Re: [PATCH 3/3] bpfilter: do not (ab)use host-program build rule

[Alexei Starovoitov]

On Sat, Jun 09, 2018 at 02:12:10AM +0900, Masahiro Yamada wrote:
> It is an ugly hack to overwrite $(HOSTCC) with $(CC) to reuse the
> build rules from scripts/Makefile.host.  It should not be tedious
> to write a build rule for its own.
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> ---
> 
>  net/bpfilter/Makefile                   | 17 +++++++++++------
>  net/bpfilter/{main.c => bpfilter_umh.c} |  0
>  2 files changed, 11 insertions(+), 6 deletions(-)
>  rename net/bpfilter/{main.c => bpfilter_umh.c} (100%)
> 
> diff --git a/net/bpfilter/Makefile b/net/bpfilter/Makefile
> index 39c6980..6571b30 100644
> --- a/net/bpfilter/Makefile
> +++ b/net/bpfilter/Makefile
> @@ -3,18 +3,23 @@
>  # Makefile for the Linux BPFILTER layer.
>  #
>  
> -hostprogs-y := bpfilter_umh
> -bpfilter_umh-objs := main.o
> -HOSTCFLAGS += -I. -Itools/include/ -Itools/include/uapi
> -HOSTCC := $(CC)

that is a hack indeed. I don't like it either, but see below.

> -
>  ifeq ($(CONFIG_BPFILTER_UMH), y)
>  # builtin bpfilter_umh should be compiled with -static
>  # since rootfs isn't mounted at the time of __init
>  # function is called and do_execv won't find elf interpreter
> -HOSTLDFLAGS += -static
> +STATIC := -static
>  endif
>  
> +quiet_cmd_cc_user = CC      $@
> +      cmd_cc_user = $(CC) -Wall -Wmissing-prototypes -O2 -std=gnu89 \
> +		    -I$(srctree) -I$(srctree)/tools/include/ \
> +		    -I$(srctree)/tools/include/uapi $(STATIC) -o $@ $<
> +
> +$(obj)/bpfilter_umh: $(src)/bpfilter_umh.c FORCE
> +	$(call if_changed,cc_user)

Does this scale?
Please see two top patches here:
https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git/log/?h=ipt_bpf
that add more meat to bpfilter and a lot more files.
Recompiling all of them at once isn't nice either.
This Makefile needs different .c -> .o rules for bpfilter_kern.c files
that get compiled into kernel module and for the rest of umh files:
main.c ctor.c init.c gen.c etc
that need to be compiled .c -> .o differently.
I don't see how to do it without ugly hacks in Makefile.
In that sense HOSTCC = CC hack looked the least ugly to me that's
why I went with it.
Better ideas?