Tuesday, October 25, 2022

[SOLVED] error: attributes should be specified before the declarator in a function definition, while building kernel LSM module

Issue

I tried to write my own Linux Security Module (LSM). I took help from this youtube video here (ignore the DEFINE_LSM(yama), it was a typo).

My code is as simple as this:

#include <linux/lsm_hooks.h>
#include <linux/kern_levels.h>
#include <linux/binfmts.h>

static int my_test_bprm_check_security(struct linux_binprm *bprm){
    printk(KERN_ERR "Hello my_test_bprm_check_security: %s\n", bprm->interp);
    return 0;
}

static struct security_hook_list my_test_hooks[] __lsm_ro_after_init = {
    LSM_HOOK_INIT(bprm_check_security, my_test_bprm_check_security),
};

static int __init my_test_init(void){
    printk(KERN_ERR "mytest: We are going to do things\n");
    security_add_hooks(my_test_hooks, ARRAY_SIZE(my_test_hooks),"my_test");
    return 0;
}

DEFINE_LSM(my_test){
    .name = "my_test",
    .init = my_test_init,
};

When I tried to compile it (as a kernel module so that I can insmod it without the need of integrating it into the kernel itself) with the following Makefile

obj-m += my_test.o

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

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

It throws the following error:

In file included from security/my_test.c:1:
./include/linux/lsm_hooks.h:1667:9: error: attributes should be specified before the declarator in a function definition
 1667 |         static struct lsm_info __lsm_##lsm                              \
      |         ^~~~~~
security/my_test.c:20:1: note: in expansion of macro ‘DEFINE_LSM’
   20 | DEFINE_LSM(my_test){
      | ^~~~~~~~~~
security/my_test.c:14:19: error: ‘my_test_init’ defined but not used [-Werror=unused-function]
   14 | static int __init my_test_init(void){
      |                   ^~~~~~~~~~~~

Looking into the lsm_hooks.h at the specified line (1667) I found the following macro defining a struct:

#define DEFINE_LSM(lsm)                         \
    static struct lsm_info __lsm_##lsm              \
        __used __section(".lsm_info.init")          \
        __aligned(sizeof(unsigned long))

I went through this post. Could anyone tell me what is the problem and how to deal with this? I am a beginner in kernel programming.

I am using the following version of GCC:

gcc (Ubuntu 11.2.0-19ubuntu1) 11.2.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Solution

You forgot the equal sign between invocation of DEFINE_LSM macro, which defines a variable, and value of that variable. Correct:

DEFINE_LSM(my_test) = {
    .name = "my_test",
    .init = my_test_init,
};


Answered By - Tsyvarev
Answer Checked By - Marilyn (WPSolving Volunteer)
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