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

[Kees Cook]

On Wed, May 16, 2018 at 11:16 PM, Masahiro Yamada
<yamada.masahiro@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>

(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@xxxxxxxxxxxx>

-Kees

> ---
>
> 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@xxxxxxxxxxxxx>
>  T:     git git://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git kconfig
>  L:     linux-kbuild@xxxxxxxxxxxxxxx
>  S:     Maintained
> -F:     Documentation/kbuild/kconfig-language.txt
> +F:     Documentation/kbuild/kconfig*
>  F:     scripts/kconfig/
>
>  KDUMP
> --
> 2.7.4
>



-- 
Kees Cook
Pixel Security