"Data modified on freelist" error

[John Stockard <john.stockard@vertsys.com>]

I've received two odd kernel error messages on my OpenBSD firewall/nat box.
The error messages are:

Data modified on freelist: word 5 of object 0xe091c700 size 96 previous type
pfil (0xe093af80 != 0xdeadbeef)
Data modified on freelist: word 5 of object 0xe0903200 size 128 previous
type pfil (0xe0928400 != 0xdeadbeef)

The kernel doesn't panic, and none of the functionality of the box seems to
be diminished when this error happens.  The error has occurred twice in the
last week (since the last reboot of this box).  The first error message
appeared about 4 days ago, and the second one appeared about 2 hours ago.

The box in question is running OpenBSD 2.7-STABLE (generic kernel) on an
i386 platform (180 MHz Pentium Pro CPU, Micronics W6-Li "Lightning" mobo
[dual-CPU capable, but only one CPU is installed]).  The dmesg report is as
follows:

----

OpenBSD 2.7-stable (GENERIC) #0: Fri Oct 20 17:33:22 CDT 2000
    john@fw:/usr/src/sys/arch/i386/compile/GENERIC
cpu0: Intel Pentium Pro ("GenuineIntel" 686-class, 256KB L2 cache) 180 MHz
cpu0: FPU,V86,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SYS,MTRR,PGE,MCA,CMOV
real mem  = 66695168 (65132K)
avail mem = 57397248 (56052K)
using 839 buffers containing 3436544 bytes (3356K) of memory
mainbus0 (root)
bios0 at mainbus0: AT/286+(ae) BIOS, date 09/20/96
pci0 at mainbus0 bus 0: configuration mode 1 (bios)
pchb0 at pci0 dev 0 function 0 "Intel 82441FX PCI/Memory (PMC)" rev 0x02
pcib0 at pci0 dev 7 function 0 "Intel 82371SB (Triton II) PCI-ISA" rev 0x01
pciide0 at pci0 dev 7 function 1 "Intel 82371SB (Triton II) IDE" rev 0x00:
DMA, channel 0 wired to compatibility, channel 1 wired to compatibility
wd0 at pciide0 channel 0 drive 0: <QUANTUM FIREBALL_TM3200A>
wd0: can use 32-bit, PIO mode 4, DMA mode 2
wd0: 16-sector PIO, LBA, 3067MB, 6232 cyl, 16 head, 63 sec, 6281856 sectors
pciide0: channel 0 interrupting at irq 14
wd0(pciide0:0:0): using PIO mode 4, DMA mode 2 (using DMA data transfers)
atapiscsi0 at pciide0 channel 1
scsibus0 at atapiscsi0: 2 targets
cd0 at scsibus0 targ 0 lun 0: <MITSUMI, CD-ROM FX120T !B, b01> SCSI0 5/cdrom
removable
cd0: can use 32-bit, PIO mode 3, DMA mode 2
wd1 at pciide0 channel 1 drive 1: <IOMEGA ZIP 100>
wd1: can use 32-bit, PIO mode 0
wd1: 1-sector PIO, LBA, 0MB, 512 cyl, 12 head, 32 sec, 0 sectors
pciide0: channel 1 interrupting at irq 15
cd0(pciide0:1:0): using PIO mode 0, DMA mode 2 (using DMA data transfers)
wd1(pciide0:1:1): using PIO mode 0
"S3 ViRGE" rev 0x05 at pci0 dev 15 function 0 not configured
ep0 at pci0 dev 16 function 0 "3Com 3c595 100Base-TX" rev 0x00: address
00:60:97:29:68:95, utp/100-TX default utp irq 11
fxp0 at pci0 dev 17 function 0 "Intel 82557" rev 0x02: irq 9, address
00:a0:c9:77:ea:71
inphy0 at fxp0 phy 1: i82555 10/100 media interface, rev. 0
dc0 at pci0 dev 18 function 0 "Lite-On PNIC" rev 0x20: irq 5 address
00:a0:cc:25:45:45
ukphy0 at dc0 phy 1: Generic IEEE 802.3u media interface
ukphy0: OUI 0x00c0b4, model 0x0000, rev. 8
isa0 at pcib0
isadma0 at isa0
pcppi0 at isa0 port 0x61
midi0 at pcppi0: <PC speaker>
sysbeep0 at pcppi0
npx0 at isa0 port 0xf0/16: using exception 16
pccom0 at isa0 port 0x3f8/8 irq 4: ns16550a, 16 byte fifo
pccom1 at isa0 port 0x2f8/8 irq 3: ns16550a, 16 byte fifo
vt0 at isa0 port 0x60/16 irq 1: vga 80 col, color, 8 scr, mf2-kbd
pms0 at vt0 irq 12
fdc0 at isa0 port 0x3f0/6 irq 6 drq 2
fd0 at fdc0 drive 0: 1.44MB 80 cyl, 2 head, 18 sec
biomask c040 netmask ca60 ttymask da62
pctr: 686-class user-level performance counters enabled
mtrr: Pentium Pro MTRR support
dkcsum: wd0 matched BIOS disk 80
wd1: no disk label
dkcsum: wd1 had no matching BIOS disk
root on wd0a
rootdev=0x0 rrootdev=0x300 rawdev=0x302

----

What is causing this error?  Is it something I should be concerned about?

I tried searching around on the OpenBSD web site for any mention of "Data
modified on freelist" error messages, but I came up empty-handed.

Thanks in advance.

John Stockard
Network Systems Engineer
Vertical Systems, Inc.
Phone:  +1 (952) 934-7533 ext. 341