The unreliability of AF_UNIX datagram sockets
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
> 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.
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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