netdev - I found a strange place while reading “net/ipv6/reassembly.c”

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Date:   Wed, 15 Aug 2018 04:38:29 +0000
From:   Ttttabcd <ttttabcd@...tonmail.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: I found a strange place while reading “net/ipv6/reassembly.c”

Hello everyone who develops the kernel.

At the beginning I was looking for the source author, but his email address has expired, so I can only come here to ask questions.

The problem is in the /net/ipv6/reassembly.c file, the author is Pedro Roque.

I found some strange places when I read the code for this file (Linux Kernel version 4.18).

In the "/net/ipv6/reassembly.c"

In the function "ip6_frag_queue"

	offset = ntohs(fhdr->frag_off) & ~0x7;
	end = offset + (ntohs(ipv6_hdr(skb)->payload_len) -
			((u8 *)(fhdr + 1) - (u8 *)(ipv6_hdr(skb) + 1)));

	if ((unsigned int)end > IPV6_MAXPLEN) {
		*prob_offset = (u8 *)&fhdr->frag_off - skb_network_header(skb);
		return -1;
	}

Here the length of the payload is judged.

And in the function "ip6_frag_reasm"

	payload_len = ((head->data - skb_network_header(head)) -
		       sizeof(struct ipv6hdr) + fq->q.len -
		       sizeof(struct frag_hdr));
	if (payload_len > IPV6_MAXPLEN)
		goto out_oversize;

	......
	out_oversize:
		net_dbg_ratelimited("ip6_frag_reasm: payload len = %d\n", payload_len);
		goto out_fail;

Here also judges the length of the payload.

Judged the payload length twice.

I tested that the code in the label "out_oversize:" does not execute at all, because it has been returned in "ip6_frag_queue".

Unless I comment out the code that judge the payload length in the function "ip6_frag_queue", the code labeled "out_oversize:" can be executed.

So, is this repeated?