copy_from_user - difficulty in copying double pointer from user space

Question

Structures involved are like the following. struct node { int a; int b; }

struct ioctl_node {
    struct node **ptr;
    int         count; //Stores the no of pointers this ptr is pointing to.
};

In user space, I have populated a structure of ioctl_node with count = 3 . And ptr[0] and ptr[2] is pointing to two structs of type node, whereas ptr[1] is NULL . I want this to be passed to kernel. I have issued ioctl call to pass this info from user space to kernel space.

I did the following in kernel space.

struct ioctl_node k_node;

copy_from_user(&k_node, arg, sizeof(struct ioctl_node)) //arg is the source

//k_node.count is showing the right value what I have set in the user space.

struct node *tmp = NULL;

I then did, copy_from_user(&tmp, k_node.ptr, sizeof(struct node *) and this call is also returning success. But, I am having difficulty, in copying the full contents of **ptr properly in kernel space. Can anybody plese help, how can I be able to do that. How should I do the next copy_from_user to copy all the contents. I tried, but its giving copy error.

Answer 1

copy_from_user copies one block of consecutive bytes from user space to kernel space. You'll need to first copy the struct ioctl_node , then copy the pointers to individual nodes, then copy the struct node 's in a loop. Be sure to allocate the right amount of memory. Since you know the number of nodes in advance, you can allocate an array of them.

struct node_data {
    size_t node_count;
    struct nodes *nodes;
};
struct node_data *read_nodes(struct ioctl_node_user_pointer *arg)
{
    struct node_data *nd = NULL;
    struct ioctl_node kin = {0};
    struct node *node_user_pointers = NULL;
    int i;
    int err;
    /* Step 1: retrieve the root node */
    nd = kmalloc(sizeof(*nd), GFP_KERNEL);
    if (!nd) {
        err = -ENOMEM;
        goto error;
    }
    if (copy_from_user(ioctl_node_user_pointer, &kin, sizeof(kin))) {
        err = -EFAULT;
        goto error;
    }
    if (kin->count < 0 || kin->count > ((size_t)-1)/sizeof(*nodes)) {
        err = -EINVAL;
        goto error;
    }
    nd->node_count = kin.count;
    /* Step 2: retrieve the pointers to individual nodes */
    node_user_pointers = kmalloc(sizeof(*node_user_pointers) * nd->node_count, GFP_KERNEL);
    if (node_user_pointers) {
        err = -ENOMEM;
        goto error;
    }
    if (copy_from_user(kin->nodes, node_user_pointers, sizeof(*node_user_pointers) * nd->node_count)) {
        err = -EFAULT;
        goto error;
    }
    /* Step 3: retrieve the nodes themselves */
    nd->nodes = kmalloc(sizeof(*nodes) * nd->node_count, GFP_KERNEL);
    if (!nd->nodes) {
        err = -ENOMEM;
        goto error;
    }
    for (i = 0; i < head.count; i++) {
        if (copy_from_user(node_user_pointers[i], nd->nodes + i, sizeof(nd->nodes[0]))) {
            err = -EFAULT;
            goto error;
        }
    }
    kfree(node_user_pointers);
    return nd;
error:
    if (nd) {
        kfree(nd->nodes);
        kfree(nd);
    }
    kfree(node_user_pointers);
    return ERR_PTR(err);
}