[libvirt] [PATCH] util: storage: Fix qcow(2) header parser according to docs

[Peter Krempa]

The backing store string location offset 0 determines that the file
isn't present. The string size shouldn't be then checked:

from qemu.git/docs/specs/qcow2.txt

== Header ==

The first cluster of a qcow2 image contains the file header:

Byte  0 -  3:   magic
                QCOW magic string ("QFI\xfb")

      4 -  7:   version
                Version number (valid values are 2 and 3)

      8 - 15:   backing_file_offset
                Offset into the image file at which the backing file name
                is stored (NB: The string is not null terminated). 0 if the
                image doesn't have a backing file.

     16 - 19:   backing_file_size
                Length of the backing file name in bytes. Must not be
                longer than 1023 bytes. Undefined if the image doesn't have
                a backing file.         ^^^^^^^^^

This patch intentionally leaves the backing file string size check in
place in case a malformatted file would be presented to libvirt. Also
according to the docs the string size is maximum 1023 bytes, thus this
patch adds a check to verify that.

I was also able to verify that the check was done the same way in the
legacy qcow fromat (in qemu's code).
---
 src/util/virstoragefile.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/src/util/virstoragefile.c b/src/util/virstoragefile.c
index 5db9184..13056a7 100644
--- a/src/util/virstoragefile.c
+++ b/src/util/virstoragefile.c
@@ -385,15 +385,22 @@ qcowXGetBackingStore(char **res,
     offset = virReadBufInt64BE(buf + QCOWX_HDR_BACKING_FILE_OFFSET);
     if (offset > buf_size)
         return BACKING_STORE_INVALID;
+
+    if (offset == 0) {
+        if (format)
+            *format = VIR_STORAGE_FILE_NONE;
+        return BACKING_STORE_OK;
+    }
+
     size = virReadBufInt32BE(buf + QCOWX_HDR_BACKING_FILE_SIZE);
     if (size == 0) {
         if (format)
             *format = VIR_STORAGE_FILE_NONE;
         return BACKING_STORE_OK;
     }
-    if (offset + size > buf_size || offset + size < offset)
+    if (size > 1023)
         return BACKING_STORE_INVALID;
-    if (size + 1 == 0)
+    if (offset + size > buf_size || offset + size < offset)
         return BACKING_STORE_INVALID;
     if (VIR_ALLOC_N(*res, size + 1) < 0)
         return BACKING_STORE_ERROR;
-- 
2.1.0