From 5bb37d151b026759ee35f04212b11b4d625c7431 Mon Sep 17 00:00:00 2001

From: Kevin Wolf <kwolf@redhat.com>

Date: Wed, 7 Dec 2011 12:42:10 +0100

Subject: [PATCH 23/25] qemu-img rebase: Fix for undersized backing files

Backing files may be smaller than the corresponding COW file. When

reading directly from the backing file, qemu-img rebase must consider

this and assume zero sectors after the end of backing files.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

---

 qemu-img.c |   42 +++++++++++++++++++++++++++++++++---------

 1 files changed, 33 insertions(+), 9 deletions(-)

diff --git a/qemu-img.c b/qemu-img.c

index 8bdae66..01cc0d3 100644

--- a/qemu-img.c

+++ b/qemu-img.c

@@ -1420,6 +1420,8 @@ static int img_rebase(int argc, char **argv)

      */

     if (!unsafe) {

         uint64_t num_sectors;

+        uint64_t old_backing_num_sectors;

+        uint64_t new_backing_num_sectors;

         uint64_t sector;

         int n;

         uint8_t * buf_old;

@@ -1430,6 +1432,8 @@ static int img_rebase(int argc, char **argv)

         buf_new = qemu_blockalign(bs, IO_BUF_SIZE);

         bdrv_get_geometry(bs, &num_sectors);

+        bdrv_get_geometry(bs_old_backing, &old_backing_num_sectors);

+        bdrv_get_geometry(bs_new_backing, &new_backing_num_sectors);

         local_progress = (float)100 /

             (num_sectors / MIN(num_sectors, IO_BUF_SIZE / 512));

@@ -1448,16 +1452,36 @@ static int img_rebase(int argc, char **argv)

                 continue;

             }

-            /* Read old and new backing file */

-            ret = bdrv_read(bs_old_backing, sector, buf_old, n);

-            if (ret < 0) {

-                error_report("error while reading from old backing file");

-                goto out;

+            /*

+             * Read old and new backing file and take into consideration that

+             * backing files may be smaller than the COW image.

+             */

+            if (sector >= old_backing_num_sectors) {

+                memset(buf_old, 0, n * BDRV_SECTOR_SIZE);

+            } else {

+                if (sector + n > old_backing_num_sectors) {

+                    n = old_backing_num_sectors - sector;

+                }

+

+                ret = bdrv_read(bs_old_backing, sector, buf_old, n);

+                if (ret < 0) {

+                    error_report("error while reading from old backing file");

+                    goto out;

+                }

             }

-            ret = bdrv_read(bs_new_backing, sector, buf_new, n);

-            if (ret < 0) {

-                error_report("error while reading from new backing file");

-                goto out;

+

+            if (sector >= new_backing_num_sectors) {

+                memset(buf_new, 0, n * BDRV_SECTOR_SIZE);

+            } else {

+                if (sector + n > new_backing_num_sectors) {

+                    n = new_backing_num_sectors - sector;

+                }

+

+                ret = bdrv_read(bs_new_backing, sector, buf_new, n);

+                if (ret < 0) {

+                    error_report("error while reading from new backing file");

+                    goto out;

+                }

             }

             /* If they differ, we need to write to the COW file */

-- 

1.7.7.5