[PATCH] xenconfig: xm: check for driver on disk format

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

When reviewing libxl vif typename series[0] I found out a bug
on xen-xm formatter where this "virsh domxml-to-native xen-xm file.xml"
can lead to a NULL dereference if the disk driver isn't specified.
Fix this by checking for driver before writing/testing it down.

[0] https://www.redhat.com/archives/libvir-list/2016-April/msg01434.html

Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
---
 src/xenconfig/xen_xm.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/xenconfig/xen_xm.c b/src/xenconfig/xen_xm.c
index 34d57de..3658c59 100644
--- a/src/xenconfig/xen_xm.c
+++ b/src/xenconfig/xen_xm.c
@@ -297,9 +297,12 @@ xenFormatXMDisk(virConfValuePtr list,
                 type = "aio";
             else
                 type = virStorageFileFormatTypeToString(format);
-            virBufferAsprintf(&buf, "%s:", driver);
-            if (STREQ(driver, "tap"))
-                virBufferAsprintf(&buf, "%s:", type);
+
+            if (driver) {
+                virBufferAsprintf(&buf, "%s:", driver);
+                if (STREQ(driver, "tap"))
+                    virBufferAsprintf(&buf, "%s:", type);
+            }
         } else {
             switch (virDomainDiskGetType(disk)) {
             case VIR_STORAGE_TYPE_FILE:
-- 
2.1.4

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