if_tap unaligned access problem

[Julian Elischer]

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 :-)

>
>> If_tun which is almost the same code has no such problem because
>> it lacks the 14 byte ethernet header. Openbsd has a combined
>> tun/tap driver, with some alignment hacks to fix this.
>
>
> yes, that is true.
>
>> Should I create a pr for this problem, or is there a simple
>> quick fix possible ?
>
>
> you probably should file pr. (1) and (2) above are quick fixes. (3) is 
> more complicated and, maybe, not desirable.
>
> max
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