Regarding - unresolved symbol

Regarding - unresolved symbol

[vinay hegde]

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

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

[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!]

Thank you,
Vinay Hegde


		
__________________________________________________________ 
Yahoo! India Matrimony: Find your partner now. Go to http://yahoo.shaadi.com

Re: Regarding - unresolved symbol

[linux-os (Dick Johnson)]

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 System.map. 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
>
>

Cheers,
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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Thank you.

Re: Regarding - unresolved symbol

[Jon Masters]

On 10/11/05, linux-os (Dick Johnson) <linux-os@analogic.com> wrote:

> On Tue, 11 Oct 2005, vinay hegde wrote:

> > [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!!!

Yummy.

> 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.

The fix is to call out to userland to get the device node cerated for
you - either allow udev/devfs to do this for you, use
call_usermode_helper to do it, send a message up to userland via
NETLINK or whatever. There's more than one way to do it.

Jon.

Re: Regarding - unresolved symbol

[Jon Masters]

On 10/11/05, vinay hegde <thisismevinay@yahoo.co.in> wrote:

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

It's a *very bad* idea to start trying to jump around in the
sys_call_table from a module. In fact, the sys_call_table is not being
exported any more for that reason.

Jon.