[PATCH] Documentation: warn users not to use select on choice options in Kconfig

[PATCH] Documentation: warn users not to use select on choice options in Kconfig

[Julian Braha]

Hengelein's master thesis about Kconfig from 2015 wrote that 'select'
does not work on member options inside of a 'choice'.

So I've re-implemented his check as part of kconfirm:
https://github.com/julianbraha/kconfirm

and discovered several instances of these dead selects in use, and
fixing them is not always trivial. For example, see:
https://lore.kernel.org/all/a3b7a752-8cc4-492a-bb62-43121b852c41@app.fastmail.com/

Let's add a warning to the documentation (a place where developers
are more likely to see it than a master's thesis...)

Signed-off-by: Julian Braha <julianbraha@gmail.com>
---
 Documentation/kbuild/kconfig-language.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index d9338407c1c6..ab2822d7ca1c 100644
--- a/Documentation/kbuild/kconfig-language.rst
+++ b/Documentation/kbuild/kconfig-language.rst
@@ -164,6 +164,11 @@ applicable everywhere (see syntax).
 	That will limit the usefulness but on the other hand avoid
 	the illegal configurations all over.
 
+	select should also not be used on the member options of a choice.
+	Selecting a choice option will do nothing (dead code). Instead,
+	consider making these a dependency, or using the defaults of the
+	choice to enable the member option when it's needed.
+
 	If "select" <symbol> is followed by "if" <expr>, <symbol> will be
 	selected by the logical AND of the value of the current menu symbol
 	and <expr>. This means, the lower limit can be downgraded due to the
-- 
2.54.0

Re: [PATCH] Documentation: warn users not to use select on choice options in Kconfig

[Kees Cook]

On Wed, Jul 15, 2026 at 11:01:49PM +0100, Julian Braha wrote:
> Hengelein's master thesis about Kconfig from 2015 wrote that 'select'
> does not work on member options inside of a 'choice'.
> 
> So I've re-implemented his check as part of kconfirm:
> https://github.com/julianbraha/kconfirm
> 
> and discovered several instances of these dead selects in use, and
> fixing them is not always trivial. For example, see:
> https://lore.kernel.org/all/a3b7a752-8cc4-492a-bb62-43121b852c41@app.fastmail.com/
> 
> Let's add a warning to the documentation (a place where developers
> are more likely to see it than a master's thesis...)
> 
> Signed-off-by: Julian Braha <julianbraha@gmail.com>

Yeah, it'd be good to add a checkpatch or even build-time check too.

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook

Re: [PATCH] Documentation: warn users not to use select on choice options in Kconfig

[Julian Braha]

Hi Kees,

On 7/16/26 05:19, Kees Cook wrote:
> Yeah, it'd be good to add a checkpatch or even build-time check too.

I'll look into adding a check for this into the kconfig interpreter.

- Julian Braha

Re: [PATCH] Documentation: warn users not to use select on choice options in Kconfig

[Nicolas Schier]

On Wed, 15 Jul 2026 23:01:49 +0100, Julian Braha wrote:
> Documentation: warn users not to use select on choice options in Kconfig

Applied to kbuild/linux.git (kbuild-next-unstable), thanks!

[1/1] Documentation: warn users not to use select on choice options in Kconfig
      https://git.kernel.org/kbuild/c/d10cfe72

Please look out for regression or issue reports or other follow up
comments, as they may result in the patch/series getting dropped,
reverted or modified (e.g. trailers).

Patches applied to the kbuild-next-unstable branch are accepted pending
wider testing in linux-next and any post-commit review; they will
generally be moved to the kbuild-next branch in about a week if no
issues are found.

Best regards,
-- 
Nicolas

Re: [PATCH] Documentation: warn users not to use select on choice options in Kconfig

[Jani Nikula]

On Thu, 16 Jul 2026, Julian Braha <julianbraha@gmail.com> wrote:
> Hi Kees,
>
> On 7/16/26 05:19, Kees Cook wrote:
>> Yeah, it'd be good to add a checkpatch or even build-time check too.
>
> I'll look into adding a check for this into the kconfig interpreter.

This. I wish there was a kconfig lint option that would whine about all
the things that kconfig-language.rst explicitly tells you *not* to use
select for. (So I could perhaps eventually stop replying to people with
that stuff.)

BR,
Jani.


-- 
Jani Nikula, Intel