2018-05-17 15:38 GMT+09:00 Kees Cook <keescook@xxxxxxxxxxxx>: > 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 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@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 -- Best Regards Masahiro Yamada -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
