EXPORT_SYMBOL解析

来源：互联网发布：如何成为程序员编辑：程序博客网时间：2024/06/03 22:56

http://www.cnblogs.com/dyllove98/p/3186967.html

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.omod1-y:=mod_a.oKERNELDIR := /lib/modules/$(shell uname -r)/buildPWD:=$(shell pwd)modules:    $(MAKE) -C $(KERNELDIR) M=$(PWD) modulesmodules_install:    $(MAKE) -C $(KERNELDIR) M=$(PWD) modules_installclean:    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.omod2-y:=mod_b.oKERNELDIR := /lib/modules/$(shell uname -r)/buildPWD:=$(shell pwd)modules:    $(MAKE) -C $(KERNELDIR) M=$(PWD) modulesmodules_install:    $(MAKE) -C $(KERNELDIR) M=$(PWD) modules_installclean:    rm -rf *.o *~core .depend .*.cmd *.ko *.ko.* *.mod.c .tmp_versions *odule* $(TARGET)

[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.omod2-y:=mod_b.o<span style="margin: 0px; padding: 0px; color: rgb(255, 0, 0);">KBUILD_EXTRA_SYMBOLS=~/fulinux/export_symbol/mod1/Module.symvers</span>KERNELDIR := /lib/modules/$(shell uname -r)/buildPWD:=$(shell pwd)modules:    $(MAKE) -C $(KERNELDIR) M=$(PWD) modulesmodules_install:    $(MAKE) -C $(KERNELDIR) M=$(PWD) modules_installclean:    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！！！

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