导出内核符号 EXPORT_SYMBOL insmod: error inserting 'mod2.ko': -1 Unknown symbol in module

一般我们编写C程序时，要调用某个文件中的函数，需要在本文件中包含声明有被调用函数的头文件，然后编译连接后，方能找到调用函数。对于模块依赖的情况，不能简单的使用上面的方法，内核提供了一个机制，就是EXPORT_SYMBOL标签内定义的函数或者符号对全部内核代码公开，不用修改内核代码就可以在您的内核模块中直接调用，即使用EXPORT_SYMBOL可以将一个函数以符号的方式导出给其他模块使用。您还可以手工修改内核源代码来导出另外的函数，用于重新编译并加载新内核后的测试。

include/module.h:

struct kernel_symbol
{
unsigned long value;
const char *name;
};
/* For every exported symbol, place a struct in the __ksymtab section */
#define __EXPORT_SYMBOL(sym, sec)               \
__CRC_SYMBOL(sym, sec)                  \
static const char __kstrtab_##sym[]         \
__attribute__((section("__ksymtab_strings")))       \
= MODULE_SYMBOL_PREFIX #sym;                        \
static const struct kernel_symbol __ksymtab_##sym   \
__attribute_used__                  \
__attribute__((section("__ksymtab" sec), unused))   \
= { (unsigned long)&sym, __kstrtab_##sym }

#define EXPORT_SYMBOL(sym)                  \
__EXPORT_SYMBOL(sym, "")

#define EXPORT_SYMBOL_GPL(sym)                  \
__EXPORT_SYMBOL(sym, "_gpl")

#endif

下面是这种方法是演示：






第一个模块文件如下：

[lingyun@localhost export_symbol]$ ls

mod1 mod2

[lingyun@localhost export_symbol]$ cd mod1/

[lingyun@localhost mod1]$ ls

Makefile mod_a.c

[lingyun@localhost mod1]$ vim mod_a.c

mod_a.c

/*********************************************************************************
*      Copyright:  (C) 2013 fulinux<fulinux@sina.com>
*                  All rights reserved.
*
*       Filename:  mod_a.c
*    Description:  This file
*
*        Version:  1.0.0(07/12/2013~)
*         Author:  fulinux <fulinux@sina.com>
*      ChangeLog:  1, Release initial version on "07/12/2013 10:06:50 AM"
*
********************************************************************************/

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

static int func1(void)
{
printk("In Func: %s...\n",__func__);
return 0;
}
EXPORT_SYMBOL(func1);

static int __init hello_init(void)
{
printk("Module 1, Init!\n");
return 0;
}

static void __exit hello_exit(void)
{
printk("Module 1, Exit!\n");
}

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");

其中EXPORT_SYMBOL(func1)导出func1函数符号，保存函数地址和名称.

这个模块的第一个Makefile文件：

[lingyun@localhost mod1]$ ls

Makefile mod_a.c

[lingyun@localhost mod1]$ vim Makefile

obj-m:=mod1.o
mod1-y:=mod_a.o

KERNELDIR := /lib/modules/$(shell uname -r)/build

PWD:=$(shell pwd)

modules:
$(MAKE) -C $(KERNELDIR) M=$(PWD) modules

modules_install:
$(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install
clean:
rm -rf *.o *~core .depend .*.cmd *.ko *.ko.* *.mod.c .tmp_versions *odule* $(TARGET)

其中内嵌对象 - obj-y，可加载模块 - obj-m， KERNELDIR指向指向内核代码目录。

编译编译并加载：

[lingyun@localhost mod1]$ ls

Makefile mod_a.c

[lingyun@localhost mod1]$ make

make -C /lib/modules/2.6.32-279.el6.x86_64/build M=/usr/local/src/lingyun/fulinux/export_symbol/mod1 modules

make[1]: Entering directory `/usr/src/kernels/2.6.32-279.el6.x86_64'

CC [M] /usr/local/src/lingyun/fulinux/export_symbol/mod1/mod_a.o

LD [M] /usr/local/src/lingyun/fulinux/export_symbol/mod1/mod1.o

Building modules, stage 2.

MODPOST 1 modules

CC /usr/local/src/lingyun/fulinux/export_symbol/mod1/mod1.mod.o

LD [M] /usr/local/src/lingyun/fulinux/export_symbol/mod1/mod1.ko.unsigned

NO SIGN [M] /usr/local/src/lingyun/fulinux/export_symbol/mod1/mod1.ko

make[1]: Leaving directory `/usr/src/kernels/2.6.32-279.el6.x86_64'



[lingyun@localhost mod1]$ sudo insmod mod1.ko



[lingyun@localhost mod1]$ cat /proc/kallsyms | grep func1

0000000000000000 r __ksymtab_func1 [mod1]

0000000000000000 r __kstrtab_func1 [mod1]

0000000000000000 r __kcrctab_func1 [mod1]

0000000000000000 T func1 [mod1]

[lingyun@localhost mod1]$



[lingyun@localhost mod1]$ dmesg | grep Module

- User ID: CentOS (Kernel Module GPG key)

Module 1, Init!



第二个模块的文件如下：

[lingyun@localhost mod1]$ cd ../mod2/

[lingyun@localhost mod2]$ vim mod_b.c

/*********************************************************************************
*      Copyright:  (C) 2013 fulinux<fulinux@sina.com>
*                  All rights reserved.
*
*       Filename:  mod_b.c
*    Description:  This file
*
*        Version:  1.0.0(07/12/2013~)
*         Author:  fulinux <fulinux@sina.com>
*      ChangeLog:  1, Release initial version on "07/12/2013 10:29:55 AM"
*
********************************************************************************/

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

static int func2(void)
{
extern int func1(void);
func1();
printk("In Func: %s...\n",__func__);
return 0;
}

static int __init hello_init(void)
{
printk("Module 2, Init!\n");
func2();
return 0;
}

static void __exit hello_exit(void)
{
printk("Module 2, Exit!\n");
}

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");

在这里调用了第一个模块中的func1函数。

对应的Makefile文件：

[lingyun@localhost mod2]$ vim Makefile

obj-m:=mod2.o
mod2-y:=mod_b.o

KERNELDIR := /lib/modules/$(shell uname -r)/build

PWD:=$(shell pwd)

modules:
$(MAKE) -C $(KERNELDIR) M=$(PWD) modules

modules_install:
$(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install
clean:
rm -rf *.o *~core .depend .*.cmd *.ko *.ko.* *.mod.c .tmp_versions *odule* $(TARGET)

[lingyun@localhost mod2]$ make

make -C /lib/modules/2.6.32-279.el6.x86_64/build M=/usr/local/src/lingyun/fulinux/export_symbol/mod2 modules

make[1]: Entering directory `/usr/src/kernels/2.6.32-279.el6.x86_64'

Building modules, stage 2.

MODPOST 1 modules

WARNING: "func1" [/usr/local/src/lingyun/fulinux/export_symbol/mod2/mod2.ko] undefined!

make[1]: Leaving directory `/usr/src/kernels/2.6.32-279.el6.x86_64'

[lingyun@localhost mod2]$

[lingyun@localhost mod2]$ sudo insmod mod2.ko

insmod: error inserting 'mod2.ko': -1 Unknown symbol in module

[lingyun@localhost mod2]$



解决上面的问题如下：

解决办法是把mod_a的Module.symvers放到mod_b的当前路径,从而编译mod_b,符号信息会自动连接进去.

或者在mod_b的makefile中使用KBUILD_EXTRA_SYMBOLS指定mod_a的Module.symvers, 如:

KBUILD_EXTRA_SYMBOLS=/mod_a/Module.symvers

编译mod_b时,搜索Module.symvers的路径是:

1, kernel source path, e.g. /usr/src/kernels/linux-2.6.28.10

2, makefile中M=所指定的路径, 它等效于变量KBUILD_EXTMOD的值

3, 变量KBUILD_EXTRA_SYMBOLS的值

此时Makefile文件如下：

obj-m:=mod2.o
mod2-y:=mod_b.o

<span style="color: rgb(255, 0, 0);">KBUILD_EXTRA_SYMBOLS=~/fulinux/export_symbol/mod1/Module.symvers</span>
KERNELDIR := /lib/modules/$(shell uname -r)/build

PWD:=$(shell pwd)

modules:
$(MAKE) -C $(KERNELDIR) M=$(PWD) modules

modules_install:
$(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install
clean:
rm -rf *.o *~core .depend .*.cmd *.ko *.ko.* *.mod.c .tmp_versions *odule* $(TARGET)

在编译加载如下：

[lingyun@localhost mod2]$ make

make -C /lib/modules/2.6.32-279.el6.x86_64/build M=/usr/local/src/lingyun/fulinux/export_symbol/mod2 modules

make[1]: Entering directory `/usr/src/kernels/2.6.32-279.el6.x86_64'

CC [M] /usr/local/src/lingyun/fulinux/export_symbol/mod2/mod_b.o

LD [M] /usr/local/src/lingyun/fulinux/export_symbol/mod2/mod2.o

Building modules, stage 2.

MODPOST 1 modules

CC /usr/local/src/lingyun/fulinux/export_symbol/mod2/mod2.mod.o

LD [M] /usr/local/src/lingyun/fulinux/export_symbol/mod2/mod2.ko.unsigned

NO SIGN [M] /usr/local/src/lingyun/fulinux/export_symbol/mod2/mod2.ko

make[1]: Leaving directory `/usr/src/kernels/2.6.32-279.el6.x86_64'

[lingyun@localhost mod2]$ sudo insmod mod2.ko

[lingyun@localhost mod2]$



[lingyun@localhost mod2]$ dmesg | grep "In Func:"

In Func: func1...

In Func: func2...



可见模块二调用模块一的func1成功！！！