Get_user running at kernel mode returns error

Question

Hi, all,

I have a problem with get_user() macro. What I did is as follows:

I run the following program

int main() 
{ 
    int a = 20; 
    printf("address of a: %p", &a); 
    sleep(200); 
    return 0; 
} 

When the program runs, it outputs the address of a, say, 0xbff91914.

Then I pass this address to a module running in Kernel Mode that retrieves the contents at this address (at the time when I did this, I also made sure the process didn't terminate, because I put it to sleep for 200 seconds... ):

The address is firstly sent as a string, and I cast them into pointer type.

int * ptr = (int*)simple_strtol(buffer, NULL,16); 
printk("address: %p",ptr); // I use this line to make sure the cast is correct. When running, it outputs bff91914, as expected. 
int val = 0; 
int res; 
res= get_user(val, (int*) ptr); 

However, res is always not 0, meaning that get_user returns error. I am wondering what is the problem....

Thank you!!

-- Fangkai

Answer 1

That is probably because you're trying to get value from a different user space. That address you got is from your simple program's address space, while you're probably using another program for passing the value to the module, aren't you?

Answer 2

The call to get_user must be made in the context of the user process.

Since you write "I also made sure the process didn't terminate, because I put it to sleep for 200 seconds..." I have a feeling you are not abiding by that rule. For the call to get_user to be in the context of the user process, you would have had to make a system call from that process and there would not have been a need to sleep the process.

So, you need to have your user process make a system call (an ioctl would be fine) and from that system call make the call to get_user.