Re: [Xen-devel] how to deal with copy_to_user returning non zero

[And Dinh <ug93tad@xxxxxxxxx>]

how does it get page fault? I made sure the output buffer at the user space is properly allocated with the correct  size.

When page fault, do I have no choice but abort? It seems calling the hypercall again does not solve it.

On 7 Nov 2014, at 6:44 pm, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:

On 07/11/14 10:38, Anh Dinh wrote:

I'm testing out a hypercall that takes in large data (many MB) from the user space and simply copy the data back. 

For copying in, I call xmalloc_array() for about 4MB at a time and call copy_from_user() successfully for the entire input. 

The problem is with copy_to_user() which returns non-zero values. I tried to vary the size of the data being copy, but still unable to copy the whole data back. 

What are the cause of copy_to_user() failure? I checked the source I got quite lost at the code. 

How could I make sure all data is copy back? Surely it is not a memory limitation problem, because I copy_from_user() succeeds for over 100MB. 

copy_to_user() returns non-zero if it encounters a fault while moving data, generally a pagefault.

******

long do_test_copy(long n, unsigned char *in, unsigned char *out){

This looks quite bogus.  You should have XEN_GUEST_HANDLE() for a pointer into guest memory.  A bare pointer like *in is completely wrong.

~Andrew

        unsigned char **tmp; 

        long ret;

        int m = n/(MAX_ALLOC); 

        tmp = xmalloc_array(unsigned char*,m); 

        for (int i=0; i<m; i++){

                tmp[i] = xmalloc_array(unsigned char, MAX_ALLOC); 

                copy_from_user(tmp[i],in+i*MAX_ALLOC,MAX_ALLOC); 

        }

        

        for (int i=0; i<m; i++){

                if (tmp[i]){

                        ret = copy_to_user(out+i*MAX_ALLOC,tmp[i],MAX_ALLOC);                 

                        xfree(tmp[i]); 

                }

        }

        xfree(tmp);

        return 0;       

}

****

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