Date:	Mon, 1 Apr 2013 05:06:53 -0700
From:	Anatol Pomozov <anatol.pomozov@...il.com>
To:	Stefani Seibold <stefani@...bold.net>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Monakhov <dmonakhov@...nvz.org>,
	Dave Young <dyoung@...hat.com>, JeffMoyer <jmoyer@...hat.com>
Subject: Re: losetup kernel crash in drivers/block/loop.c kernel 3.4.11

Hi

On Wed, Oct 3, 2012 at 1:51 AM, Stefani Seibold <stefani@...bold.net> wrote:
> Hi,
>
> i am faced with a strange kernel crash while removing a loopback device
> with losetup, during a software update of my embedded device, which was
> introduced between 3.0 and 3.4. All other used kernels 2.6.39, 2.6.35,
> 2.6.33, 2.6.29, 2.6.27 and 2.6.20 works well.
>
> BUG: unable to handle kernel NULL pointer derference at 00000041
> IP: [<c019faef>] invalidate_bdev+0x4/0x26
> *pde = 00000000
> Ooops: 0000 I#11 PREEMNT SMP
> Modules linked in: vfat fat i915 drm_kms_helper drm intel_agp i2c_algo_bit intel_gtt agpgart video backlight e1000e usb_storage
>
> Pid: 869, comm: losetup Tainted G            8.3.4
> EIP: 0060:[<c0194aef>] EFLAGS: 00010282 CPU: 1
> EIP is at invalidate_bdev+0x4/0x26
> EAX: 00000029 EBX: f63c1c00 ECX: 00000000 EDX: f63c1e20
> ESI: f5c6bc80 EDI: f63c1c60 EBP: f596e500 ESP: f5053e54
>  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
> CR0: 8005003b CR2: 00000041 CR3: 324ae000 CR4: 000407d0
> DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
> DR6: ffff0ff0 DR7: 00000400
> Process losetup (pid: 869, ti=f5052000 task=f616c0c0 task.ti=f5052000)
> Stack:
>  f63c1c00 c0277449 000200da f63c1c00 ffffffe7 00004c01 f5c39900 c02784d0
>  f5d750a4 00000000 f5053efc f5d750a4 f5269900 c017dda6 0000001d 00008000
>  f63c1cfc c027897b ffffffe7 00004c01 f5053f10 c0202021 00000000 f5c39900
> Call Trace:
>  [<c0277449>] ? loop_clr_fd+0x11/0x1d6
>  [<c02784d0>] ? lo_ioctl+0x455/0x62b
>  [<c017dda6>] ? do_last.clone.32+0x55b/0x5d5
>  [<c027807b>] ? loop_switch.clone.13+0x67/0x67
>  [<c0202021>] ? __blkdev_driver_ioctl+0x1d/0x25
>  [<c0202905>] ? blkdev_ioctl+0x6a3/0x6c2
>  [<c016800d>] ? handle_pte_fault+0x21d/0x7ad
>  [<c017e19b>] ? do_file_open+0x21/0x5d
>  [<c019425b>] ? block_ioctl+0x2f/0x34
>  [<c019425b>] ? block_ioctl+0x2f/0x34
>  [<c019422c>] ? bd_set_size+0x60/0x60
>  [<c017fe00>] ? do_vfs_ioctl+0x455/0x492
>  [<c01181d3>] ? do_page_fault+0x30f/0x32c
>  [<c017293a>] ? fd_install+0x1e/0x3d
>  [<c0173865>] ? do_sys_open+0x17e/0x188
>  [<c017feea>] ? sys_ioctl+0x2d/0x47
>  [<c033f7c1>] ? syscall+0x7/0xb
> Code: 00 89 f0 5b 5e 5f c3 53 8b 40 08 8b 58 18 83 7b 3c 00 74 11 e8 3f b9 ff ff 89 d8 31 d2 31 c9 5b e9 ba 8e fc ff 5b c3 53 8b 40 08 (8b) 58 18 83 7b 3c 00 74 17 e8  1f b9 ff ff e8 4e 88 fc ff 89 d8
> EIP: [<c019eaef>] invalidate_bdev+0x4/0x26 SS:ESP 0068:f5053e54
> CR2: 0000000000000041
>
> This dump was copied by hand from a smart phone screenshot, i hope there
> are no typos.
>
> It is not possible to write a demo program which reproduce this bug due
> the complexity, so i will explain what going on.
>
> First mount a kernel which include a initramfs doing the following:
>
> /bin/mount -t proc none /proc
> /bin/mount -o rw,data=journal,barrier=1,errors=remount-ro /dev/sda3 /mnt
> /bin/mount -o loop /mnt/rootfs.squashfs /rootfs
> /bin/mount -o loop modules.squashfs /rootfs/lib/modules
> /bin/mount -o move /mnt /rootfs/rw
> /bin/umount /proc
> exec /rootfs/bin/sh -c 'exec /sbin/switch_root -c /dev/console /rootfs /sbin/init'
> exec /bin/sh
>
> The Squashfs-Image will be mounted and will be the new root filesystem,
> the file system of /dev/sda3 will be then mounted under /rw.
>
> The reason to do this is, that is is very easy to exchange the root
> filesystem, since it it only a plain image file. And there is no extra
> partition necessary which can be to small in the future.
>
> Also the kernel modules will be a squashfs image as a part of the
> initramfs. This make it safe to exchange the kernel, because it will
> change togehter with the modules.
>
> After starting the new init process of the rootfs.squashfs the firmware
> image opfs.squashfs will be mounted also via loopback block device
> at /opt.
>
> When the user decide to do an update, a new rootfs.squashf will be
> copied into a ramdisk and the following script (snippet) will be
> executed:
>
> cat <<EOF >/tmp/init
> #!/bin/sh
> exec </dev/console
> exec >/dev/console
> exec 2>/dev/console
> umount /init/opt
> umount -l -r /init/rw
> umount -l -r /init
> umount /etc
> rm -rf /tmp/etc
> sync
> for i in /dev/loop*
> do
>         losetup -d $i 2>/dev/null
> done
> rm \$0
> exec /tmp/update.sh "$1" "$2"
> reboot -f
> EOF
>         chmod a+x /tmp/init
>
>         echo "::restart:/tmp/init" >/tmp/etc/inittab
>
>         mount -o ro /dev/ramdisk /mnt
>         cd /mnt
>         /sbin/pivot_root . init
>
>         mount -o move /init/tmp /tmp
>         mount -o move /init/proc /proc
>         mount -o move /init/sys /sys
>         mount -o move /init/dev/pts /dev/pts
>         mount -o move /init/dev/shm /dev/shm
>         mount -o bind /tmp/etc /etc
>
>         init -q
>         sleep 1
>         kill -SIGQUIT 1
>         exit
>
> Now the update.sh script has the control over the system, no more
> application or daemons will running and all mass storages should be
> unmounted.
>
> Till this everything is working fine, than the update.sh will execute
> the following code:
>
> rm -f /rw/optfs.squashfs
>
> for i in /dev/loop*
> do
>         losetup -d $i 2>/dev/null
> done
>
> This will remove the old firmware and all possible loopback devices.
> Executing the losetup will crash the kernel and will produce the Oops
> above.
>
> This is independent to the underlying file system or the processor
> architecture, it will happen on x86 or ppc and ext3fs and yaffs2 as
> well.
>
> Any idea?

Here is proposed fix http://marc.info/?l=linux-kernel&m=136481752606623&w=2
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