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vladimiroltean edited this page Apr 13, 2015 · 8 revisions

Using Syntastic Vim plugin with the Linux kernel

Vim can be a hard-to-use editor, especially in large projects like the Linux kernel, but certain plugins and distros like Janus (which I personally use) can make it much easier, especially when configured correctly. By default, Syntastic won't work out of the box with such a project because of the complex flags and include directories that the Linux Makefile uses, but Syntastic has no clue how to reproduce them when invoking gcc. So this tutorial is going to be precisely about this.

Setup

First of all, we will have to build something using the Linux kernel Makefile. We can either do this in-tree, executing make in the root directory of the Linux sources, or write an out-of-tree kernel module, which I will further assume.

If you don't know how to do this, you will need two special files (if you already have the sources set up for a kernel module, feel free to skip this chapter):

  • a Makefile containing:

      KDIR = /usr/src/linux-so2
  kbuild:
  	make -C $(KDIR) M=`pwd`

  • a Kbuild file with contents in the likes of:

      EXTRA_CFLAGS = -Wall -g
  obj-m = my-module.o

Of course, you also need some kernel sources (which you can get from here). In this example, I am using some Linux 3.13 sources symbolically linked to the folder /usr/src/linux-so2:

# ln -s /home/aldi/so2/linux-3.13 /usr/src/linux-so2

What is happening is that, by invoking Makefile, in return it changes the directory to that of the kernel source (KDIR) and invokes the Makefile found there. The Linux kernel Makefile is a huge beast that, amongst other things, checks for the M environment variable (set to pwd) so that it knows to return to our folder when compiling our module. The second file (Kbuild) contains further details about our module (how it needs to be compiled, which source files are to be used, etc). Without further ado, we will see that EXTRA_CFLAGS will be appended to the set of CFLAGS already provided by the current kernel source we are using.

Just so we can check functionality easier, write a simple module that contains an error (missing semicolon in my_init):

#include <linux/init.h>   /* for __init and __exit */
#include <linux/module.h> /* for module_init and module_exit */
#include <linux/printk.h> /* for printk call */
 
MODULE_AUTHOR("Syntastic");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Syntastic test module");
 
static int __init my_init(void)
{
        printk(KERN_DEBUG "It works!\n")    /* missing semicolon */
        return 0;
}

static void __exit my_exit(void)
{
        printk(KERN_DEBUG "Goodbye!\n");
}

module_init(my_init);
module_exit(my_exit);

Save the module to a C file:

:w my-module.c

Makefile inspection

So the Makefile of our module does little more than call the kernel one, so let's explore that a bit. What we're looking to find are the CFLAGS that gcc is using, so we can copy and pass them to syntastic and it can emulate what gcc is doing. It turns out that the CFLAGS are dynamically set, so they can't really be determined by simply reading the Makefile, but the following comment is useful:

# Use 'make V=1' to see the full commands

In our module folder, we can activate verbose output for module compilation. giving us access to all parameters used by gcc. A sample output may be:

$ make V=1
make -C /usr/src/linux-so2 M=/home/aldi/so2/my-module
make[1]: Entering directory `/home/aldi/so2/linux-3.13'
mkdir -p /home/aldi/so2/my-module/.tmp_versions ; rm -f /home/aldi/so2/my-module/.tmp_versions/*
make -f scripts/Makefile.build obj=/home/aldi/so2/my-module
  gcc -Wp,-MD,/home/aldi/so2/my-module/.my-module.o.d  -nostdinc -isystem /usr/lib/gcc/x86_64-linux-gnu/4.8/include -I/home/aldi/so2/linux-3.13/arch/x86/include -Iarch/x86/include/generated  -Iinclude -I/home/aldi/so2/linux-3.13/arch/x86/include/uapi -Iarch/x86/include/generated/uapi -I/home/aldi/so2/linux-3.13/include/uapi -Iinclude/generated/uapi -include /home/aldi/so2/linux-3.13/include/linux/kconfig.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -m32 -msoft-float -mregparm=3 -freg-struct-return -mno-mmx -mno-sse -fno-pic -mpreferred-stack-boundary=2 -march=i686 -maccumulate-outgoing-args -Wa,-mtune=generic32 -ffreestanding -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_AVX=1 -DCONFIG_AS_AVX2=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -Wframe-larger-than=2048 -fno-stack-protector -Wno-unused-but-set-variable -fno-omit-frame-pointer -fno-optimize-sibling-calls -g -pg -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fconserve-stack -Werror=implicit-int -Werror=strict-prototypes -DCC_HAVE_ASM_GOTO -Wall -g -Wno-unused  -DMODULE  -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(my-module)"  -D"KBUILD_MODNAME=KBUILD_STR(my-module)" -c -o /home/aldi/so2/my-module/my-module.o /home/aldi/so2/my-module/my-module.c
  if [ "-pg" = "-pg" ]; then if [ /home/aldi/so2/my-module.o != "scripts/mod/empty.o" ]; then /home/aldi/so2/linux-3.13/scripts/recordmcount  "/home/aldi/so2/my-module/my-module.o"; fi; fi;
(cat /dev/null;   echo kernel//home/aldi/so2/my-module/my-module.ko;) > /home/aldi/so2/my-module/modules.order
make -f /home/aldi/so2/linux-3.13/scripts/Makefile.modpost
  find /home/aldi/so2/my-module/.tmp_versions -name '*.mod' | xargs -r grep -h '\.ko$' | sort -u | sed 's/\.ko$/.o/' | scripts/mod/modpost   -i /home/aldi/so2/linux-3.13/Module.symvers -I /home/aldi/so2/my-module/Module.symvers  -o /home/aldi/so2/my-module/Module.symvers -S -w  -s -T -
  gcc -Wp,-MD,/home/aldi/so2/my-module/.my-module.mod.o.d  -nostdinc -isystem /usr/lib/gcc/x86_64-linux-gnu/4.8/include -I/home/aldi/so2/linux-3.13/arch/x86/include -Iarch/x86/include/generated  -Iinclude -I/home/aldi/so2/linux-3.13/arch/x86/include/uapi -Iarch/x86/include/generated/uapi -I/home/aldi/so2/linux-3.13/include/uapi -Iinclude/generated/uapi -include /home/aldi/so2/linux-3.13/include/linux/kconfig.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -m32 -msoft-float -mregparm=3 -freg-struct-return -mno-mmx -mno-sse -fno-pic -mpreferred-stack-boundary=2 -march=i686 -maccumulate-outgoing-args -Wa,-mtune=generic32 -ffreestanding -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_AVX=1 -DCONFIG_AS_AVX2=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -Wframe-larger-than=2048 -fno-stack-protector -Wno-unused-but-set-variable -fno-omit-frame-pointer -fno-optimize-sibling-calls -g -pg -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fconserve-stack -Werror=implicit-int -Werror=strict-prototypes -DCC_HAVE_ASM_GOTO -Wall -g -Wno-unused  -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(my-module.mod)"  -D"KBUILD_MODNAME=KBUILD_STR(my-module)" -DMODULE  -c -o /home/aldi/so2/my-module/my-module.mod.o /home/aldi/so2/my-module/my-module.mod.c
  ld -r -m elf_i386 -T /home/aldi/so2/linux-3.13/scripts/module-common.lds --build-id  -o /home/aldi/so2/my-module/my-module.ko /home/aldi/so2/my-module/my-module.o /home/aldi/so2/my-module/my-module.mod.o
make[1]: Leaving directory `/home/aldi/so2/linux-3.13'

Makefile output parsing

From the above generated output, only parts of one line are of interest to us, the one in which our source code (/home/aldi/so2/my-module/my-module.c) is turned into object code. We need to copy the CFLAGS of the first gcc command into a file called .syntastic_c_config:

-nostdinc -isystem /usr/lib/gcc/x86_64-linux-gnu/4.8/include -I/home/aldi/so2/linux-3.13/arch/x86/include -Iarch/x86/include/generated  -Iinclude -I/home/aldi/so2/linux-3.13/arch/x86/include/uapi -Iarch/x86/include/generated/uapi -I/home/aldi/so2/linux-3.13/include/uapi -Iinclude/generated/uapi -include /home/aldi/so2/linux-3.13/include/linux/kconfig.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -m32 -msoft-float -mregparm=3 -freg-struct-return -mno-mmx -mno-sse -fno-pic -mpreferred-stack-boundary=2 -march=i686 -maccumulate-outgoing-args -Wa,-mtune=generic32 -ffreestanding -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -DCONFIG_AS_AVX=1 -DCONFIG_AS_AVX2=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -Wframe-larger-than=2048 -fno-stack-protector -Wno-unused-but-set-variable -fno-omit-frame-pointer -fno-optimize-sibling-calls -g -pg -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fconserve-stack -Werror=implicit-int -Werror=strict-prototypes -DCC_HAVE_ASM_GOTO -Wall -g -Wno-unused  -DMODULE  -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(my-module)"  -D"KBUILD_MODNAME=KBUILD_STR(my-module)"

Now we need to do some parsing on this output, because Syntastic wants each parameter to be on a separate line. We can sed the text, replacing each whitespace with a new line. In Vim:

:%s/ /\r/g

We have two more things to do:

  • by default, Syntastic treats include paths as relative to the configuration file. Obviously, we need to change those paths so that they are absolute (prefix them with $(KDIR) as described above, in our case /usr/linux/so2/). We can also obtain this with sed. Replace /usr/src/linux-so2/ with your appropriate KDIR in the following Vim command:

    :%s!\m^-I\zs\ze[^/]!/usr/src/linux-so2/!

  • Syntastic's interpretation of -D macro predefinitions is a little different from gcc, because of the way parameters in the config file are treated. It can be seen in the last 3 parameters that KBUILD_BASENAME and KBUILD_MODNAME depend upon KBUILD_STR. To avoid shell expansion of these parameters, they are defined with quotes in the Linux Makefile, but Syntastic automatically encloses each parameter in quotes. The end result is that these parameters get escaped twice, which is not what we want. In order to solve this we must unquote everything that came from the Linux Makefile:

    :%s/"//g

The final version should look like this:

    -nostdinc
    -isystem
    /usr/lib/gcc/x86_64-linux-gnu/4.8/include
    -I/usr/src/linux-so2/arch/x86/include
    -I/usr/src/linux-so2/arch/x86/include/generated
    
    -I/usr/src/linux-so2/include
    -I/usr/src/linux-so2/arch/x86/include/uapi
    -I/usr/src/linux-so2/arch/x86/include/generated/uapi
    -I/usr/src/linux-so2/include/uapi
    -I/usr/src/linux-so2/include/generated/uapi
    -include
    /usr/src/linux-so2/include/linux/kconfig.h
    -D__KERNEL__
    -Wall
    -Wundef
    -Wstrict-prototypes
    -Wno-trigraphs
    -fno-strict-aliasing
    -fno-common
    -Werror-implicit-function-declaration
    -Wno-format-security
    -fno-delete-null-pointer-checks
    -O2
    -m32
    -msoft-float
    -mregparm=3
    -freg-struct-return
    -mno-mmx
    -mno-sse
    -fno-pic
    -mpreferred-stack-boundary=2
    -march=i686
    -maccumulate-outgoing-args
    -Wa,-mtune=generic32
    -ffreestanding
    -DCONFIG_AS_CFI=1
    -DCONFIG_AS_CFI_SIGNAL_FRAME=1
    -DCONFIG_AS_CFI_SECTIONS=1
    -DCONFIG_AS_AVX=1
    -DCONFIG_AS_AVX2=1
    -pipe
    -Wno-sign-compare
    -fno-asynchronous-unwind-tables
    -mno-sse
    -mno-mmx
    -mno-sse2
    -mno-3dnow
    -mno-avx
    -Wframe-larger-than=2048
    -fno-stack-protector
    -Wno-unused-but-set-variable
    -fno-omit-frame-pointer
    -fno-optimize-sibling-calls
    -g
    -pg
    -Wdeclaration-after-statement
    -Wno-pointer-sign
    -fno-strict-overflow
    -fconserve-stack
    -Werror=implicit-int
    -Werror=strict-prototypes
    -DCC_HAVE_ASM_GOTO
    -Wall
    -g
    
    -DMODULE
    
    -DKBUILD_STR(s)=#s
    -DKBUILD_BASENAME=my-module
    -DKBUILD_MODNAME=my-module

Testing

We have to save this into a file named .syntastic_c_config and reference it in our .vimrc (or .vimrc.after, in the case of Janus):

let g:syntastic_c_config_file = '.syntastic_c_config'

By default, Syntastic starts looking for a config file in the current folder, then upwards until either it hits the root, or it finds a valid config file. As such, it makes sense to only place the configuration file where it is valid (in our example, /home/aldi/so2/), so it won't interfere with other projects.

Now open the file my-module.c we wrote at the beginning. If you don't have g:syntastic_check_on_open enabled, then either type :w or :SyntasticCheck. By now, Syntastic should have run and found the error (missing semicolon). If it seems like nothing happens, run :Errors in Vim to see the checker's output. You can also set g:syntastic_debug to 1 in your vimrc, and check what parameters got passed from Syntastic to gcc by using the command :messages.

Congratulations, if you have come this far then you now have Syntastic working with the Linux kernel sources!

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