if_tap unaligned access problem

[Maksim Yevmenkin]

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

> 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