NetBSD Problem Report #4597

Received: (qmail 4048 invoked from network); 28 Nov 1997 21:38:26 -0000
Message-Id: <>
Date: Fri, 28 Nov 1997 16:37:34 -0500 (EST)
Subject: cyclic union mount causes kernel panic
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>Number:         4597
>Category:       kern
>Synopsis:       A Cyclic union mount causes a kernel panic
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Nov 28 13:50:03 +0000 1997
>Originator:     Rick Byers
>Release:        Nov 25, 97
Rick Byers                                      Internet Access Worldwide                                		     System Admin
University of Waterloo, Computer Science                    (905)714-1400               


System: NetBSD rickb 1.3_ALPHA NetBSD 1.3_ALPHA (RICKB) #21: Wed Nov 26 00:35:24 EST 1997 root@rickb:/usr/src/sys/arch/i386/compile/RICKB i386

	If A is union mounted onto B, then union mounting B back onto A
	causes a kernel panic.  Obviously, this should never happen, unless
	someone is deliberatly crashing you system (happened to us).
	I wouldn't be surprised if there are some similar problems with
	mount_null also.

panic: locking against myself
#0  0xf0963c00 in ?? ()
#1  0x2a10000 in ?? ()
#2  0xf01fe577 in cpu_reboot ()
#3  0xf0125032 in panic ()
#4  0xf01ee91a in ufs_lock ()
#5  0xf01499ee in union_root ()
#6  0xf0139798 in lookup ()
#7  0xf0139186 in namei ()
#8  0xf0149614 in union_mount ()
#9  0xf013c07f in sys_mount ()
#10 0xf0202e34 in syscall ()

	mkdir a b
	mount -t union a b
	mount -t union b a
If you're concerned about people crashing your system, you should take 
UNIONFS out of your kernel for the time being.  However, I don't understand
the filesystem code well enough to come up with a fix that "does the right 

From: Rick Byers <>
Cc:  Subject: kern/4597
Date: Sun, 23 May 1999 17:58:02 -0400

 This is still a problem in NetBSD-1.4.

 IMHO, the UNION file system should be removed from the GENERIC kernel
 (possibly along with null and umap) with a comment indicating that any
 machine with the UNION filesystem in the kernel can be crashed by any
 user.  Experienced administrators know not to trust unionfs, but
 newbies do not (I was bitten by this 2 years ago).

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