Here are five possible code completions for the given function, addressing the CWE-476 (NULL Pointer Dereference) vulnerability:

1. Check for NULL pointers before use:

```c
static int remove_device_files(struct super_block *sb,
			       struct qib_devdata *dd)
{
	struct dentry *dir;
	char unit[10];

	if (!sb || !dd) {
		pr_err("Invalid input: NULL pointer detected\n");
		return -EINVAL;
	}

	snprintf(unit, sizeof(unit), "%u", dd->unit);
	dir = sb->s_root;
	if (!dir) {
		pr_err("Root directory not found\n");
		return -ENOENT;
	}

	// Continue with device file removal logic
	// ...
}
```

2. Use early return for invalid input:

```c
static int remove_device_files(struct super_block *sb,
			       struct qib_devdata *dd)
{
	struct dentry *dir;
	char unit[10];

	if (!sb || !dd)
		return -EINVAL;

	snprintf(unit, sizeof(unit), "%u", dd->unit);
	dir = sb->s_root;
	if (!dir)
		return -ENOENT;

	// Continue with device file removal logic
	// ...
}
```

3. Use assertions for debugging:

```c
static int remove_device_files(struct super_block *sb,
			       struct qib_devdata *dd)
{
	struct dentry *dir;
	char unit[10];

	assert(sb != NULL);
	assert(dd != NULL);

	snprintf(unit, sizeof(unit), "%u", dd->unit);
	dir = sb->s_root;
	assert(dir != NULL);

	// Continue with device file removal logic
	// ...
}
```

4. Use a helper function for NULL checks:

```c
static inline int check_null_pointers(struct super_block *sb,
				      struct qib_devdata *dd,
				      struct dentry *dir)
{
	if (!sb || !dd || !dir) {
		pr_err("NULL pointer detected\n");
		return -EINVAL;
	}
	return 0;
}

static int remove_device_files(struct super_block *sb,
			       struct qib_devdata *dd)
{
	struct dentry *dir;
	char unit[10];

	snprintf(unit, sizeof(unit), "%u", dd->unit);
	dir = sb->s_root;

	if (check_null_pointers(sb, dd, dir) != 0)
		return -EINVAL;

	// Continue with device file removal logic
	// ...
}
```

5. Use goto for error handling:

```c
static int remove_device_files(struct super_block *sb,
			       struct qib_devdata *dd)
{
	struct dentry *dir;
	char unit[10];
	int ret = 0;

	if (!sb || !dd) {
		pr_err("Invalid input: NULL pointer detected\n");
		ret = -EINVAL;
		goto out;
	}

	snprintf(unit, sizeof(unit), "%u", dd->unit);
	dir = sb->s_root;
	if (!dir) {
		pr_err("Root directory not found\n");
		ret = -ENOENT;
		goto out;
	}

	// Continue with device file removal logic
	// ...

out:
	return ret;
}
```

These completions demonstrate different approaches to handling NULL pointers and preventing the CWE-476 vulnerability. Choose the one that best fits your coding style and project requirements.