Re: [PATCH] loopback device can't act as its backing store

From: Andrew Morton (akpm_at_osdl.org)
Date: 12/03/04

  • Next message: Andrew Morton: "Re: [PATCH] loopback device can't act as its backing store" 
    Date:	Fri, 3 Dec 2004 14:50:56 -0800
To: franz_pletz@t-online.de (Franz Pletz)

    franz_pletz@t-online.de (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@happyjack.org>
    To: linux-kernel@vger.kernel.org
    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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