[RFC PATCH] virtio_blk: Checking "private_data" to avoid kernel panic when hotplugging

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On guest with upstream's kernel(3.3.0-rc7), I
mounted virtblk as:
  a) # mkfs /dev/vda
  b) # mount /dev/vda /mnt
  c) # cd /mnt

Then I did hotplug for virtblk via virsh on host as:
  a) # sudo virsh detach-disk guest vda
  b) # sudo virsh attach-disk guest /media/data/test.img vda

I encountered guest's kernel panic (*probability *
*event*)whose backtrace liked this:

PID: 2496   TASK: ffff88001f5de080  CPU: 0   COMMAND: "blkid"
 #0 [ffff88001afdbb00] machine_kexec at ffffffff81031fcb
 #1 [ffff88001afdbb60] crash_kexec at ffffffff810b8f72
 #2 [ffff88001afdbc30] oops_end at ffffffff814f04b0
 #3 [ffff88001afdbc60] die at ffffffff8100f26b
 #4 [ffff88001afdbc90] do_general_protection at ffffffff814f0042
 #5 [ffff88001afdbcc0] general_protection at ffffffff814ef815
    [exception RIP: virtio_check_driver_offered_feature+27]
    RIP: ffffffffa00540cb  RSP: ffff88001afdbd78  RFLAGS: 00010206
    RAX: ffffffff810ffde0  RBX: ffff88001f14a800  RCX: 000000004cf0758b
    RDX: 4ce86d8b4ce0658b  RSI: 0000000000000007  RDI: ffffffff81a970a0
    RBP: ffff88001afdbd78   R8: ffffffffa009d060   R9: 0000000000000100
    R10: 0000000000000006  R11: 0000000000000246  R12: 000000000000101d
    R13: 0000000000005331  R14: ffffffff81a970a0  R15: 0000000000000000
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 #6 [ffff88001afdbd80] virtblk_ioctl at ffffffffa009c459 [virtio_blk]
 #7 [ffff88001afdbdc0] __blkdev_driver_ioctl at ffffffff81257317
 #8 [ffff88001afdbe00] blkdev_ioctl at ffffffff8125779d
 #9 [ffff88001afdbe50] block_ioctl at ffffffff811aec5c
#10 [ffff88001afdbe60] vfs_ioctl at ffffffff81189342
#11 [ffff88001afdbea0] do_vfs_ioctl at ffffffff811894e4
#12 [ffff88001afdbf30] sys_ioctl at ffffffff81189a61
#13 [ffff88001afdbf80] system_call_fastpath at ffffffff8100b0f2
    RIP: 0000003f566dd847  RSP: 00007fffa23e6130  RFLAGS: 00010202
    RAX: 0000000000000010  RBX: ffffffff8100b0f2  RCX: 0000000000000008
    RDX: 0000000000000000  RSI: 0000000000005331  RDI: 0000000000000003
    RBP: 0000000040000000   R8: 0000003f5699b580   R9: 0000000000000100
    R10: 0000000000000006  R11: 0000000000000246  R12: 0000000000000000
    R13: 0000003f59020ba0  R14: 0000000000000003  R15: 000000000241f030
    ORIG_RAX: 0000000000000010  CS: 0033  SS: 002b

This panic was triggered by the command of "blkid" in
udev's rule "60-persistent-storage.rules".

So, when the virtblk was reattached, command "blkid"
probed filesystem metadata of disks. At that moment,
virtblk_ioctl() was called, but the data of "virtio_blk"
"bdev->bd_disk->private_data" pointing to may not be
updated for the sync reason.

This patch do this check to avoid panic.

Any comment will be appreciated.

Signed-off-by: Ren Mingxin <renmx@xxxxxxxxxxxxxx>
 virtio_blk.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index c4a60ba..4ac81f8 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -245,6 +245,12 @@ static int virtblk_ioctl(struct block_device *bdev, fmode_t mode,
        struct virtio_blk *vblk = disk->private_data;

+        * Check whether the private data pointer has been updated.
+        */
+       if (vblk != vblk->vdev->priv)
+               return -EFAULT;
+       /*
         * Only allow the generic SCSI ioctls if the host can support it.
        if (!virtio_has_feature(vblk->vdev, VIRTIO_BLK_F_SCSI))

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