Re: [PATCH] fix race in AF_UNIX

From: Miklos Szeredi <miklos@szeredi.hu>
Date: Mon, 04 Jun 2007 11:45:32 +0200

quoted

A recv() on an AF_UNIX, SOCK_STREAM socket can race with a
send()+close() on the peer, causing recv() to return zero, even though
the sent data should be received.

This happens if the send() and the close() is performed between
skb_dequeue() and checking sk->sk_shutdown in unix_stream_recvmsg():

process A  skb_dequeue() returns NULL, there's no data in the socket queue
process B  new data is inserted onto the queue by unix_stream_sendmsg()
process B  sk->sk_shutdown is set to SHUTDOWN_MASK by unix_release_sock()
process A  sk->sk_shutdown is checked, unix_release_sock() returns zero

This is only part of the story.  It turns out, there are other races
involving the garbage collector, that can throw away perfectly good
packets with AF_UNIX sockets in them.

The problems arise when a socket goes from installed to in-flight or
vica versa during garbage collection.  Since gc is done with a
spinlock held, this only shows up on SMP.

The following patch fixes it for me, but it's possibly the wrong
approach.

Signed-off-by: Miklos Szeredi <redacted>

I haven't seen a repost of the first patch, which is necessary because
that first patch doesn't apply to the current tree.  Please don't
ignore Arnaldo's feedback like that, or else I'll ignore you just the
same. :-)