我在 Ubuntu 中工作。我正在尝试制作两个相互使用功能的内核模块。我的问题是我得到了正确编译的模块，但其中一个的符号没有解析。

为简单起见，我们将这些模块称为m1 和m2。

m2 正在导出函数 void func_m2(void)。 m1 正在调用此函数。两个模块都可以正确编译。

全部编译完成后，我需要先加载 m2 模块(因为它导出了 func_m2 函数)，然后再加载 m1 模块。那么，让我们来吧:

volodymyr@sv1:~/development/kmodules/m2$ sudo insmod ./m2.ko

现在，让我们加载试图使用 func_m2 的 m1 模块:

volodymyr@sv1:~/development/kmodules/m1$ sudo insmod ./m1.ko
insmod: error inserting './m1.ko': -1 Unknown symbol in module

以下是我在日志中看到的内容:

volodymyr@sv1:~/development/kmodules/m1$ dmesg | tail
[ 3938.166616] Loading m2 module ...
[ 3963.078055] m1: no symbol version for func_m2
[ 3963.078059] m1: Unknown symbol func_m2

因此，似乎对符号 func_m2 的引用未解析。有趣的。让我们检查它是否存在于符号表中:

volodymyr@sv1:~/development/kmodules$ cat /proc/kallsyms | grep 'func_m2'
ffffffffa00530d0 r __ksymtab_func_m2    [m2]
ffffffffa00530e8 r __kstrtab_func_m2    [m2]
ffffffffa00530e0 r __kcrctab_func_m2    [m2]
ffffffffa0053000 T func_m2      [m2]
000000004edd543f a __crc_func_m2        [m2]

如您所见，func_m2 实际上存在于符号表中。那为什么m1加载不出来呢？

我已经为我的内核和 Linux 源安装了正确的 Linux 头文件。我没有在内核中做任何修改，它是原封不动的，它的版本是:2.6.31-16-generic(我运行 x64)

现在，为了向您展示全貌，我将用于此测试的 m1 和 m2 模块的源代码和 Makefile 放在这里。

m1模块:

m1.c:

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

extern void func_m2(void);

int hello_start(void)
{
    printk(KERN_INFO "Loading m1 module ...\n");

    func_m2();

    return 0;
 }

 void hello_end(void)
 {
    printk(KERN_INFO "Unloading m1 ...\n");
 }

module_init(hello_start);
module_exit(hello_end);

MODULE_LICENSE("GPL");

生成文件:

obj-m := m1.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

m2模块:

m2.c:

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

int hello_start(void)
{
    printk(KERN_INFO "Loading m2 module ...\n");

    return 0;
}

void hello_end(void)
{
    printk(KERN_INFO "Unloading m2 ...\n");
}

void func_m2(void)
{
    printk(KERN_INFO "This a function in m2\n");
}

module_init(hello_start);
module_exit(hello_end);

MODULE_LICENSE("GPL");
EXPORT_SYMBOL(func_m2);

生成文件:

obj-m := m2.o
export-objs := m2.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

基本上我的问题是:为什么不能加载m1？

如果有人能回答会很有帮助。

最佳答案

当您构建 m2 时，它会创建一个 Module.symvers 文件。

将此文件复制到您正在构建 m1 的位置。然后制作m1，并insmod它。

您之前构建 m1 时可能会收到警告，例如:

WARNING: "func_m2" [/tmp/m1/m1.ko] undefined!

一旦您使用 m2 模块中的 Module.symvers，它就会消失。

来自 http://www.kernel.org/doc/Documentation/kbuild/modules.txt :

--- 6.2 Symbols and External Modules

When building an external module, the build system needs access to the symbols from the kernel to check if all external symbols are defined. This is done in the MODPOST step. modpost obtains the symbols by reading Module.symvers from the kernel source tree. If a Module.symvers file is present in the directory where the external module is being built, this file will be read too. During the MODPOST step, a new Module.symvers file will be written containing all exported symbols that were not defined in the kernel.

这也值得一读，来自同一个文件:

--- 6.3 Symbols From Another External Module

Sometimes, an external module uses exported symbols from another external module. kbuild needs to have full knowledge of all symbols to avoid spitting out warnings about undefined symbols. Three solutions exist for this situation.

NOTE: The method with a top-level kbuild file is recommended but may be impractical in certain situations.

Use a top-level kbuild file If you have two modules, foo.ko and bar.ko, where foo.ko needs symbols from bar.ko, you can use a common top-level kbuild file so both modules are compiled in the same build. Consider the following directory layout:

./foo/ <= contains foo.ko ./bar/ <= contains bar.ko

The top-level kbuild file would then look like:

$ ./Kbuild (or ./Makefile): obj-y := foo/ bar/

And executing

$ make -C $KDIR M=$PWD

will then do the expected and compile both modules with full knowledge of symbols from either module.

Use an extra Module.symvers file When an external module is built, a Module.symvers file is generated containing all exported symbols which are not defined in the kernel. To get access to symbols from bar.ko, copy the Module.symvers file from the compilation of bar.ko to the directory where foo.ko is built. During the module build, kbuild will read the Module.symvers file in the directory of the external module, and when the build is finished, a new Module.symvers file is created containing the sum of all symbols defined and not part of the kernel.

Use "make" variable KBUILD_EXTRA_SYMBOLS If it is impractical to copy Module.symvers from another module, you can assign a space separated list of files to KBUILD_EXTRA_SYMBOLS in your build file. These files will be loaded by modpost during the initialization of its symbol tables.

关于c - 对于另一个模块中定义的符号，insmod 失败并显示 "Unknown symbol in module"，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/33718358/