[PATCH] kconfig: default to zero if int/hex symbol lacks default property

[Tom Rini]

On Sun, May 05, 2024 at 03:53:53PM +0200, Yoann Congal wrote:

> From: Masahiro Yamada <masahiroy at kernel.org>
> 
> This is a cherry-pick from the kernel commit:
> 6262afa10ef7c (kconfig: default to zero if int/hex symbol lacks default property, 2023-11-26)
> 
> When a default property is missing in an int or hex symbol, it defaults
> to an empty string, which is not a valid symbol value.
> 
> It results in an incorrect .config, and can also lead to an infinite
> loop in scripting.
> 
> Use "0" for int and "0x0" for hex as a default value.
> 
> Signed-off-by: Masahiro Yamada <masahiroy at kernel.org>
> Reviewed-by: Yoann Congal <yoann.congal at smile.fr>
> 
> Signed-off-by: Yoann Congal <yoann.congal at smile.fr>
> ---
> Added context that was not in the upstream commit:
> The infinite loop case happens with a configuration defined like this
> (a hex config without a valid default value):
>   config HEX_TEST
>   	hex "Hex config without default"
> 
> And using:
>   $ make oldconfig < /dev/null
>   scripts/kconfig/conf  --oldconfig Kconfig
>   *
>   * General setup
>   *
> 
>   Error in reading or end of file.
> 
>   Error in reading or end of file.
>   Hex config without default (HEX_TEST) [] (NEW)
> 
>   Error in reading or end of file.
>   Hex config without default (HEX_TEST) [] (NEW)
>   # This loops forever
> 
> NB: Scripted config manipulation often call make with /dev/null as
> stdin (Yocto recipe, CI build, ...)
> 
> This was discovered when working on Yocto bug:
> https://bugzilla.yoctoproject.org/show_bug.cgi?id=14136

I'm surprised this was accepted. In the past I've wanted to avoid this
kind of change in Kconfig because while the empty string can be easily
be checked in the code as "user didn't really configure this, do
nothing" a value of zero is a valid option in these cases and so then in
the code we need a bool symbol to decide if the hex/int symbol is set or
not.

Today this is less of an issue than it used to be in U-Boot with
everything CONFIG-related migrated to Kconfig and so there's no longer
the question of if we missed migrating a file that defined the value but
there's still places we have in the code where hex symbol is undefined
is not the same thing as hex symbol is 0x0.

Is there a specific use case you have for this in U-Boot? It's been a
while, but it's also been cases of newly introduced symbols in Kconfig
files with incorrect dependencies, where the infinite loop in kconfig
happened, CI failed and we caught the problem.

-- 
Tom
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