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Date: Fri, 11 Jan 2013 09:34:32 +0200
Message-ID: <CALX0vxAhRz--NTG1yLHTf1xQxkiqixyWEndeYnuxgsTzctq5-g@mail.gmail.com>
Subject: Possible bug in m_split() when splitting M_EXT mbufs
From: Jacques Fourie <jacques.fourie@gmail.com>
To: Hackers freeBSD <freebsd-hackers@freebsd.org>
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Hi,

Could someone please verify if m_split as in svn rev 245286 is doing the
right thing in the scenario where a mbuf chain is split with len0 falling
on a mbuf boundary and the mbuf in question being a M_EXT mbuf? Consider
the following example where m0 is a mbuf chain consisting of 2 M_EXT mbufs,
both 1448 bytes in length. Let len0 be 1448. The 'len0 > m->m_len' check
will be false so the for loop will not be entered in this case. We now have
len = 1448 and remain = 0 and m still points to the first mbuf in the
chain. Also assume that m0 is a pkthdr mbuf. A new pkthdr mbuf n will be
allocated and initialized before the following piece of code is executed :

extpacket:
        if (m->m_flags & M_EXT) {
                n->m_data = m->m_data + len;
                mb_dupcl(n, m);
        } else {
                bcopy(mtod(m, caddr_t) + len, mtod(n, caddr_t), remain);
        }
        n->m_len = remain;
        m->m_len = len;
        n->m_next = m->m_next;
        m->m_next = NULL;
        return (n);

As m is a M_EXT mbuf the code in the if() clause will be executed. The
problem is that m still points to the first mbuf so effectively the data
pointer for n is assigned to the end of m's data pointer. It should
actually point to the start of the data pointer of the next mbuf in the
original m0 chain, right?