Access user address from the kernel

[ClassAct]

Hi,
Can someone kindly let me know if it is possible to access the user process' address space from a kernel thread.
I tried changing the mm pointer from NULL to a user space mm but it OOPSed giving page fault error.(Or is there anything else i need to apart from resetting the mm pointer and active_mm pointer to user thread's mm and active_mm?)

I also tried to tried to reset the cr3 register of the processor executing the kernel thread to the user thread's cr3(My machine has 2 processors) but it didnt help either.
I am stuck with this for a loong time now.Kindly let me know if this is possible.
Thanks a lot,
KA

[stevexyz]

Although I have no idea why you might be trying to do this, is there a very good reason why you can't use one of the established mechanisms for user <-> kernel communication?

[burntfuse]

I'm not a kernel hacker, but I think there's a function called copy_to_user that's for copying from kernelspace memory to userspace memory.

[allenkliu-lx]

Hi,

I used copy_to_user in kernel 2.6.18.1.2798-fc6. I made update to 2.6.20.1 and reboot, my program start to fail on this particular call. Anyone has idea?

Thanks!

[dilbert]

For copy_to_user() I queried always access_ok() before. Not for copy_from_usser. I don't remember anymore why. I only remember that I spent some time with this.
(BTW, the example is from a 2.4.x driver.)


if (copy_from_user (&hxx_afe_if, rq->ifr_data, sizeof(hxx_afe_if)))
{
return -EFAULT;
}

if (0 != access_ok(VERIFY_WRITE, &rq->ifr_data, sizeof(data)))
{
return -EFAULT;
}
else
{
if ((result = copy_to_user (rq->ifr_data, &hxx_afe_if, sizeof(hxx_afe_if))))
{
return -EFAULT;
}
}

[allenkliu-lx]

Thanks Dilbert for your response!

I made mistake. It may not be copy_to_user problem. I think I copied my driver into wrong place (for older version of kernel) that caused the ioctl function not work which used copy_to_user in the ioctl. However, I am having trouble to insmod or modprobe my driver. I got "Invalid module format" after my mobprobe.

Anyone has idea?

Allen