From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin-developers@cygwin.com
Subject: Re: The unreliability of AF_UNIX datagram sockets
Date: Mon, 3 May 2021 17:45:31 +0200 [thread overview]
Message-ID: <YJAam1OhVVkRIYu+@calimero.vinschen.de> (raw)
In-Reply-To: <YI/Q1ZQ0cllEqFUB@calimero.vinschen.de>
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.
Corinna
next prev parent reply other threads:[~2021-05-03 15:45 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-27 15:47 Ken Brown
2021-04-29 11:05 ` Corinna Vinschen
2021-04-29 11:16 ` Corinna Vinschen
2021-04-29 14:38 ` Ken Brown
2021-04-29 15:05 ` Corinna Vinschen
2021-04-29 15:18 ` Corinna Vinschen
2021-04-29 16:44 ` Ken Brown
2021-04-29 17:39 ` Corinna Vinschen
2021-05-01 21:41 ` Ken Brown
2021-05-03 10:30 ` Corinna Vinschen
2021-05-03 15:45 ` Corinna Vinschen [this message]
2021-05-03 16:56 ` Ken Brown
2021-05-03 18:40 ` Corinna Vinschen
2021-05-03 19:48 ` Ken Brown
2021-05-03 20:50 ` Ken Brown
2021-05-04 11:06 ` Corinna Vinschen
2021-05-13 14:30 ` Ken Brown
2021-05-17 10:26 ` Corinna Vinschen
2021-05-17 13:02 ` Ken Brown
2021-05-17 13:02 ` Ken Brown
2021-05-20 13:46 ` Ken Brown
2021-05-20 19:25 ` Corinna Vinschen
2021-05-21 21:54 ` Ken Brown
2021-05-22 15:49 ` Corinna Vinschen
2021-05-22 16:50 ` Ken Brown
2021-05-22 18:21 ` Ken Brown
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