The unreliability of AF_UNIX datagram sockets

Corinna Vinschen
Mon May 3 15:45:31 GMT 2021

On May  3 12:30, Corinna Vinschen wrote:
> 1. As you noticed, the socket descriptors are inherited by exec'ed
>    children, but the mqueue isn't.  So we need at least some kind of
>    fixup_after_exec for mqueues used as part of AF_UNIX sockets.
> 2. While none of the mqueue structures are propagated to child
>    processes, the handles to the synchronization objects accidentally
>    are.
> 3. Note 1 and 2 can only be implemented, if we introduce a new
>    superstructure keeping track of all mdq_t/mq_info structure
>    pointers in an application.  Oh well.  Bummer, I was SOO happy
>    that the posix_ipc stuff didn't need it yet...
> 4. As stated in the code comment leading the mqueue implementation,
>    I used Stevens code as the basis.  What I didn't realize so far is
>    that Stevens simplified the implementation in some ways.  The code
>    works for real POSIX mqueues, but needs some more fixing before it
>    can be used for AF_UNIX at all.
> 5. I hacked a bit on an mq-only mmap call, which is supposed to allow
>    creating/opening of named shared memeory areas, but that's a tricky
>    extension to the mmap scenario.  I have a gut feeling that it's
>    better to avoid using mmap at all and use Windows section mapping
>    directly in mq_open/mq_close, especially if we have to implement
>    fixup_after_exec semantics anyway.
> 6. Ultimately, AF_UNIX sockets should not run file-backed at all,
>    anyway.  Given that sockets can't be bound multiple times, there's
>    no persistency requirement for the mqueue.

Got it:

7. The idea of _mq_recv partial reads is entirely broken.  Given that
   the information in the queue consists of header info plus payload,
   the entire block has to be read, and then a new block with fixed
   header and shortened payload has to be rewritten with bumped priority.
   This in turn can only be performed by the AF_UNIX code, unless we
   expect knowledge of the AF_UNIX packet layout in the mqueue code.


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