Re: Regarding - unresolved symbol

From: linux-os (Dick Johnson)
Date: Tue Oct 11 2005 - 10:16:53 EST

On Tue, 11 Oct 2005, vinay hegde wrote:

> Hi,
> I am developing a device driver module related to the
> real time clock on Linux 2.4 kernel. [This is done
> with regard to changing one of the existing timer chip
> on the system.] For some reason, I am getting into an
> unresolved symbol.
> I am following sequence of code in the module (not
> necessarily in the sequential manner though).
> ----------
> extern void *sys_call_table[];
> static int (*timer_mknod)(const char *, ... );
> timer_mknod = sys_call_table[__NR_mknod];
> ----------
> Whenever I try to insert the module into the kernel, I
> get the following error:
> timer.o: unresolved symbol sys_call_table

sys_call_table isn't an exported symbol.

> All the necessary headers are present and I am able to
> see the symbol 'sys_call_table' in I do
> not see any error in this regard. Can somebody help me
> in pointing out the flaw?

The flaw is in attempting to modify the sys_call_table
with a module.

> [I am able to fix this problem by simply using the
> 'sys_mknod()' call in my module, but I really would
> like to know why the above piece of code can not
> work!]

Even sys_mknod() should not work from a module; Who's
context did you steal to create that device-file? What
happens next to that poor sucker? Who's data-space
did you corrupt for the file-name string? I note that,
from the names, it's likely that you intend to do this
dastardly deed from a timer-queue!!!

File operations require a process context. The kernel doesn't
have one. All file operations should (read must) be done
from user space. The kernel is designed to do things
on behalf of user-space callers. Executing sys_* from
a module will result in somebody saying; "See, you are
wrong... It works fine!". Later on mysterious errors
occur which are un-trackable.

> Thank you,
> Vinay Hegde

Dick Johnson
Penguin : Linux version 2.6.13 on an i686 machine (5589.54 BogoMips).
Warning : 98.36% of all statistics are fiction.

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