Re: [PATCH v4 19/31] Documentation: kconfig: document a new Kconfig macro language

Masahiro Yamada
2018-05-17T06:56:09Z

2018-05-17 15:38 GMT+09:00 Kees Cook <keescook@chromium.org>:
> On Wed, May 16, 2018 at 11:16 PM, Masahiro Yamada
> <yamada.masahiro@socionext.com> wrote:
>> Add a document for the macro language introduced to Kconfig.
>>
>> The motivation of this work is to move the compiler option tests to
>> Kconfig from Makefile. A number of kernel features require the
>> compiler support. Enabling such features blindly in Kconfig ends up
>> with a lot of nasty build-time testing in Makefiles. If a chosen
>> feature turns out unsupported by the compiler, what the build system
>> can do is either to disable it (silently!) or to forcibly break the
>> build, despite Kconfig has let the user to enable it. By moving the
>> compiler capability tests to Kconfig, features unsupported by the
>> compiler will be hidden automatically.
>>
>> This change was strongly prompted by Linus Torvalds. You can find
>> his suggestions [1] [2] in ML. The original idea was to add a new
>> attribute with 'option shell=...', but I found more generalized text
>> expansion would make Kconfig more powerful and lovely. The basic
>> ideas are from Make, but there are some differences.
>>
>> [1]: https://lkml.org/lkml/2016/12/9/577
>> [2]: https://lkml.org/lkml/2018/2/7/527
>>
>> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
>
> (Added Randy, Jon, and linux-doc to CC for more review)
>
> This should likely be written in .rst and linked to from the developer index...
>
> https://www.kernel.org/doc/html/latest/doc-guide/sphinx.html#writing-documentation
>
> As for the content, though:
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
>
> -Kees

At least, nothing in Documentation/kbuild/ has not been
converted to ReST yet.

>> ---
>>
>> Changes in v4:
>> - Update according to the syntax change
>>
>> Changes in v3:
>> - Newly added
>>
>> Changes in v2: None
>>
>> Documentation/kbuild/kconfig-macro-language.txt | 252 ++++++++++++++++++++++++
>> MAINTAINERS | 2 +-
>> 2 files changed, 253 insertions(+), 1 deletion(-)
>> create mode 100644 Documentation/kbuild/kconfig-macro-language.txt
>>
>> diff --git a/Documentation/kbuild/kconfig-macro-language.txt b/Documentation/kbuild/kconfig-macro-language.txt
>> new file mode 100644
>> index 0000000..a8dc792
>> --- /dev/null
>> +++ b/Documentation/kbuild/kconfig-macro-language.txt
>> @@ -0,0 +1,252 @@
>> +Concept
>> +-------
>> +
>> +The basic idea was inspired by Make. When we look at Make, we notice sort of
>> +two languages in one. One language describes dependency graphs consisting of
>> +targets and prerequisites. The other is a macro language for performing textual
>> +substitution.
>> +
>> +There is clear distinction between the two language stages. For example, you
>> +can write a makefile like follows:
>> +
>> + APP := foo
>> + SRC := foo.c
>> + CC := gcc
>> +
>> + $(APP): $(SRC)
>> + $(CC) -o $(APP) $(SRC)
>> +
>> +The macro language replaces the variable references with their expanded form,
>> +and handles as if the source file were input like follows:
>> +
>> + foo: foo.c
>> + gcc -o foo foo.c
>> +
>> +Then, Make analyzes the dependency graph and determines the targets to be
>> +updated.
>> +
>> +The idea is quite similar in Kconfig - it is possible to describe a Kconfig
>> +file like this:
>> +
>> + CC := gcc
>> +
>> + config CC_HAS_FOO
>> + def_bool $(shell, $(srctree)/scripts/gcc-check-foo.sh $(CC))
>> +
>> +The macro language in Kconfig processes the source file into the following
>> +intermediate:
>> +
>> + config CC_HAS_FOO
>> + def_bool y
>> +
>> +Then, Kconfig moves onto the evaluation stage to resolve inter-symbol
>> +dependency as explained in kconfig-language.txt.
>> +
>> +
>> +Variables
>> +---------
>> +
>> +Like in Make, a variable in Kconfig works as a macro variable. A macro
>> +variable is expanded "in place" to yield a text string that may then be
>> +expanded further. To get the value of a variable, enclose the variable name in
>> +$( ). The parentheses are required even for single-letter variable names; $X is
>> +a syntax error. The curly brace form as in ${CC} is not supported either.
>> +
>> +There are two types of variables: simply expanded variables and recursively
>> +expanded variables.
>> +
>> +A simply expanded variable is defined using the := assignment operator. Its
>> +righthand side is expanded immediately upon reading the line from the Kconfig
>> +file.
>> +
>> +A recursively expanded variable is defined using the = assignment operator.
>> +Its righthand side is simply stored as the value of the variable without
>> +expanding it in any way. Instead, the expansion is performed when the variable
>> +is used.
>> +
>> +There is another type of assignment operator; += is used to append text to a
>> +variable. The righthand side of += is expanded immediately if the lefthand
>> +side was originally defined as a simple variable. Otherwise, its evaluation is
>> +deferred.
>> +
>> +The variable reference can take parameters, in the following form:
>> +
>> + $(name,arg1,arg2,arg3)
>> +
>> +You can consider the parameterized reference as a function. (more precisely,
>> +"user-defined function" in the contrast to "built-in function" listed below).
>> +
>> +Useful functions must be expanded when they are used since the same function is
>> +expanded differently if different parameters are passed. Hence, a user-defined
>> +function is defined using the = assignment operator. The parameters are
>> +referenced within the body definition with $(1), $(2), etc.
>> +
>> +In fact, recursively expanded variables and user-defined functions are the same
>> +internally. (In other words, "variable" is "function with zero argument".)
>> +When we say "variable" in a broad sense, it includes "user-defined function".
>> +
>> +
>> +Built-in functions
>> +------------------
>> +
>> +Like Make, Kconfig provides several built-in functions. Every function takes a
>> +particular number of arguments.
>> +
>> +In Make, every built-in function takes at least one argument. Kconfig allows
>> +zero argument for built-in functions, such as $(fileno), $(lineno). You could
>> +consider those as "built-in variable", but it is just a matter of how we call
>> +it after all. Let's say "built-in function" here to refer to natively supported
>> +functionality.
>> +
>> +Kconfig currently supports the following built-in functions.
>> +
>> + - $(shell,command)
>> +
>> + The "shell" function accepts a single argument that is expanded and passed
>> + to a subshell for execution. The standard output of the command is then read
>> + and returned as the value of the function. Every newline in the output is
>> + replaced with a space. Any trailing newlines are deleted. The standard error
>> + is not returned, nor is any program exit status.
>> +
>> + - $(info,text)
>> +
>> + The "info" function takes a single argument and prints it to stdout.
>> + It evaluates to an empty string.
>> +
>> + - $(warning,text)
>> +
>> + The "warning" function is similar to "info" except that it sends its argument
>> + to stderr and prefixes the output with the name of the current Kconfig file
>> + and the current line number.
>> +
>> + - $(error,text)
>> +
>> + The "error" function is similar to "warning", but it terminates the parsing
>> + immediately.
>> +
>> + - $(if,condition,then-part[,else-part])
>> +
>> + The "if" function takes two or three arguments ('else-part' is optional).
>> + Depending on the value of the condition part, the argument to be expanded
>> + is selected. The condition is true if its expansion contains any characters
>> + except whitespaces. In this case, the then-part is expanded. Otherwise, the
>> + else-part is expanded.
>> +
>> + Note:
>> + In Make, the condition is true if it contains any characters including
>> + whitespaces, which is why $(strip ...) is sometimes necessary in the
>> + condition part. Kconfig changed the behavior to make it handier.
>> +
>> + - $(filename)
>> +
>> + The 'filename' takes no argument, and $(filename) is expanded to a file name
>> + being parsed.
>> +
>> + - $(lineno)
>> +
>> + The 'lineno' takes no argument, and $(lineno) is expanded to a line number
>> + being parsed.
>> +
>> +
>> +Difference of function call syntax
>> +----------------------------------
>> +
>> +Kconfig adopts Make-like macro language, but the function call syntax is
>> +slightly different.
>> +
>> +A function call in Make looks like follows:
>> +
>> + $(func-name arg1,arg2,arg3)
>> +
>> +The function name and the first argument are separated by at least one
>> +whitespace. Then, leading whitespaces are trimmed from the first argument,
>> +but whitespaces in the other arguments are kept. You need to use a kind of
>> +trick to start the first parameter with spaces. For example, if you want
>> +to make "info" function print " hello", you can write like follows:
>> +
>> + $(info $(space)$(space)hello)
>> +
>> +Kconfig uses only commas for delimiters, and keeps all whitespaces in the
>> +function call. Some people prefer putting a space after each comma delimiter:
>> +
>> + $(func-name, arg1, arg2, arg3)
>> +
>> +In this case, "func-name" will receive " arg1", " arg2", " arg3". The presence
>> +of leading spaces may really matter depending on the function. The same applies
>> +to Make - for example, $(subst .c, .o, $(sources)) is a typical mistake.
>> +
>> +In Make, a user-defined function is referenced by using a built-in function,
>> +'call', like this:
>> +
>> + $(call my-func,arg1,arg2,arg3)
>> +
>> +Kconfig invokes user-defined functions and built-in functions in the same way.
>> +The omission of 'call' makes the syntax shorter.
>> +
>> +In Make, some functions exceptionally treat commas verbatim instead of argument
>> +separators. For example, $(shell echo hello, world) evaluates to "hello, world".
>> +Likewise, $(info hello, world) prints "hello, world" to stdout. You could say
>> +this is _useful_ inconsistency.
>> +
>> +For simpler implementation and grammatical consistency, Kconfig always treats
>> +commas that appear in the $( ) form as delimiters. It means
>> +
>> + $(shell, echo hello, world)
>> +
>> +is an error because it is passing two parameters where the 'shell' function
>> +accepts only one. To pass commas in arguments, you can use the following trick:
>> +
>> + comma := ,
>> + $(shell, echo hello$(comma) world)
>> +
>> +
>> +Caveats
>> +-------
>> +
>> +A variable (or function) cannot be expanded across tokens. So, you cannot use
>> +a variable as a shorthand for an expression that consists of multiple tokens.
>> +The following works:
>> +
>> + RANGE_MIN := 1
>> + RANGE_MAX := 3
>> +
>> + config FOO
>> + int "foo"
>> + range $(RANGE_MIN) $(RANGE_MAX)
>> +
>> +But, the following does not work:
>> +
>> + RANGES := 1 3
>> +
>> + config FOO
>> + int "foo"
>> + range $(RANGES)
>> +
>> +A variable cannot be expanded to any keyword in Kconfig. The following does
>> +not work:
>> +
>> + MY_TYPE := tristate
>> +
>> + config FOO
>> + $(MY_TYPE) "foo"
>> + default y
>> +
>> +Obviously from the design, $(shell command) is expanded in the textual
>> +substitution phase. You cannot pass symbols to the 'shell' function.
>> +The following does not work as expected.
>> +
>> + config ENDIAN_FLAG
>> + string
>> + default "-mbig-endian" if CPU_BIG_ENDIAN
>> + default "-mlittle-endian" if CPU_LITTLE_ENDIAN
>> +
>> + config CC_HAS_ENDIAN_FLAG
>> + def_bool $(shell $(srctree)/scripts/gcc-check-flag ENDIAN_FLAG)
>> +
>> +Instead, you can do like follows so that any function call is statically
>> +expanded.
>> +
>> + config CC_HAS_ENDIAN_FLAG
>> + bool
>> + default $(shell $(srctree)/scripts/gcc-check-flag -mbig-endian) if CPU_BIG_ENDIAN
>> + default $(shell $(srctree)/scripts/gcc-check-flag -mlittle-endian) if CPU_LITTLE_ENDIAN
>> diff --git a/MAINTAINERS b/MAINTAINERS
>> index 58b9861..b7d7ae61 100644
>> --- a/MAINTAINERS
>> +++ b/MAINTAINERS
>> @@ -7632,7 +7632,7 @@ M: Masahiro Yamada <yamada.masahiro@socionext.com>
>> T: git git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git kconfig
>> L: linux-kbuild@vger.kernel.org
>> S: Maintained
>> -F: Documentation/kbuild/kconfig-language.txt
>> +F: Documentation/kbuild/kconfig*
>> F: scripts/kconfig/
>>
>> KDUMP
>> --
>> 2.7.4
>>
>
>
>
> --
> Kees Cook
> Pixel Security

--
Best Regards
Masahiro Yamada