Re: [PATCH 04/16] net: bpfilter: use 'userprogs' syntax to build bpfilter_umh

Alexei Starovoitov
2020-06-30T16:47:15Z

On Tue, Jun 30, 2020 at 03:30:04PM +0900, Masahiro Yamada wrote:
> Hi Michal, Alexei,
>
> On Mon, Jun 8, 2020 at 8:56 PM Michal Kubecek <mkubecek@suse.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@kernel.org>
> > > Reported-by: kbuild test robot <lkp@intel.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

That was done to make sure both static and dynamic linking work.
For production -static is necessary.
For debugging of usermode blob dynamic is beneficial.

> 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.

Currently I'm working on adding bpf_iter to use 'user mode driver'
(old user mode blob) facility on top of Eric's patches.
So there will be quite a bit more complexity to build system.
Folks who don't want to deal with -static requirement should
just disable the feature.

> 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.

I don't think it's making it much simpler.
It's a tiny change to makefile.
I could be missing something.
Requiring -static for =y and =m is fine.

> 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.

absolutely not.
Both =y and =m are mandatory.

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

The default for BPFILTER is =N.
Distros should NOT be turning that to =y

Same thing with upcoming bpf_iter. It will default to =n.