Compile Kernel Modules with inline assembly in Intel Syntax

Question

I wan't to write a kernel module which uses quite a lot of inline assembly. Since I am used to Intel Syntax I would like to avoid AT&T Syntax completely. One way of doing this is shown in the following minimal example:

samplemodule.c

#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>

MODULE_LICENSE("GPL");

unsigned long foo(void) {
    unsigned long ret = 0;

    asm (
        ".intel_syntax noprefix\n"
        "mov rbx, 1337\n"
        "mov %0, rbx\n"
        ".att_syntax noprefix\n"
        :"=r"(ret)
        :
        :"rbx"
    );

    return ret;
}

static int init_routine(void) {
    printk(KERN_INFO "Sample Module init\n");
    printk(KERN_INFO "Test: %lu\n", foo());

    return 0;
}

static void exit_routine(void) {
    printk(KERN_INFO "Sample Module exit\n");
}

module_init(init_routine);
module_exit(exit_routine);

Makefile

obj-m += samplemodule.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

So, whenever I want to inline assembly I have two write .intel_syntax noprefix\n...\n.att_syntax noprefix\n. Are there other ways of accomplishing this? When compiling with gcc I used to simply pass the -masm=intel argument to gcc which allowed me to freely use Intel Syntax. Is something similar possible in this case?

Or you could, you know, just learn the AT&T syntax... You'd probably be better served by doing that in the long run... — twalberg, Jan 09 '15 at 20:07

user1466329 · Accepted Answer · 2015-01-14T11:45:13.713

According to https://www.kernel.org/doc/Documentation/kbuild/makefiles.txt, there are several variables to pass specific options to the gcc toolchain: the one you should try is KBUILD_CFLAGS_MODULE. In this way, you can still use the option -masm=intel. Your all target should be like

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) KBUILD_CFLAGS_MODULE='-masm=intel' modules

Problems arise if you include assembler code from kernel headers (which, I think, is not infrequent), which use AT&T syntax: in that case, the final code will mix both syntaxes, and the compilation will fail. In this last case, I think the only way is to manually specify the Intel syntax in each asm directive, as you did so far.

Workaround

Since changing compilation parameters or the scripts is not a clean job and is prone to errors, I would suggest a simple workaround: your main issue is avoiding to type ".intel_syntax noprefix\n" and ".att_syntax noprefix\n" every time, right? You can place these directives inside a macro, maybe place the macro in a header and include that header everywhere you need. For example, you can define a macro like

#define INTEL_ASM(a,b,c,d)          \
    asm (                           \
    ".intel_syntax noprefix\n"  \
    a                           \
    ".att_syntax noprefix\n"    \
    :b                          \
    :c                          \
    :d                          \
)

so that your code becomes

unsigned long foo(void) {
    unsigned long ret = 0;

    INTEL_ASM (
        "mov rbx, 1337\n"
        "mov %0, rbx\n"
        ,"=r"(ret)
        ,
        ,"rbx"
    );

    return ret;
}

Thx, this almost works. When using it exactly like you suggested the module cannot be loaded anymore (`insmod` says `Invalid module format` most likely because `-DMODULE` is overwritten). I fixed it by adding `KBUILD_CFLAGS_MODULE += '-masm=intel'` at the beginning of the Makefile. — user1658887, Jan 09 '15 at 23:45

Compile Kernel Modules with inline assembly in Intel Syntax

1 Answers1

Workaround