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[PATCH v2 1/4] qcow2: Require that the virtual size is a multiple of the

From: Alberto Garcia
Subject: [PATCH v2 1/4] qcow2: Require that the virtual size is a multiple of the sector size
Date: Thu, 9 Jan 2020 20:12:59 +0100

The qcow2 header specifies the virtual size of the image in bytes, but
BlockDriverState stores it as a number of 512-byte sectors.

If the user tries to create an image with a size that is not a
multiple of the sector size then this is fixed on creation by
silently rounding the image size up (see commit c2eb918e32).
qcow2_co_truncate() is more strict and returns an error instead.

However when an image is opened the virtual size is rounded down,
which means that trying to access the last few advertised bytes will
result in an error. As seen above QEMU cannot create such images and
there's no good use case that would require us to try to handle them
so let's just treat them as unsupported.

Signed-off-by: Alberto Garcia <address@hidden>
 block/qcow2.c              | 7 +++++++
 docs/interop/qcow2.txt     | 3 ++-
 tests/qemu-iotests/080     | 8 ++++++++
 tests/qemu-iotests/080.out | 5 +++++
 4 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 7fbaac8457..87ca2832f0 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1326,6 +1326,13 @@ static int coroutine_fn qcow2_do_open(BlockDriverState 
*bs, QDict *options,
         goto fail;
+    if (!QEMU_IS_ALIGNED(header.size, BDRV_SECTOR_SIZE)) {
+        error_setg(errp, "Virtual size is not a multiple of %u",
+                   (unsigned) BDRV_SECTOR_SIZE);
+        ret = -EINVAL;
+        goto fail;
+    }
     if (header.header_length > sizeof(header)) {
         s->unknown_header_fields_size = header.header_length - sizeof(header);
         s->unknown_header_fields = g_malloc(s->unknown_header_fields_size);
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index af5711e533..891f5662d8 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -40,7 +40,8 @@ The first cluster of a qcow2 image contains the file header:
                     with larger cluster sizes.
          24 - 31:   size
-                    Virtual disk size in bytes.
+                    Virtual disk size in bytes. qemu can only handle
+                    sizes that are a multiple of 512 bytes.
                     Note: qemu has an implementation limit of 32 MB as
                     the maximum L1 table size.  With a 2 MB cluster
diff --git a/tests/qemu-iotests/080 b/tests/qemu-iotests/080
index 4bcb5021e8..6f136d616f 100755
--- a/tests/qemu-iotests/080
+++ b/tests/qemu-iotests/080
@@ -48,6 +48,7 @@ header_size=104
@@ -197,6 +198,13 @@ poke_file "$TEST_IMG" "$offset_snap1_l1_size" 
 { $QEMU_IMG snapshot -d test $TEST_IMG; } 2>&1 | _filter_testdir
+echo "== Image size not a multiple of the sector size =="
+_make_test_img 64k
+echo "Modifying virtual size to 65535 bytes"
+poke_file "$TEST_IMG" "$offset_virtual_size" "\x00\x00\x00\x00\x00\x00\xff\xff"
+{ $QEMU_IMG info $TEST_IMG; } 2>&1 | _filter_testdir | _filter_imgfmt
 # success, all done
 echo "*** done"
 rm -f $seq.full
diff --git a/tests/qemu-iotests/080.out b/tests/qemu-iotests/080.out
index 45ab01db8e..aadc817339 100644
--- a/tests/qemu-iotests/080.out
+++ b/tests/qemu-iotests/080.out
@@ -104,4 +104,9 @@ Data may be corrupted, or further writes to the image may 
corrupt it.
 3 leaked clusters were found on the image.
 This means waste of disk space, but no harm to data.
+== Image size not a multiple of the sector size ==
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=65536
+Modifying virtual size to 65535 bytes
+qemu-img: Could not open 'TEST_DIR/t.IMGFMT': Virtual size is not a multiple 
of 512
 *** done

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