🚀 go-pugleaf

In article <588@dtscp1.UUCP> scott@dtscp1.UUCP (Scott Barman) writes:
> I am trying to write some software for mail that will interact with our
> internal network and must co-exist with the mailers on our bsd-based
> systems (actually, SunOS 3.5 and SunOS 4.0) and with a System V Release
> 3.2 system.  I need to know how these systems lock the mail file.
> Does /usr/.../*.lock work on both of these?  I know that our v7-like
> system uses the execute bit, but I do not have the sources to the other
> systems.
>
> scott barman  {gatech, emory}!dtscp1!scott

Scott,
I had the unfortunate occasion to study mailbox locking under SunOS 4.0; my
findings might be of interest to you and others on the net.

This was originally submitted to Sun as a bug report.  They didn't follow
up on it, so I can't say whether things are likely to change in the future.
	
	Jeff Stearns			jeff@tc.fluke.COM
	John Fluke Mfg. Co, Inc.	(206) 356-5064

------------------------------------------------------------------------

Date: Thu, 25 Aug 88 16:42:44 PDT
From: jeff (Jeff Stearns)
To: rdk@sun.com
Subject: /usr/ucb/mail doesn't follow /bin/mail lock protocol
Cc: jhc@sun.com
Status: R
 
Name:   Jeff Stearns
 
Phone Number:   206/356-5064
 
Site:   John Fluke Mfg. Co., Inc.
 
PAL #: FLUKE-MF01
 
Subject: /usr/ucb/mail doesn't follow /bin/mail lock protocol
 
Index:  usr.ucb/mail SunOS 4.0
 
Description:
        - /bin/mail locks a user's mailbox with flock("/var/spool/mail/user")
          before delivery.  This is good.
                 
        - /usr/ucb/mail follows a different strategy which is guaranteed to
          fail:  
                creat("/var/spool/mail/user.lock");
          This can never succeed because the permissions of /var/spool/mail
          forbid file creation.  (This is for security reasons.)
                 
        - Thus /usr/ucb/mail manipulates mailboxes without the benefit of any
          locking.  This leads to garbled messages and mangled mailboxes if
          /bin/mail delivers a message at the same time that /usr/ucb/mail is
          manipulating the mailbox.

          /bin/mail delivers a message at the same time that /usr/ucb/mail is
          manipulating the mailbox.

Repeat-By:
        Nondeterminism forbids an exact recipe.  Receive a lot of mail while
        you are running /usr/ucb/mail.  Better yet, run trace(1) or inspect the
        code.

Related-bug:
        - You may note that /bin/mail actually tries to follow two schemes for
          locking a mailbox.  It repeatedly tries the
                creat("/var/spool/mail/user.lock");
          of /usr/ucb/mail, but this fails (/bin/mail runs as the target uid
          at the instant of delivery).  After this fails seven times, it
          gives up and simply flocks the mailbox file.

          /bin/mail's repeated fiddling with /var/spool/mail/user.lock is
          probably also a bug.
-- 
    Jeff Stearns        John Fluke Mfg. Co, Inc.               (206) 356-5064
    jeff@tc.fluke.COM   {uw-beaver,microsoft,sun}!fluke!jeff
						  
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