[Qemu-devel] [PATCH] qemu-img: refuse to compress files with invalid size

[Stefan Hajnoczi]

Image file compression works at cluster granularity.  It is not possible
to compress less than a cluster of data at a time.

Print an error when attempting qemu-img convert -c with an input file
that is not a multiple of the cluster size.

I considered automatically adjusting the output file size but think it's
better to be explicit.  This avoids confusion when users notice that
image file size changed after conversion.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 qemu-img.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/qemu-img.c b/qemu-img.c
index 31627b0..2273851 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1370,6 +1370,13 @@ static int img_convert(int argc, char **argv)
             goto out;
         }
         cluster_sectors = cluster_size >> 9;
+        if (total_sectors % cluster_sectors) {
+            error_report("compression requires that input file size is a "
+                         "multiple of %d bytes",
+                         cluster_size);
+            ret = -1;
+            goto out;
+        }
         sector_num = 0;
 
         nb_sectors = total_sectors;
-- 
1.8.1.4