Here are five possible code completions for the `skb_copy_expand` function:

**Option 1: Using kmalloc and memcpy**
```c
struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
				int newheadroom, int newtailroom,
				gfp_t gfp_mask)
{
	struct sk_buff *new_skb;
	void *new_head;

	/* Allocate the copy buffer */
	new_skb = kmalloc(sizeof(struct sk_buff), gfp_mask);
	if (!new_skb)
		return NULL;

	/* Allocate the new head and tail buffers */
	new_head = kmalloc(skb->len + newheadroom + newtailroom, gfp_mask);
	if (!new_head) {
		kfree(new_skb);
		return NULL;
	}

	/* Copy the original skb data to the new buffer */
	memcpy(new_head + newheadroom, skb->data, skb->len);

	/* Initialize the new skb */
	new_skb->data = new_head + newheadroom;
	new_skb->len = skb->len;
	new_skb->tail = new_skb->data + skb->len;
	new_skb->end = new_skb->tail + newtailroom;

	return new_skb;
}
```

**Option 2: Using kmemdup and skb_put**
```c
struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
				int newheadroom, int newtailroom,
				gfp_t gfp_mask)
{
	struct sk_buff *new_skb;

	/* Allocate the copy buffer */
	new_skb = kmemdup(skb, sizeof(struct sk_buff), gfp_mask);
	if (!new_skb)
		return NULL;

	/* Allocate the new head and tail buffers */
	skb_put(new_skb, skb->len + newheadroom + newtailroom);

	/* Copy the original skb data to the new buffer */
	memcpy(skb_put(new_skb, skb->len), skb->data, skb->len);

	/* Initialize the new skb */
	new_skb->data = new_skb->head + newheadroom;
	new_skb->len = skb->len;
	new_skb->tail = new_skb->data + skb->len;
	new_skb->end = new_skb->tail + newtailroom;

	return new_skb;
}
```

**Option 3: Using skb_clone and skb_put**
```c
struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
				int newheadroom, int newtailroom,
				gfp_t gfp_mask)
{
	struct sk_buff *new_skb;

	/* Allocate the copy buffer */
	new_skb = skb_clone(skb, gfp_mask);
	if (!new_skb)
		return NULL;

	/* Allocate the new head and tail buffers */
	skb_put(new_skb, newheadroom + newtailroom);

	/* Copy the original skb data to the new buffer */
	memcpy(skb_put(new_skb, skb->len), skb->data, skb->len);

	/* Initialize the new skb */
	new_skb->data = new_skb->head + newheadroom;
	new_skb->len = skb->len;
	new_skb->tail = new_skb->data + skb->len;
	new_skb->end = new_skb->tail + newtailroom;

	return new_skb;
}
```

**Option 4: Using skb_realloc_headroom and skb_put**
```c
struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
				int newheadroom, int newtailroom,
				gfp_t gfp_mask)
{
	struct sk_buff *new_skb;

	/* Allocate the copy buffer */
	new_skb = skb_realloc_headroom(skb, newheadroom, gfp_mask);
	if (!new_skb)
		return NULL;

	/* Allocate the new tail buffer */
	skb_put(new_skb, newtailroom);

	/* Copy the original skb data to the new buffer */
	memcpy(skb_put(new_skb, skb->len), skb->data, skb->len);

	/* Initialize the new skb */
	new_skb->len = skb->len;
	new_skb->tail = new_skb->data + skb->len;
	new_skb->end = new_skb->tail + newtailroom;

	return new_skb;
}
```

**Option 5: Using skb_copy and skb_put**
```c
struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
				int newheadroom, int newtailroom,
				gfp_t gfp_mask)
{
	struct sk_buff *new_skb;

	/* Allocate the copy buffer */
	new_skb = skb_copy(skb, gfp_mask);
	if (!new_skb)
		return NULL;

	/* Allocate the new head and tail buffers */
	skb_put(new_skb, newheadroom + newtailroom);

	/* Copy the original skb data to the new buffer */
	memcpy(skb_put(new_skb, skb->len), skb->data, skb->len);

	/* Initialize the new skb */
	new_skb->data = new_skb->head + newheadroom;
	new_skb->len = skb->len;
	new_skb->tail = new_skb->data + skb->len;
	new_skb->end = new_skb->tail + newtailroom;

	return new_skb;
}
```

Note that these options may have different performance characteristics and may not all be suitable for your specific use case. Additionally, some options may require additional error checking or handling.