illumos gate

Evaluation:

The problem here is due to failures in sendmsg(2) when
sending messages on a "connected" datagram socket,
where the sendmsg call is not specifying a destination.

The AF_UNIX transport has two forms of "address" to identify
endpoints: (1) the struct sockaddr_un exposed to applications,
and (2) an internal equivalent struct so_ux_addr. The internal
form is just a vnode pointer and a magic number. I'll refer to
these as external form and internal form, respectively.

Several functions supporting AF_UNIX accept either form,
and (as necessary) convert the external form to internal form
calling so_ux_addr_xlate() to do the conversion. Note that
this conversion needs to lookup the vnode for the pathname
in the external form AF_UNIX address.

That lookup is the cause of problems in this issue. When
sendmsg is called on a "connected" datagram socket (one
where a connect call has recorded a destination address)
and when that particular sendmsg call does not specify a
destination name, the AF_UNIX support code uses the
address stored by connect (as it should) but is uses the
external form address. This implies that, even though
this call did not specify an address, it ends up trying to
do a lookup to convert external to internal form. This can
fail when the process has dropped privileges.

The fix for this is fairly simple. When the connect call
stores the external form of address, it also stores the
internal form. In the cases where sendmsg is called
without a destination address, we should be using the
internal form of address we stored, and avoid doing
another (unnecessary) so_ux_addr_xlate() call.

Proposed fix out for review:
https://www.illumos.org/rb/r/269/