EXPORT_SYMBOL解析
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
* All rights reserved.
*
* Filename: mod_a.c
* Description: This file
*
* Version: 1.0.0(07/12/2013~)
* Author: fulinux
* ChangeLog: 1, Release initial version on "07/12/2013 10:06:50 AM"
*
********************************************************************************/
#include
#include
#include
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
* All rights reserved.
*
* Filename: mod_b.c
* Description: This file
*
* Version: 1.0.0(07/12/2013~)
* Author: fulinux
* ChangeLog: 1, Release initial version on "07/12/2013 10:29:55 AM"
*
********************************************************************************/
#include
#include
#include
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
KBUILD_EXTRA_SYMBOLS=~/fulinux/export_symbol/mod1/Module.symvers
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！！！