Re: [PATCH] loopback device can't act as its backing store
From: Andrew Morton
Date: Fri Dec 03 2004 - 17:49:25 EST
franz_pletz@xxxxxxxxxxx (Franz Pletz) wrote:
>
> The patch below fixes a bug in loop which apparently causes the kernel to call
> the initialization routine of a loopback device recursively while trying to set
> the backing store to the loopback device it's being mapped to.
Your patch addresses direct loop0-on-loop0 recursion, but does it fix the
more complex loop-stacks which Chris Spiegel identified?
I don't think there's any actual infinite recursion in Chris's example - in
his case we simply stacked loop deveces too deep. But a fix for Chris's
scenario will also fix the one which you identify, I think. Andries posted
such a patch but I have not yet got around to looking at it.
Begin forwarded message:
Date: Fri, 12 Nov 2004 02:49:34 -0800
From: Chris Spiegel <lkml@xxxxxxxxxxxxx>
To: linux-kernel@xxxxxxxxxxxxxxx
Subject: Oops with loop devices on 2.6.9
Hi,
While playing around with loop mounts on kernel 2.6.9 I managed to get
a kernel panic. After messing around with it I can reproduce the
problem reliably. The sequence I came up with to cause the problem:
mount -o loop /dev/loop/0 /mnt
mount -o loop /dev/loop/1 /mnt
mount -o loop /dev/loop/2 /mnt
mount /dev/loop/0 /mnt -t ext2
I know the above is silly and contrived.
An example oops is as follows (I copied this down on paper and then
back, so hopefully I made no transcription errors):
Unable to handle kernel paging request at virtual address 98858a6f
printing eip:
c011345a
*pde = 00000000
Oops: 0000 [#1]
SMP
Modules linked in:
CPU 0
EIP 0060:[<c011345a>] Not tainted VLI
EFLAGS 00010083 (2.6.9)
EIP is at do_page_fault+0x99/0x599
eax: c9100000 ebx: 65642f3c ecx: 0000007b edx: f7d4858b
esi: 00000000 edi: c01133c1 ebp: 988589ff esp: c9100108
ds: 007b es: 007b ss: 0068
Unable to handle kernel NULL pointer dereference at virtual address 00000070
printing eip:
c011345a
*pde = 00000000
I ran this through ksymoops, but it just spit it back at me with the
following tacked on:
Warning (Oops_read): Code line not seen, dumping what data is available
>>eax; c9100000 <pg0+8b9f000/3fa9d400>
>>edx; f7d4858b <pg0+377e758b/3fa9d400>
>>edi; c01133c1 <do_page_fault+0/599>
>>esp; c9100108 <pg0+8b9f108/3fa9d400>
1 warning issued. Results may not be reliable.
So I'm not sure if that's useful. I could also get my system to lock up
if I did the above, but without the loop1 and 2 devices. One time it
just froze, no messages. Another time I got:
double fault, gdt at c1408260 [255 bytes]
I'm attaching my kernel config if that's of any help. If you'd like me
to reply to the list, please CC me so I can set the In-Reply-To header
properly.
Chris
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