Patchwork [4/5] qcow2: Fix corruption after refblock allocation

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Submitter Kevin Wolf
Date May 28, 2010, 11:22 a.m.
Message ID <1275045773-26963-5-git-send-email-kwolf@redhat.com>
Download mbox | patch
Permalink /patch/53882/
State New
Headers show

Comments

Kevin Wolf - May 28, 2010, 11:22 a.m.
Refblock allocation code needs to take into consideration that update_refcount
will load a different refcount block into the cache, so it must initialize the
cache for a new refcount block only afterwards. Not doing this means that not
only the refcount in the wrong block is updated, but also that the caller will
work on the wrong block.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2-refcount.c |   11 +++++++++--
 1 files changed, 9 insertions(+), 2 deletions(-)
Stefan Hajnoczi - May 28, 2010, 3:42 p.m.
I haven't figured out this function yet so I can't review this one sorry.

Stefan

Patch

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index a7f295d..5b7cda4 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -221,8 +221,6 @@  static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
 
     /* Allocate the refcount block itself and mark it as used */
     uint64_t new_block = alloc_clusters_noref(bs, s->cluster_size);
-    memset(s->refcount_block_cache, 0, s->cluster_size);
-    s->refcount_block_cache_offset = new_block;
 
 #ifdef DEBUG_ALLOC2
     fprintf(stderr, "qcow2: Allocate refcount block %d for %" PRIx64
@@ -231,6 +229,10 @@  static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
 #endif
 
     if (in_same_refcount_block(s, new_block, cluster_index << s->cluster_bits)) {
+        /* Zero the new refcount block before updating it */
+        memset(s->refcount_block_cache, 0, s->cluster_size);
+        s->refcount_block_cache_offset = new_block;
+
         /* The block describes itself, need to update the cache */
         int block_index = (new_block >> s->cluster_bits) &
             ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
@@ -242,6 +244,11 @@  static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
         if (ret < 0) {
             goto fail_block;
         }
+
+        /* Initialize the new refcount block only after updating its refcount,
+         * update_refcount uses the refcount cache itself */
+        memset(s->refcount_block_cache, 0, s->cluster_size);
+        s->refcount_block_cache_offset = new_block;
     }
 
     /* Now the new refcount block needs to be written to disk */