[Qemu-devel] [PATCH] qcow2: Fix fail path in realloc_refcount_block()

[Max Reitz <mreitz@redhat.com>]

If qcow2_alloc_clusters() fails, new_offset and ret will both be
negative after the fail label, thus passing the first if condition and
subsequently resulting in a call of qcow2_free_clusters() with an
invalid (negative) offset parameter. Fix this by checking for new_offset
being positive instead.

While we're at it, clean up the whole fail path. qcow2_cache_put()
actually can never fail, hence the return value can safely be ignored
(aside from asserting that it indeed did not fail).

Furthermore, there is no reason to give QCOW2_DISCARD_ALWAYS to
qcow2_free_clusters(), a mere QCOW2_DISCARD_OTHER will suffice.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Suggested-by: Laszlo Ersek <lersek@redhat.com>
---
 block/qcow2-refcount.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 6151148..b111319 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -1440,20 +1440,22 @@ static int64_t realloc_refcount_block(BlockDriverState *bs, int reftable_index,
     }
 
 fail:
-    if (new_offset && (ret < 0)) {
-        qcow2_free_clusters(bs, new_offset, s->cluster_size,
-                QCOW2_DISCARD_ALWAYS);
-    }
     if (refcount_block) {
-        if (ret < 0) {
-            qcow2_cache_put(bs, s->refcount_block_cache, &refcount_block);
-        } else {
-            ret = qcow2_cache_put(bs, s->refcount_block_cache, &refcount_block);
-        }
+        /* This should never fail, as it would only do so if the given refcount
+         * block cannot be found in the cache. As this is impossible as long as
+         * there are no bugs, assert the success. */
+        int tmp = qcow2_cache_put(bs, s->refcount_block_cache, &refcount_block);
+        assert(tmp == 0);
     }
+
     if (ret < 0) {
+        if (new_offset > 0) {
+            qcow2_free_clusters(bs, new_offset, s->cluster_size,
+                                QCOW2_DISCARD_OTHER);
+        }
         return ret;
     }
+
     return new_offset;
 }
 
-- 
1.9.0