Here are five possible code completions for the given function, addressing the CWE-787 (Out-of-bounds Write) vulnerability:

1. Using memcpy with size check:
```c
if (copy_from_user(range, (void __user *)arg, sizeof(range)))
    return -EFAULT;

if (range[1] > bdev->bd_inode->i_size)
    return -EINVAL;

return blkdev_issue_discard(bdev, range[0], range[1], GFP_KERNEL, mode);
```

2. Using get_user for each element:
```c
if (get_user(range[0], (uint64_t __user *)arg) ||
    get_user(range[1], ((uint64_t __user *)arg) + 1))
    return -EFAULT;

if (range[1] > bdev->bd_inode->i_size)
    return -EINVAL;

return blkdev_issue_discard(bdev, range[0], range[1], GFP_KERNEL, mode);
```

3. Using a temporary struct:
```c
struct discard_range {
    uint64_t start;
    uint64_t len;
};
struct discard_range r;

if (copy_from_user(&r, (void __user *)arg, sizeof(r)))
    return -EFAULT;

if (r.len > bdev->bd_inode->i_size)
    return -EINVAL;

return blkdev_issue_discard(bdev, r.start, r.len, GFP_KERNEL, mode);
```

4. Using __get_user macro:
```c
uint64_t start, len;

if (__get_user(start, (uint64_t __user *)arg) ||
    __get_user(len, ((uint64_t __user *)arg) + 1))
    return -EFAULT;

if (len > bdev->bd_inode->i_size)
    return -EINVAL;

return blkdev_issue_discard(bdev, start, len, GFP_KERNEL, mode);
```

5. Using access_ok and __copy_from_user:
```c
if (!access_ok(VERIFY_READ, (void __user *)arg, sizeof(range)))
    return -EFAULT;

if (__copy_from_user(range, (void __user *)arg, sizeof(range)))
    return -EFAULT;

if (range[1] > bdev->bd_inode->i_size)
    return -EINVAL;

return blkdev_issue_discard(bdev, range[0], range[1], GFP_KERNEL, mode);
```

Each of these completions includes checks to prevent out-of-bounds writes and ensures proper handling of user-space data, addressing the CWE-787 vulnerability.