Date: Fri, 11 Jan 2013 10:53:06 +0200 From: Jacques Fourie <jacques.fourie@gmail.com> To: Gleb Smirnoff <glebius@freebsd.org> Cc: Hackers freeBSD <freebsd-hackers@freebsd.org> Subject: Re: Possible bug in m_split() when splitting M_EXT mbufs Message-ID: <CALX0vxAqOwxrBwh_%2Bj4=A2m6qXuKWbd_mu9Zpg-boXE5dBVuTQ@mail.gmail.com> In-Reply-To: <20130111081254.GG82815@FreeBSD.org> References: <CALX0vxAhRz--NTG1yLHTf1xQxkiqixyWEndeYnuxgsTzctq5-g@mail.gmail.com> <20130111081254.GG82815@FreeBSD.org>
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On Fri, Jan 11, 2013 at 10:12 AM, Gleb Smirnoff <glebius@freebsd.org> wrote:
> Jacques,
>
> On Fri, Jan 11, 2013 at 09:34:32AM +0200, Jacques Fourie wrote:
> J> Could someone please verify if m_split as in svn rev 245286 is doing the
> J> right thing in the scenario where a mbuf chain is split with len0
> falling
> J> on a mbuf boundary and the mbuf in question being a M_EXT mbuf? Consider
> J> the following example where m0 is a mbuf chain consisting of 2 M_EXT
> mbufs,
> J> both 1448 bytes in length. Let len0 be 1448. The 'len0 > m->m_len' check
> J> will be false so the for loop will not be entered in this case. We now
> have
> J> len = 1448 and remain = 0 and m still points to the first mbuf in the
> J> chain. Also assume that m0 is a pkthdr mbuf. A new pkthdr mbuf n will be
> J> allocated and initialized before the following piece of code is
> executed :
> J>
> J> extpacket:
> J> if (m->m_flags & M_EXT) {
> J> n->m_data = m->m_data + len;
> J> mb_dupcl(n, m);
> J> } else {
> J> bcopy(mtod(m, caddr_t) + len, mtod(n, caddr_t), remain);
> J> }
> J> n->m_len = remain;
> J> m->m_len = len;
> J> n->m_next = m->m_next;
> J> m->m_next = NULL;
> J> return (n);
> J>
> J> As m is a M_EXT mbuf the code in the if() clause will be executed. The
> J> problem is that m still points to the first mbuf so effectively the data
> J> pointer for n is assigned to the end of m's data pointer. It should
> J> actually point to the start of the data pointer of the next mbuf in the
> J> original m0 chain, right?
>
> Thanks for analysis, Jacques.
>
> IMHO, the following patch should suffice and won't break anything:
>
> Index: uipc_mbuf.c
> ===================================================================
> --- uipc_mbuf.c (revision 245223)
> +++ uipc_mbuf.c (working copy)
> @@ -1126,7 +1126,7 @@
> u_int len = len0, remain;
>
> MBUF_CHECKSLEEP(wait);
> - for (m = m0; m && len > m->m_len; m = m->m_next)
> + for (m = m0; m && len >= m->m_len; m = m->m_next)
> len -= m->m_len;
> if (m == NULL)
> return (NULL);
>
> Can you please test it?
I think that your patch may cause other issues - m now points to the first
mbuf in the tail portion. The final piece of code under the extpacket:
label will then set m->m_len to 0 for example.
>
> --
> Totus tuus, Glebius.
>
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