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

1. Add bounds checking before writing:

```c
if (addr < mm->mmap_base || end > mm->mmap_base + mm->task_size) {
    return ERR_PTR(-EINVAL);
}
```

2. Use safer memory allocation:

```c
vma = kmalloc(sizeof(struct vm_area_struct), GFP_KERNEL);
if (!vma)
    return ERR_PTR(-ENOMEM);
memset(vma, 0, sizeof(struct vm_area_struct));
```

3. Validate input parameters:

```c
if (end <= addr || pgoff + pglen < pgoff) {
    return ERR_PTR(-EINVAL);
}
```

4. Use a helper function to safely adjust pointers:

```c
static inline void safe_adjust_vm_start(struct vm_area_struct *vma, unsigned long new_start)
{
    if (new_start < vma->vm_end)
        vma->vm_start = new_start;
}
```

5. Implement a size-limited copy for string data:

```c
if (anon_name && anon_name->name) {
    size_t name_len = strnlen(anon_name->name, MAX_ANON_NAME_LEN);
    vma->anon_name = kmalloc(name_len + 1, GFP_KERNEL);
    if (vma->anon_name)
        strncpy(vma->anon_name, anon_name->name, name_len + 1);
}
```

These completions aim to prevent out-of-bounds writes by adding input validation, using safer memory allocation methods, and implementing bounds-checked operations.