kern/159663: sockets don't work though nullfs mounts
rmh at freebsd.org
Sun Sep 25 15:32:28 UTC 2011
2011/9/24 Robert Millan <rmh at freebsd.org>:
> I found a thread from 2007 with further discussion about this problem:
I've looked at the situation in a bit more detail, for now only with
sockets in mind (not named pipes). My understanding is (please
correct me if I'm wrong):
- nullfs holds reference counts for each vnode, but sockets have their
own mechanism for reference counting (so_count / soref / sorele).
vnode reference counting doesn't protect against socket being closed,
which would leave a stale pointer in the upper nullfs layer.
- Increasing the reference count of the socket itself can't be done in
null_nodeget() because this function is merely a getter whose call
doesn't indicate any meaningful event.
- It's not clear to me that there's any event in time where the socket
reference can be increased. If mounting a nullfs were that event,
then all existing sockets would be soref'ed but we wouldn't be
soref'ing future sockets created in the lower layer after the mount.
This doesn't seem correct.
- Possible solution: null_nodeget() semantics are replaced with
something that actually allows vnodes in the upper layer to be created
- Possible solution: upper layer has a memory structure to keep track
of which sockets in the lower layer have been soref'ed.
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