You are getting a compile error because the symbol you are trying to export is not complete. Step back and think about what you are trying to do. You were initially trying to export (with EXPORT_SYMBOL) a function that is supposed to be implemented by your module, but you state that its implementation in another object file that will be linked later (with the keyword "extern"). You don't provide that implementation, and that is the reason why your compilation fails. With a function pointer, however, you are exporting a complete object, that is a pointer to a function with a certain signature. This might be confusing at first, specially if you don't know how compilers work, but I hope this helps. -- Henrique Rodrigues http://www.dcc.ufmg.br/~hsr -- Henrique Rodrigues http://www.dcc.ufmg.br/~hsr On Mon, Jan 13, 2014 at 11:08 PM, Le Tan <tamlokveer@gmail.com> wrote:
Thanks very much! I will try that later. What you mentioned is another amazing sight. But I am confused that why it can't work if I export a function from my module and call it in the kvm? Do I have to modify the makefile of kvm? How? I have referenced to the doc of KBuild. It said that : Sometimes, an external module uses exported symbols from another external module. kbuild needs to have full knowledge of all symbols to avoid spliitting 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.
But I have tried those methods. They don't work. Why? Thanks for your help very much!
2014/1/14 Henrique Rodrigues <henriquesilvar@gmail.com>
Hi Le,
You can do that by exporting a function pointer from the kvm code that is only called if it is not null. Then, when you load your module, you set that exported function pointer (I'm assuming that you want to call your module's function from the kvm code... ). Here is a post on how to do that:
http://stackoverflow.com/questions/11463184/how-to-use-exported-symbols-in-t...
http://stackoverflow.com/questions/1196944/can-i-replace-a-linux-kernel-func...
Best, -- Henrique Rodrigues http://www.dcc.ufmg.br/~hsr
On Mon, Jan 13, 2014 at 2:47 AM, Le Tan <tamlokveer@gmail.com> wrote:
Hello! I am writing a device driver module. I define some functions in the module, for example print_record(). I am doing something in the kvm, so I want to call print_record() in the file of kvm module, for example, I may call print_record() in file /arch/x86/kvm/x86.c to put something into my device driver module. Now comes the question. 1. Where should I put the codes of my device driver module? It's the first time I write the device driver. 2. After compiling my module, I encounter an error when I compile the kvm module. ERROR: "print_record" [arch/x86/kvm/kvm.ko] undefined! I use EXPORT_SYMBOL(print_record) in my module file. I use "extern" to declare print_record() and then call print_record() in file x86.c. To solve this problem, I have tried to copy the Module.symvers from my module folder to /arch/x86/kvm/. But it doesn't work. I have also tried to add *KBUILD_EXTRA_SYMBOLS *to the Makefile of kvm. It doesn't work either. I cat /proc/kallsyms and find that the type of symbol "print_record" is "t" ( local text). What should I do? How to call functions defined in my own module from kvm? Maybe there is something wrong in Makefiles?
Any suggestion is appreciated! Thanks!
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