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Date:   Sun, 3 Jan 2021 15:30:30 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Valdis Klētnieks <valdis.kletnieks@...edu>
Cc:     Michal Marek <michal.lkml@...kovi.net>,
        "David S. Miller" <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Kconfig, DEFAULT_NETSCH, and shooting yourself in the foot..

On Sun, Jan 3, 2021 at 5:14 AM Valdis Klētnieks <valdis.kletnieks@...edu> wrote:
>
> Consider the following own goal I just discovered I scored:
>
> [~] zgrep -i fq_codel /proc/config.gz
> CONFIG_NET_SCH_FQ_CODEL=m
> CONFIG_DEFAULT_FQ_CODEL=y
> CONFIG_DEFAULT_NET_SCH="fq_codel"
>
> Obviously, fq_codel didn't get set as the default, because that happens
> before the module gets loaded (which may never happen if the sysadmin
> thinks the DEFAULT_NET_SCH already made it happen)
>
> Whoops. My bad, probably - but....
>
> The deeper question, part 1:
>
> There's this chunk in net/sched/Kconfig:
>
> config DEFAULT_NET_SCH
>         string
>         default "pfifo_fast" if DEFAULT_PFIFO_FAST
>         default "fq" if DEFAULT_FQ
>         default "fq_codel" if DEFAULT_FQ_CODEL
>         default "fq_pie" if DEFAULT_FQ_PIE
>         default "sfq" if DEFAULT_SFQ
>         default "pfifo_fast"
> endif
>
> (And a similar chunk right above it with a similar issue)
>
> Should those be "if (foo=y)" so =m can't be chosen? (I'll be
> happy to write the patch if that's what we want)
>
> Deeper question, part 2:
>
> Should there be a way in the Kconfig language to ensure that
> these two chunks can't accidentally get out of sync?  There's other
> places in the kernel where similar issues arise - a few days ago I was
> chasing a CPU governor issue where it looked like it was possible
> to set a default that was a module and thus possibly not actually loaded.
>


If there is a restriction where a modular discipline cannot be the default,
I think you can add 'depends on FOO = y'.



For example,


choice
           prompt "Default"

           config DEFAULT_FOO
                  bool "Use foo for default"
                  depends on FOO = y

           config DEFAULT_BAR
                  bool "Use bar for default"
                  depends on BAR = y

           config DEFAULT_FALLBACK
                  bool "fallback when nothing else is builtin"

endchoice





-- 
Best Regards
Masahiro Yamada

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