Re: [PATCH] fix race in AF_UNIX

[ebiederm@xmission.com (Eric W. Biederman)]

Miklos Szeredi <miklos@szeredi.hu> writes:

> Right.  But the devil is in the details, and (as you correctly point
> out later) to implement this, the whole locking scheme needs to be
> overhauled.  Problems:
>
>  - Using the queue lock to make the dequeue and the fd detach atomic
>    wrt the GC is difficult, if not impossible: they are are far from
>    each other with various magic in between.  It would need thorough
>    understanding of these functions and _big_ changes to implement.
>
>  - Sleeping on u->readlock in GC is currently not possible, since that
>    could deadlock with unix_dgram_recvmsg().  That function could
>    probably be modified to release u->readlock, while waiting for
>    data, similarly to unix_stream_recvmsg() at the cost of some added
>    complexity.
>
>  - Sleeping on u->readlock is also impossible, because GC is holding
>    unix_table_lock for the whole operation.  We could release
>    unix_table_lock, but then would have to cope with sockets coming
>    and going, making the current socket iterator unworkable.
>
> So theoretically it's quite simple, but it needs big changes.  And
> this wouldn't even solve all the problems with the GC, like being a
> possible DoS vector.

Making the GC fully incremental will solve the DoS vector problem as
well.  Basically you do a fixed amount of reclaim in the new socket
allocation code.

It appears clear that since we can't stop the world and garbage
collect we need an incremental collector.

Eric