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

[Anh Dinh <ug93tad@xxxxxxxxx>]

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

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

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.Â

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

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.Â

******

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

    unsigned char **tmp;Â

    long ret;

    int m = n/(MAX_ALLOC);Â

    tmp = xmalloc_array(unsigned char*,m);Â

    for (int i=0; i<m; i++){

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

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

    }

   ÂÂ

    for (int i=0; i<m; i++){

        if (tmp[i]){

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

            xfree(tmp[i]);Â

        }

    }

    xfree(tmp);

    return 0;   ÂÂ

}

****

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