Re: netfilter Kconfig question

[Ulf Magnusson <ulfalizer@gmail.com>]

On Tue, Nov 27, 2018 at 2:33 AM Masahiro Yamada
<yamada.masahiro@socionext.com> wrote:
>
> On Tue, Nov 27, 2018 at 4:49 AM Ulf Magnusson <ulfalizer@gmail.com> wrote:
> >
> > On Mon, Nov 26, 2018 at 8:31 AM Masahiro Yamada
> > <yamada.masahiro@socionext.com> wrote:
> > >
> > > Hi Randy,
> > >
> > >
> > > On Tue, Nov 20, 2018 at 5:51 AM Randy Dunlap <rdunlap@infradead.org> wrote:
> > > >
> > > > Hi Yamada-san,
> > > >
> > > > There was recently a netfilter build error in linux-next for which
> > > > a patch was posted by Taehee Yoo.  The patch works (fixes the build
> > > > error) but I don't see how or why.
> > > >
> > > > My build error report is here:
> > > > https://lore.kernel.org/lkml/fa7a7824-44df-c058-dba2-ec29c5028361@infradead.org/T/#u
> > > >
> > > >
> > > > Would you take a look at it and try to explain why the patch works?
> > > >
> > > > The patch's email thread begins here:
> > > > https://www.spinics.net/lists/netfilter-devel/msg56985.html
> > >
> > >
> > > I think this is a bug of Kconfig.
> > >
> > > I sent a patch.
> > > https://patchwork.kernel.org/patch/10697637/
> > >
> > >
> > > --
> > > Best Regards
> > > Masahiro Yamada
> >
> > I always understood this behavior as intentional.
> >
> > Say that a high-level arch symbol has 'select FOO_HELPERS if FOO' for
> > example, where both FOO and FOO_HELPERS can be compiled as modules.
> >
> > The intention there is probably to make sure that FOO_HELPERS is
> > available whenever FOO is (FOO_HELPERS must be >= FOO). If FOO is y,
> > then FOO_HELPERS must be y. If FOO is m, then it's fine for
> > FOO_HELPERS to be m (but not n), because it can be loaded as needed
> > whenever FOO is.
>
> If we find such an example, it should be rewritten to:
>
>
> config FOO
>         tristate "foo"
>         select FOO_HELPERS

Yup, almost always cleaner ways to do it.

>
>
>
>
> > Forcing FOO_HELPERS to y would remove the possibility to compile it as
> > a module.
>
> This depends on the selector if you write like this:
>
>
> config OTHER_OPTION
>        tristate "?"
>        select FOO_HELPERS if FOO
>
>
> although "FOO_HELPERS >= FOO" is not met.

Yeah, that's why I mentioned "high-level arch symbol", because it gets
more confusing for other symbols. Think boards and the like too.

If you think of 'config A ... select C if B' as "A needs C if B", then
the current behavior still does the right thing in all cases:

  A B     | C
  --------+-----
  n n/m/y | n(+)  (A not enabled, irrelevant)
  m n     | n(+)  (B not enabled, irrelevant)
  m m     | m(+)  (Fine, C can be loaded if A and B are)
  m y     | m(+)  (Ditto)
  y n     | n(+)  (B not enabled, irrelevant)
  y m     | m(+)  (Fine, C can be loaded if B is)
  y y     | y     (Fine, C is available whenever B is)

The big if there is whether you think of select that way. Just trying
to show why I think it was designed that way.

Cheers,
Ulf