[Xen-devel] [PATCH] xen/blkfront: Fix crash if backend doesn't follow the right states.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

We have split the setting up of all the resources in two steps:
1) talk_to_blkback  - which figures out the num_ring_pages (from
   the default value of zero), sets up shadow and so
2) blkfront_connect - does the real part of filling out the
   internal structures.

The problem is if we bypass the 1) step and go straight to 2)
and call blkfront_setup_indirect where we use the macro
BLK_RING_SIZE - which returns an negative value (because
sz is zero  - since num_ring_pages is zero - since it has never
been set).

We can fix this by making sure that we always have called
talk_to_blkback before going to blkfront_connect.

Or we could set in blkfront_probe info->nr_ring_pages = 1
to have a default value. But that looks odd - as we haven't
actually negotiated any ring size.

This patch changes XenbusStateConnected state to detect if
we haven't done the initial handshake - and if so continue
on as if were in XenbusStateInitWait state.

We also roll the error recovery (freeing the structure) into
talk_to_blkback error path - which is safe since that function
is only called from blkback_changed.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 drivers/block/xen-blkfront.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index f3d0d47..8a8dc91 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1867,6 +1867,9 @@ again:
  destroy_blkring:
        blkif_free(info, 0);
 
+       kfree(info);
+       dev_set_drvdata(&dev->dev, NULL);
+
        return err;
 }
 
@@ -2453,11 +2456,8 @@ static void blkback_changed(struct xenbus_device *dev,
        case XenbusStateInitWait:
                if (dev->state != XenbusStateInitialising)
                        break;
-               if (talk_to_blkback(dev, info)) {
-                       kfree(info);
-                       dev_set_drvdata(&dev->dev, NULL);
+               if (talk_to_blkback(dev, info))
                        break;
-               }
        case XenbusStateInitialising:
        case XenbusStateInitialised:
        case XenbusStateReconfiguring:
@@ -2466,6 +2466,10 @@ static void blkback_changed(struct xenbus_device *dev,
                break;
 
        case XenbusStateConnected:
+               if (dev->state != XenbusStateInitialised) {
+                       if (talk_to_blkback(dev, info))
+                               break;
+               }
                blkfront_connect(info);
                break;
 
-- 
2.1.0


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