[Qemu-devel] [PATCH 2/2] qcow2: Fix qcow2_get_cluster_offset()

[Max Reitz <mreitz@redhat.com>]

Recently, qcow2_get_cluster_offset() has been changed to work with bytes
instead of sectors. This invalidated some assertions and introduced a
possible integer multiplication overflow.

This could be reproduced using e.g.

$ qemu-img create -f qcow2 -o cluster_size=1M blub.qcow2 8G
Formatting 'foo.qcow2', fmt=qcow2 size=8589934592 encryption=off
cluster_size=1048576 lazy_refcounts=off refcount_bits=16
$ qemu-io -c map blub.qcow2
qemu-io: qemu/block/qcow2-cluster.c:504: qcow2_get_cluster_offset:
Assertion `bytes_needed <= INT_MAX' failed.
[1]    20775 abort (core dumped)  qemu-io -c map foo.qcow2

This patch removes the now wrong assertion, adding comments and more
assertions to prove its correctness (and fixing the overflow which would
become apparent with the original assertion removed).

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2-cluster.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 893ddf6..b942fb5 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -484,8 +484,8 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
     unsigned int l2_index;
     uint64_t l1_index, l2_offset, *l2_table;
     int l1_bits, c;
-    unsigned int offset_in_cluster, nb_clusters;
-    uint64_t bytes_available, bytes_needed;
+    unsigned int offset_in_cluster;
+    uint64_t bytes_available, bytes_needed, nb_clusters;
     int ret;
 
     offset_in_cluster = offset_into_cluster(s, offset);
@@ -501,7 +501,6 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
     if (bytes_needed > bytes_available) {
         bytes_needed = bytes_available;
     }
-    assert(bytes_needed <= INT_MAX);
 
     *cluster_offset = 0;
 
@@ -538,8 +537,11 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
     l2_index = (offset >> s->cluster_bits) & (s->l2_size - 1);
     *cluster_offset = be64_to_cpu(l2_table[l2_index]);
 
-    /* nb_needed <= INT_MAX, thus nb_clusters <= INT_MAX, too */
     nb_clusters = size_to_clusters(s, bytes_needed);
+    /* bytes_needed <= *bytes + offset_in_cluster, both of which are unsigned
+     * integers; the minimum cluster size is 512, so this assertion is always
+     * true */
+    assert(nb_clusters <= INT_MAX);
 
     ret = qcow2_get_cluster_type(*cluster_offset);
     switch (ret) {
@@ -586,13 +588,17 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
 
     qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
 
-    bytes_available = (c * s->cluster_size);
+    bytes_available = (int64_t)c * s->cluster_size;
 
 out:
     if (bytes_available > bytes_needed) {
         bytes_available = bytes_needed;
     }
 
+    /* bytes_available <= bytes_needed <= *bytes + offset_in_cluster;
+     * subtracting offset_in_cluster will therefore definitely yield something
+     * not exceeding UINT_MAX */
+    assert(bytes_available - offset_in_cluster <= UINT_MAX);
     *bytes = bytes_available - offset_in_cluster;
 
     return ret;
-- 
2.8.3