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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Fri, 13 Apr 2018 14:32:20 +0900
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Subject: Re: [PATCH v2 07/21] kconfig: add function support and implement
 'shell' function
To:     Kees Cook <keescook@chromium.org>
Cc:     linux-kbuild <linux-kbuild@vger.kernel.org>,
        Sam Ravnborg <sam@ravnborg.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Ulf Magnusson <ulfalizer@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Luis R . Rodriguez" <mcgrof@kernel.org>,
        Nicolas Pitre <nico@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>
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2018-03-28 12:41 GMT+09:00 Kees Cook <keescook@chromium.org>:
> On Mon, Mar 26, 2018 at 10:29 PM, Masahiro Yamada
> <yamada.masahiro@socionext.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@socionext.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.


The current implementation cannot do this.

The $(shell ...) has already expanded
before the parser receives tokens.

There is no way to know whether a token came from a source file as-is,
or it was derived from textual substitution.


> 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?


For example, it would be possible to dump the result of $(shell ...)
evaluation into the console in debug mode.


> Beyond that, looks good!
>
> -Kees
>
> --
> Kees Cook
> Pixel Security
> --
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-- 
Best Regards
Masahiro Yamada