linux-kernel - Re: [PATCH v2 07/21] kconfig: add function support and implement 'shell' function

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Date:   Tue, 27 Mar 2018 20:41:18 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     linux-kbuild <linux-kbuild@...r.kernel.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Ulf Magnusson <ulfalizer@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Nicolas Pitre <nico@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 07/21] kconfig: add function support and implement
 'shell' function

On Mon, Mar 26, 2018 at 10:29 PM, Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
> This commit adds a new concept 'function' to do more text processing
> in Kconfig.
>
> A function call looks like this:
>
>   $(function arg1, arg2, arg3, ...)
>
> (Actually, this syntax was inspired by make.)
>
> Real examples will look like this:
>
>   $(shell echo hello world)
>   $(cc-option -fstackprotector)
>
> This commit adds the basic infrastructure to add, delete, evaluate
> functions, and also the first built-in function $(shell ...).  This
> accepts a single command to execute.  It returns the standard output
> from it.
>
> [Example code]
>
>   config HELLO
>           string
>           default "$(shell echo hello world)"
>
>   config Y
>           def_bool $(shell echo y)
>
> [Result]
>
>   $ make -s alldefconfig && tail -n 2 .config
>   CONFIG_HELLO="hello world"
>   CONFIG_Y=y
>
> Caveat:
> Like environments, functions are expanded in the lexer.  You cannot
> pass symbols to function arguments.  This is a limitation to simplify
> the implementation.  I want to avoid the dynamic function evaluation,
> which would introduce much more complexity.
>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> ---
>
> Reminder for myself:
> Update Documentation/kbuild/kconfig-language.txt

Yeah, this needs to be included here, especially given the "cannot
pass symbols" aspect which might surprise people.

> [...]
> +/* built-in functions */
> +static char *do_shell(struct function *f, int argc, char *argv[])
> +{
> +       static const char *pre = "(";
> +       static const char *post = ") 2>/dev/null";

Right now the search and help screens in menuconfig just show the line
a config is defined and nothing more. I think it would be extremely
handy to include shell output here in some way. Especially when trying
to answer questions like "why aren't GCC plugins available?" it's got
quite a bit harder to debug.

Could we capture the output (especially stderr) for these kinds of hints?

Beyond that, looks good!

-Kees

-- 
Kees Cook
Pixel Security