Re: if_tap unaligned access problem

From: Sam Leffler (sam_at_errno.com)
Date: 04/28/05

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    Date: Thu, 28 Apr 2005 14:15:21 -0700
To: Julian Elischer <julian@elischer.org>

    Julian Elischer wrote:
    >
    >
    > Maksim Yevmenkin wrote:
    >
    >> Hello,
    >>
    >>> A quick question about if_tap, the tapwrite function
    >>> ( which copies an ethernet frame into an mbuf using uiotombuf )
    >>> is broken on alpha and sparc64.
    >>> The 14 byte ethernet header causes the rest of the frame to
    >>> be misaligned on 4 byte boundaries. This causes crashes in
    >>> various other parts of the kernel. The solution would be to
    >>> shift the mbuf by two bytes, but I am not quite sure where
    >>> this should happen.
    >>
    >>
    >>
    >> i think we have few options here:
    >>
    >> 1) revert back original tapwrite function that was changed in v. 1.48
    >> and set offset to 2 bytes in top mbuf
    >>
    >> 2) change current version of tapwrite so it would m_prepend and
    >> m_pullup mbuf after m_uiotombuf
    >>
    >> 3) change m_uiotombuf to accept one more parameter - mbuf offset at
    >> which data should be copied. there are not that many users of m_uiotombuf
    >>
    >> /sys/kern/uipc_syscalls.c
    >> /sys/net/if_tap.c
    >> /sys/net/if_tun.c
    >> /sys/netgraph/ng_device.c
    >
    >
    >
    > I think we should defien a MAC header that looks like:
    > char dummy[2]
    > char dest[6]
    > char src[6]
    > char type_len[2]
    > and make ether net cards start writing 2 bytes into the buffer :-)

    The whole alignment thing is currently handled quite poorly. For
    example, are frequently aligning to the front of a cluster for ethernet
    receives which means that frames bridged to a non-802.3 device often
    have to prepend an mbuf because there's not enough space for their
    larger header. We need to fix the alignment on receive based on
    multiple considerations and not just ip alignment. There's a similar
    can-of-worms on the tx side that's a bit more involved.

    Dealing with rx-side alignment is getting high on my todo list but
    various other problems keep cropping up.

            Sam
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