxxxxxxxxx>
>
> The runqslower tool refuses to build without a file to read vmlinux BTF
> from. The build fails with an error message to override the location by
> setting the VMLINUX_BTF variable if autodetection fails. However, the
> Makefile doesn't actually work with that override - the error message is
> still emitted.
>
> Fix this by including the value of VMLINUX_BTF in the expansion, and only
> emitting the error message if the *result* is empty. Also permit running
> 'make clean' even though no VMLINUX_BTF is set.
>
> Fixes: 9c01546d26d2 ("tools/bpf: Add runqslower tool to tools/bpf")
> Signed-off-by: Toke HÃiland-JÃrgensen <toke@xxxxxxxxxx>
> ---
>  tools/bpf/runqslower/Makefile |   18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/tools/bpf/runqslower/Makefile b/tools/bpf/runqslower/Makefile
> index cff2fbcd29a8..89fb7cd30f1a 100644
> --- a/tools/bpf/runqslower/Makefile
> +++ b/tools/bpf/runqslower/Makefile
> @@ -10,12 +10,14 @@ CFLAGS := -g -Wall
>
>  # Try to detect best kernel BTF source
>  KERNEL_REL := $(shell uname -r)
> -ifneq ("$(wildcard /sys/kernel/btf/vmlinux)","")
> -VMLINUX_BTF := /sys/kernel/btf/vmlinux
> -else ifneq ("$(wildcard /boot/vmlinux-$(KERNEL_REL))","")
> -VMLINUX_BTF := /boot/vmlinux-$(KERNEL_REL)
> -else
> -$(error "Can't detect kernel BTF, use VMLINUX_BTF to specify it explicitly")
> +VMLINUX_BTF_PATHS := $(VMLINUX_BTF) /sys/kernel/btf/vmlinux /boot/vmlinux-$(KERNEL_REL)
> +VMLINUX_BTF_PATH := $(firstword $(wildcard $(VMLINUX_BTF_PATHS)))

If user specifies VMLINUX_BTF pointing to non-existing file, but the
system has /sys/kernel/btf/vmlinux, the latter will still be used,
which is a very surprising behavior.

Also MAKECMDGOALS feels like a fragile hack to me. How about we move
this VMLINUX_BTF guessing (without $(error)) into vmlinux.h rule
itself and use shell if conditional after it to check for file
existance and print nice error. That way we'll be checking VMLINUX_BTF
only when it's really needed.

> +
> +ifeq ("$(VMLINUX_BTF_PATH)","")
> +ifneq ($(MAKECMDGOALS),clean)
> +$(error Could not find kernel BTF file (tried: $(VMLINUX_BTF_PATHS)). \
> +       Try setting $$VMLINUX_BTF)
> +endif
>  endif
>
>  abs_out := $(abspath $(OUTPUT))
> @@ -67,9 +69,9 @@ $(OUTPUT):
>         $(call msg,MKDIR,$@)
>         $(Q)mkdir -p $(OUTPUT)
>
> -$(OUTPUT)/vmlinux.h: $(VMLINUX_BTF) | $(OUTPUT) $(BPFTOOL)
> +$(OUTPUT)/vmlinux.h: $(VMLINUX_BTF_PATH) | $(OUTPUT) $(BPFTOOL)
>         $(call msg,GEN,$@)
> -       $(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@
> +       $(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF_PATH) format c > $@
>
>  $(OUTPUT)/libbpf.a: | $(OUTPUT)
>         $(Q)$(MAKE) $(submake_extras) -C $(LIBBPF_SRC)                         \
>