From: Nolan Leake <nolan <at> sigbus.net>
Subject: [PATCH] Fix (at least one cause of) qcow2 corruption.

qcow2's get_cluster_offset() scans forward in the l2 table to find other
clusters that have the same allocation status as the first cluster.
This is used by (among others) qcow_is_allocated().

Unfortunately, it was not checking to be sure that it didn't fall off
the end of the l2 table.  This patch adds that check.

The symptom that motivated me to look into this was that
bdrv_is_allocated() was returning false when there was in fact data
there.  This is one of many ways this bug could lead to data corruption.

I checked the other place that scans for consecutive unallocated blocks
(alloc_cluster_offset()) and it appears to be OK:
    nb_clusters = MIN(nb_clusters, s->l2_size - l2_index);
appears to prevent the same problem from occurring.

Signed-off-by: Nolan Leake <nolan <at> sigbus.net>

---

From: Kevin Wolf <kwolf@redhat.com>
Subject: [PATCH] qcow2 corruption: Fix alloc_cluster_link_l2

This patch fixes a qcow2 corruption bug introduced in SVN Rev 5861. L2 tables
are big endian, so entries must be converted before being passed to functions.

This bug is easy to trigger. The following script will create and destroy a
qcow2 image (the header is gone after three loop iterations):

    #!/bin/bash
    qemu-img create -f qcow2 test.qcow 1M
    for i in $(seq 1 10); do
    qemu-system-x86_64 -hda test.qcow -monitor stdio > /dev/null 2>&1 <<EOF
    savevm test-$i
    quit
    EOF
    done

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff -up qemu-kvm-0.10/qemu/block-qcow2.c.qcow2-corruption qemu-kvm-0.10/qemu/block-qcow2.c
diff -up qemu-kvm-0.10/qemu/block-qcow2.c.qcow2-corruption qemu-kvm-0.10/qemu/block-qcow2.c
--- qemu-kvm-0.10/qemu/block-qcow2.c.qcow2-corruption	2009-04-21 09:57:21.000000000 +0100
+++ qemu-kvm-0.10/qemu/block-qcow2.c	2009-04-21 09:58:27.000000000 +0100
@@ -670,6 +670,10 @@ static uint64_t get_cluster_offset(Block
 
     nb_available = (nb_available >> 9) + index_in_cluster;
 
+    if (nb_needed > nb_available) {
+        nb_needed = nb_available;
+    }
+
     cluster_offset = 0;
 
     /* seek the the l2 offset in the l1 table */
@@ -912,7 +916,7 @@ static int alloc_cluster_link_l2(BlockDr
         goto err;
 
     for (i = 0; i < j; i++)
-        free_any_clusters(bs, old_cluster[i], 1);
+        free_any_clusters(bs, be64_to_cpu(old_cluster[i]), 1);
 
     ret = 0;
 err: