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Date:   Tue, 30 Jun 2020 15:30:04 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Michal Kubecek <mkubecek@...e.cz>,
        Alexei Starovoitov <ast@...nel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Sam Ravnborg <sam@...nborg.org>,
        Andrii Nakryiko <andriin@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH 04/16] net: bpfilter: use 'userprogs' syntax to build bpfilter_umh

Hi Michal, Alexei,

On Mon, Jun 8, 2020 at 8:56 PM Michal Kubecek <mkubecek@...e.cz> wrote:
>
> On Thu, Apr 23, 2020 at 04:39:17PM +0900, Masahiro Yamada wrote:
> > The user mode helper should be compiled for the same architecture as
> > the kernel.
> >
> > This Makefile reuses the 'hostprogs' syntax by overriding HOSTCC with CC.
> >
> > Now that Kbuild provides the syntax 'userprogs', use it to fix the
> > Makefile mess.
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> > Reported-by: kbuild test robot <lkp@...el.com>
> > ---
> >
> >  net/bpfilter/Makefile | 11 ++++-------
> >  1 file changed, 4 insertions(+), 7 deletions(-)
> >
> > diff --git a/net/bpfilter/Makefile b/net/bpfilter/Makefile
> > index 36580301da70..6ee650c6badb 100644
> > --- a/net/bpfilter/Makefile
> > +++ b/net/bpfilter/Makefile
> > @@ -3,17 +3,14 @@
> >  # Makefile for the Linux BPFILTER layer.
> >  #
> >
> > -hostprogs := bpfilter_umh
> > +userprogs := bpfilter_umh
> >  bpfilter_umh-objs := main.o
> > -KBUILD_HOSTCFLAGS += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi
> > -HOSTCC := $(CC)
> > +user-ccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi
> >
> > -ifeq ($(CONFIG_BPFILTER_UMH), y)
> > -# builtin bpfilter_umh should be compiled with -static
> > +# builtin bpfilter_umh should be linked with -static
> >  # since rootfs isn't mounted at the time of __init
> >  # function is called and do_execv won't find elf interpreter
> > -KBUILD_HOSTLDFLAGS += -static
> > -endif
> > +bpfilter_umh-ldflags += -static
> >
> >  $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
>
> Hello,
>
> I just noticed that this patch (now in mainline as commit 8a2cc0505cc4)
> drops the test if CONFIG_BPFILTER_UMH is "y" so that -static is now
> passed to the linker even if bpfilter_umh is built as a module which
> wasn't the case in v5.7.
>
> This is not mentioned in the commit message and the comment still says
> "*builtin* bpfilter_umh should be linked with -static" so this change
> doesn't seem to be intentional. Did I miss something?
>
> Michal Kubecek

I was away for a while from this because I saw long discussion in
"net/bpfilter: Remove this broken and apparently unmaintained"


Please let me resume this topic now.


The original behavior of linking umh was like this:
  - If CONFIG_BPFILTER_UMH=y, bpfilter_umh was linked with -static
  - If CONFIG_BPFILTER_UMH=m, bpfilter_umh was linked without -static



Restoring the original behavior will add more complexity because
now we have CONFIG_CC_CAN_LINK and CONFIG_CC_CAN_LINK_STATIC
since commit b1183b6dca3e0d5

If CONFIG_BPFILTER_UMH=y, we need to check CONFIG_CC_CAN_LINK_STATIC.
If CONFIG_BPFILTER_UMH=m, we need to check CONFIG_CC_CAN_LINK.
This would make the Kconfig dependency logic too complicated.


To make it simpler, I'd like to suggest two options.



Idea 1:

  Always use -static irrespective of whether
  CONFIG_BPFILTER_UMH is y or m.

  Add two more lines to clarify this
  in the comment in net/bpfilter/Makefile:

  # builtin bpfilter_umh should be linked with -static
  # since rootfs isn't mounted at the time of __init
  # function is called and do_execv won't find elf interpreter.
  # Static linking is not required when bpfilter is modular, but
  # we always pass -static to keep the 'depends on' in Kconfig simple.



Idea 2:

   Allow umh to become only modular,
   and drop -static flag entirely.

   If you look at net/bpfilter/Kconfig,
   BPFILTER_UMH already has 'default m'.
   So, I assume the most expected use-case
   is modular.

   My suggestion is to replace 'default m' with 'depends on m'.

   config BPFILTER_UMH
           tristate "bpfilter kernel module with user mode helper"
           depends on CC_CAN_LINK
           depends on m

   Then BPFILTER_UMH will be restricted to either m or n.
   Link umh dynamically because we can expect rootfs
   is already mounted for the module case.






Comments are appreciated.


-- 
Best Regards
Masahiro Yamada

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