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To: cygwin AT cygwin DOT com
From: "D. Boland" <daniel AT boland DOT nl>
Subject: Re: syslog function: Bad file descriptor
Date: Wed, 06 Aug 2014 13:30:37 +0200
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Message-ID: <53E211DD.12F8D5C8@boland.nl>
References: <53E10628 DOT 3C42FD6F AT boland DOT nl> <20140805164842 DOT GA13601 AT calimero DOT vinschen DOT de> <53E117B7 DOT 4C81186A AT boland DOT nl> <20140805190014 DOT GD13601 AT calimero DOT vinschen DOT de> <53E13FFE DOT E7A33C0D AT boland DOT nl> <20140806091941 DOT GK13601 AT calimero DOT vinschen DOT de>
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Hi Corinna,

Corinna Vinschen wrote:
> 
> On Aug  5 22:35, D. Boland wrote:
> > Corinna Vinschen wrote:
> > >
> > > Can you produce another strace for the overwriting case (non-R/O aliases)
> > > for comparison?  Also, can you do the same strace with no syslogd running?
> > >
> > > It might be necessary to create a few test versions of Cygwin with more
> > > debug output, but let's please see these straces first.
> >
> > I attached all three of them in a zipped file.
> 
> Thanks.  I got it now.  AFAICS it's a bug in sendmail.  Take a look
> into your newaliases.strace.txt file.  Start at line 260 (stripping
> off timestamp, thread and process info):
> 
>   260:  normalize_posix_path: src /dev/log
> 
> Here the syslog() function tries to open a connection to a syslogd
> listening on /dev/log.
> 
>   282:  cygwin_socket: 3 = socket(1, 2 (flags 0x0), 0)
> 
> Socket created, file descriptor is 3.
> 
>   296:  connect_syslogd: found /dev/log, fd = 3, type = DGRAM
> 
> Yes, there's a listener on /dev/log.  Now Cygwin stores the info that fd
> 3 is the connection to the syslog daemon.
> 
>   332:  close: close(3)
> 
> And at line 332, a file descriptor close orgy starts.  sendmail closes
> all descriptors from 3 to 255.  This obviously closes fd 3, but how's
> Cygwin's syslog() function to know?
> 
>  2263:  open: 3 = open(/etc/mail/aliases, 0x8000)
> 
> Uh oh!  Now fd 3 is reused for the aliases file.  Things certainly go
> downhill.
> 
>  2651:  writev: -1 = writev(3, 0x2287F0, 2), errno 9
> 
> This is syslog trying to write the log to the descriptor it knows
> is connected to /dev/log.  Fortunately the aliases file is R/O at
> this point, but it's pretty much working as designed.  Syslog()
> doesn't know the application broke its connection to the syslog
> daemon.  It dutyfully writes to the syslog descriptor it knows
> about.
> 
> As for using a file descriptor inside of syslog, that's perfectly
> valid behaviour, see
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/closelog.html:
> 
>   "The openlog() and syslog() functions may allocate a file descriptor."
> 
> Without looking into the sources, I'd assume there's a closelog()
> call missing prior to the descriptor close orgy.  This closelog()
> call should fix the problem.

It is exactly as you say. I found the close() orgy and put a closelog() prior to it.
Now it works perfectly without corrupting the aliases file (writable to sendmail).

I'm asking myself if this closing of 253 file descriptors is a sensible thing to do.
What would Sendmail be trying to accomplish there? It comments "Be shure we have
enough file descriptors". And: "in 4.4BSD, the table ([of fd's]) can be huge; impose
a reasonable limit". Bizarre.

Could it be that incoming e-mail is such a volatile process that previous opened
file descriptors are not closed quick enough? This feels like a crude hack.

Can you give your opinion on this?

Thanks for the quick response & the time you put into this.

Daniel


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