在Linux2.6.26下用EXPORT_SYMBOL时出现Unknown symbol?

http://bugzilla.kernel.org/show_bug.cgi?id=12446

 

在Linux2.6.26下用EXPORT_SYMBOL时出现Unknown symbol?

如何解决!!

这是linux kernel 2.6.26 之后版本的bug (详细描述, 请看http://bugzilla.kernel.org/show_bug.cgi?id=12446)
并且这个bug不会被fix

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

 

 

Bug 12446 - Unable to insmod module. Unknwon symbol found

Summary:	Unable to insmod module. Unknwon symbol found

Status: REJECTED WILL_NOT_FIX   Product: Drivers Component: Other Version: 2.5 Platform: All Linux   Importance: P1 low Assigned To: [email protected]   URL:     Depends on:   Blocks:     Show dependency tree / graph		Reported: 2009-01-14 05:53 by amit jain Modified: 2009-06-04 06:54 (History)   Kernel Version: 2.6.28 Tree: Mainline Regression: No

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Description From amit jain 2009-01-14 05:53:57 Latest working kernel version:2.6.28 Earliest failing kernel version:2.6.26 Distribution: Hardware Environment: Software Environment: Problem Description: When you use driver dependent on other driver. It should load cleanely, if other is loaded. But in kernel 2.6.26 onwards its give error, unknown symbol found. Actually find_symbol able to find symbol but check_version unable to match crc. Because crc is not know to other driver. Steps to reproduce: 1) create two device drivers in two separate directories. let say hello and bye 2) Export Symbol from hello module and try to use that in bye module. 3) Compile both the driver separately. 4) insmod hello.ko 3) While insmod(ing) bye.ko, gives error "Unknown Symbol found" ------- Comment #1 From Roland Kletzing 2009-01-17 02:44:19 ------- what about using modprobe instead ? (as this looks in modules.dep to find the dependend modules and load them before) ------- Comment #2 From sucheta 2009-01-18 21:40:43 ------- Hi Roland, using modprobe doesn't work. The entries in modules.dep are added. Still it ends up showing the same prints - "no symbol version for " and "Unknown symbol ". And modprobe fails. ------- Comment #3 From amit jain 2009-01-19 00:20:32 ------- Hi Roland, Thanks a lot for replying. Below I have written details of experiments we did and our understanding. Problem: insmod failure for externally compiled module :- Experiments: (1) Compiling 2 modules a.ko and b.ko ( dependent on a.ko ) together :- Works (2) Copying Module.symvers from module "a" dir to the module "b" dir, before compiling b.ko :- Works. (3) Modprobe after appending following lines in /lib/modules/modules.dep /lib/modules/2.6.27.7-smp/kernel/drivers/net/a.ko /lib/modules/2.6.27.7-smp/kernel/drivers/net/b.ko: /lib/modules/2.6.27.7-smp/kernel/drivers/net/a.ko :- Fails (4) After compiling b.ko, just modifying b.mod.c file to include the undefined symbol in its version table doesn't work (didn't expect to work ). (5) export_objs (doesn't work): In Makefile of a.ko: export_objs := a.ko / export-objs := a.ko / exportobjs := a.ko. (6) Adding "#define EXPORT_SYMTAB" in a.c file (doesn't work). From above experiments, we found that In .mod.c file it maintains __versions array, which contains export symbol name and its crc. We see symbols of modules which are compiled with kernel. No symbols of externally compiled modules. The call trace is load_module -> simplify_symbols -> resolve_symbol -> find_symbol and check_version ( if find_symbol succeeds ). check_version behavior comparison in 2.6.26 and earlier version kernels :- In earlier versions of kernel also, symbol couldn't be found in its version table. Still, check_version used to return 1 (success) and the dependent module could be insmod(ed) successfully. However, in kernel 2.6.26 onwards, behavior has changed. check_version on not finding the reqd. symbol in its version table returns 0 (fail) and the dependent external module can't be inserted anymore. Waiting eagerly for your reply. Thanks in advance. ------- Comment #4 From amit jain 2009-01-28 21:04:34 ------- Any updates ? ------- Comment #5 From Alan 2009-03-19 10:19:16 ------- No but this is not a support facility, just a bug tracker and as a problem only you've reported and nobody else has duplicated its a very very low priority, especially as its only out of tree code seeing it ------- Comment #6 From amit jain 2009-03-19 21:04:12 ------- It should not be very very low priotity, Its easy to reproduce. Its should be blocker bug, because anybody how will try to compile module dependent on other module, will fail. Strange, Why didn't nobody reply on it. I am really stuck, because of this problem. ------- Comment #7 From Tejun Heo 2009-03-19 21:48:40 ------- cc'ing Rusty Russell. ------- Comment #8 From Roland Kletzing 2009-03-20 12:11:23 ------- hello amit, i think alan is right. this seems a very specific, personal problem. anyway, if you think it`s a general kernel problem, please post an as simple as possible repro-case (i.e. some sourcecode) for your problem, so chances will raise that someone will look at your problem. i`d recommend bringing this up on kernel related mailing lists ------- Comment #9 From Rusty Russell 2009-03-22 23:00:51 ------- This is true; a5dd69707424a35d2d2cc094e870f595ad61e916 changed this. The argument is that modversions is supposed to version symbols used in a module, and this doesn't really work for out-of-tree modules (unless you copy Module.symvers across, as done above). Otherwise module B doesn't know the version of symbol A; we changed such a missing version to fail; the user *did* want us to check versions. Currently you have to modprobe --force-modversion to load such a module. I think the new behavior is correct. ------- Comment #10 From Qihua Dai 2009-06-04 05:28:02 ------- I also met the same problem on kernel 2.6.28.10. "modprobe --force-modversion" does work for me, it will report "invalid module format" Besides the solution of copy Module.symvers across, below solution can workaround it: In the makefile of "b", define KBUILD_EXTRA_SYMBOLS which points to Module.symvers in "a" Dir so that "b" will find Module.symvers of "a" e.g. KBUILD_EXTRA_SYMBOLS=/mod_a/Module.symvers During building "b", the search path of Module.symvers will be. Please correct me if my understanding is incorrect. 1, kernel source path, e.g. /usr/src/kernels/linux-2.6.28.10 2, the value of $(M) defined in makefile, which is the same as the value of KBUILD_EXTMOD 3, the value of KBUILD_EXTRA_SYMBOLS I think it's a generic kernel problem. It's better to fix it. Below attached a simple case which is from below link: http://topic.csdn.net/u/20081009/11/6fb7295c-0d30-4d4f-b390-af4413aa7f7e.html?seed=1582387931 =============C/C++ code // Module A (mod_a.c) #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 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); =============C/C++ code // Module B (mod_b.c) #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); =============Makefile for Module A BatchFile code obj-m += mod1.o mod1-y := mod_a.o KVERSION = $(shell uname -r) all: make -C /lib/modules/$(KVERSION)/build M=$(PWD) modules clean: make -C /lib/modules/$(KVERSION)/build M=$(PWD) clean rm -f *.o *.ko *.cmd =============Makefile for Module B BatchFile code obj-m += mod2.o mod2-y := mod_b.o KVERSION = $(shell uname -r) all: make -C /lib/modules/$(KVERSION)/build M=$(PWD) modules clean: make -C /lib/modules/$(KVERSION)/build M=$(PWD) clean rm -f *.o *.ko *.cmd ------- Comment #11 From Qihua Dai 2009-06-04 05:31:40 ------- A typo, it should be ""modprobe --force-modversion" does NOT work for me, it will report "invalid module format"" ------- Comment #12 From Rusty Russell 2009-06-04 06:54:15 ------- You can't modprobe --force if CONFIG_MODULE_FORCE_LOAD isn't set. That was added in 2.6.26, and is off by default. Hope that clarifies, Rusty. Note You need to log in before you can comment on or make changes to this bug.