From: Ken Brown <kbrown@cornell.edu>
To: cygwin-developers@cygwin.com
Subject: Re: The unreliability of AF_UNIX datagram sockets
Date: Mon, 3 May 2021 12:56:02 -0400 [thread overview]
Message-ID: <5564e10e-9099-fc5a-3a8d-c2ffb8ca4cff@cornell.edu> (raw)
In-Reply-To: <YJAam1OhVVkRIYu+@calimero.vinschen.de>
On 5/3/2021 11:45 AM, Corinna Vinschen wrote:
> 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.
The partial read is actually OK as is, since it's comparable to what happens on
a partial read from a pipe. I already have AF_UNIX code (on the topic/af_unix
branch) that deals with that. A boolean variable _unread keeps track of whether
there's unread data from a previous partial read. If so, the next read just
reads data without expecting a header.
Ken
next prev parent reply other threads:[~2021-05-03 16:56 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
2021-05-03 16:56 ` Ken Brown [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=5564e10e-9099-fc5a-3a8d-c2ffb8ca4cff@cornell.edu \
--to=kbrown@cornell.edu \
--cc=cygwin-developers@cygwin.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).