static unsigned int hns_nic_get_headlen(unsigned char *data, u32 flag,
					unsigned int max_size)
{
	unsigned char *network;
	u8 hlen;

	/* this should never happen, but better safe than sorry */
	if (max_size < ETH_HLEN)
		return max_size;

	/* initialize network frame pointer */
	network = data;

	/* set first protocol and move network header forward */
	network += ETH_HLEN;

	/* handle any vlan tag if present */
	if (hnae_get_field(flag, HNS_RXD_VLAN_M, HNS_RXD_VLAN_S)
		== HNS_RX_FLAG_VLAN_PRESENT) {
		if ((typeof(max_size))(network - data) > (max_size - VLAN_HLEN))
			return max_size;

		network += VLAN_HLEN;
	}

	/* handle L3 protocols */
	if (hnae_get_field(flag, HNS_RXD_L3ID_M, HNS_RXD_L3ID_S)
		== HNS_RX_FLAG_L3ID_IPV4) {
		if ((typeof(max_size))(network - data) >
		    (max_size - sizeof(struct iphdr)))
			return max_size;

		/* access ihl as a u8 to avoid unaligned access on ia64 */
		hlen = (network[0] & 0x0F) << 2;

		/* verify hlen meets minimum size requirements */
		if (hlen < sizeof(struct iphdr))
			return network - data;

		/* record next protocol if header is present */
	} else if (hnae_get_field(flag, HNS_RXD_L3ID_M, HNS_RXD_L3ID_S)
		== HNS_RX_FLAG_L3ID_IPV6) {
		if ((typeof(max_size))(network - data) >
		    (max_size - sizeof(struct ipv6hdr)))
			return max_size;

		/* record next protocol */
		hlen = sizeof(struct ipv6hdr);
	} else {
		return network - data;
	}

	/* relocate pointer to start of L4 header */
	network += hlen;

	/* finally sort out TCP/UDP */
	if (hnae_get_field(flag, HNS_RXD_L4ID_M, HNS_RXD_L4ID_S)
		== HNS_RX_FLAG_L4ID_TCP) {
		if ((typeof(max_size))(network - data) >
		    (max_size - sizeof(struct tcphdr)))
			return max_size;

		/* access doff as a u8 to avoid unaligned access on ia64 */
		hlen = (network[12] & 0xF0) >> 2;

		/* verify hlen meets minimum size requirements */
		if (hlen < sizeof(struct tcphdr))
			return network - data;

		network += hlen;
	} else if (hnae_get_field(flag, HNS_RXD_L4ID_M, HNS_RXD_L4ID_S)
		== HNS_RX_FLAG_L4ID_UDP) {
		if ((typeof(max_size))(network - data) >
		    (max_size - sizeof(struct udphdr)))
			return max_size;

		network += sizeof(struct udphdr);
	}

	/* If everything has gone correctly network should be the
	 * data section of the packet and will be the end of the header.
	 * If not then it probably represents the end of the last recognized
	 * header.
	 */
	if ((typeof(max_size))(network - data) < max_size)
		return network - data;
	else
		return max_size;
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.