misc/118160: unable to mount / rw while booting 7.0-BETA3

[Bruce Evans]

The following reply was made to PR misc/118160; it has been noted by GNATS.

From: Bruce Evans <brde at optusnet.com.au>
To: Bruce Evans <brde at optusnet.com.au>
Cc: Yuri <yuri at tsoft.com>, freebsd-gnats-submit at freebsd.org,
        freebsd-bugs at freebsd.org
Subject: Re: misc/118160: unable to mount / rw while booting 7.0-BETA3
Date: Wed, 21 Nov 2007 18:42:27 +1100 (EST)

 On Wed, 21 Nov 2007, Bruce Evans wrote:
 
 > I can now see a plausible way to reach the bad state:
 > - after booting, the root device is mounted on / r/o with no problems
 > - mistype a mount command or have $true generate a wrong mount command,
 >  so that the root device is mounted somewhere else (I don't know how
 >  it can be on "", but it could be on " " or on any valid pathname).
 >  If you preemptively mount it r/w, then this other mount will fail
 >  -- look in the logs for messages about this.
 > - now try to remount / r/w normally.  This will fail due to the r/o mount
 >  not on /.
 > - if there is only 1 extra r/o mount of /, then the r/w mount should work
 >  after unmounting the extra.  If there are several extras, then unmounting
 >  them in a certain order should give the bufobj panic.
 
 The bufobj panic actually happens in the only possible order for trying to
 fix the simulated problem: after booting with -s:
 
      # mount -o ro /dev/ad0s2a /mnt     # create extra mount (ad0s2a is root dev)
      #                                  # like a runaway script might
      # umount /mnt                      # get rid of the extra mount
      # fsck -p /
      --- panic
 
 and
 
      # mount /dev/ad0s2a /mnt           # I forgot the ro
      --- this succeeds due to the hack that lets fsck succeed, but it shouldn't
 
 Bruce