Here are five possible code completions for the given function, addressing the CWE-787 vulnerability:

1. Buffer allocation with size check:
```c
buf = kmalloc(bufsize, GFP_KERNEL);
if (!buf)
    return -ENOMEM;

while (len >= size) {
    for (i = 0; i < size; i++)
        buf[i] = *pg++;
    buf[size] = '\0';
    ret = fun(buf, priv, size);
    if (ret)
        break;
    len -= size;
}

kfree(buf);
return ret;
```

2. Using strlcpy for safe string copying:
```c
buf = kmalloc(bufsize, GFP_KERNEL);
if (!buf)
    return -ENOMEM;

while (len >= size) {
    strlcpy(buf, pg, size + 1);
    ret = fun(buf, priv, size);
    if (ret)
        break;
    pg += size;
    len -= size;
}

kfree(buf);
return ret;
```

3. Using snprintf for safe formatting:
```c
buf = kmalloc(bufsize, GFP_KERNEL);
if (!buf)
    return -ENOMEM;

while (len >= size) {
    snprintf(buf, bufsize, "%.*s", size, pg);
    ret = fun(buf, priv, size);
    if (ret)
        break;
    pg += size;
    len -= size;
}

kfree(buf);
return ret;
```

4. Using memcpy with explicit size check:
```c
buf = kmalloc(bufsize, GFP_KERNEL);
if (!buf)
    return -ENOMEM;

while (len >= size) {
    memcpy(buf, pg, size);
    buf[size] = '\0';
    ret = fun(buf, priv, size);
    if (ret)
        break;
    pg += size;
    len -= size;
}

kfree(buf);
return ret;
```

5. Using a fixed-size array instead of dynamic allocation:
```c
char buf[256];  // Assuming 256 is sufficient for all cases

if (bufsize > sizeof(buf))
    return -EINVAL;

while (len >= size) {
    memcpy(buf, pg, size);
    buf[size] = '\0';
    ret = fun(buf, priv, size);
    if (ret)
        break;
    pg += size;
    len -= size;
}

return ret;
```