[libvirt] [PATCH 3/4] snapshot: qemu: Fix segfault and vanishing snapshots when redefining

[Peter Krempa]

When the disk alignment check done while redefining an existing snapshot
failed, the qemu driver attempted to free the existing snapshot. As in
the cleanup path the definition of the snapshot wasn't assigned, the
cleanup code dereferenced a NULL pointer.

This patch changes the behavior on error paths while redefining snapshot
in two ways:

1) On failure, modifications done on the snapshot definiton object are
rolled back.

2) The previous definition of the data isn't freed until it's certain it
won't be needed any more.

This change avoids the segfault and additionaly the snapshot doesn't
vanish if re-definiton fails for some reason.
---
 src/qemu/qemu_driver.c | 51 +++++++++++++++++++++++++++++++++++---------------
 1 file changed, 36 insertions(+), 15 deletions(-)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index 0883a56..4c7558d 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -11408,6 +11408,24 @@ qemuDomainSnapshotCreateXML(virDomainPtr domain,
                 }
             }

+            if (def->dom) {
+                if (def->state == VIR_DOMAIN_DISK_SNAPSHOT ||
+                    def->memory == VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL) {
+                    align_location = VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL;
+                    align_match = false;
+                }
+
+                if (virDomainSnapshotAlignDisks(def, align_location,
+                                                align_match) < 0) {
+                    /* revert stealing of the snapshot domain definition */
+                    if (def->dom && !other->def->dom) {
+                        other->def->dom = def->dom;
+                        def->dom = NULL;
+                    }
+                    goto cleanup;
+                }
+            }
+
             if (other == vm->current_snapshot) {
                 update_current = true;
                 vm->current_snapshot = NULL;
@@ -11417,18 +11435,20 @@ qemuDomainSnapshotCreateXML(virDomainPtr domain,
              * child relations by reusing snap.  */
             virDomainSnapshotDropParent(other);
             virDomainSnapshotDefFree(other->def);
-            other->def = NULL;
+            other->def = def;
+            def = NULL;
             snap = other;
-        }
-        if (def->dom) {
-            if (def->state == VIR_DOMAIN_DISK_SNAPSHOT ||
-                def->memory == VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL) {
-                align_location = VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL;
-                align_match = false;
+        } else {
+            if (def->dom) {
+                if (def->state == VIR_DOMAIN_DISK_SNAPSHOT ||
+                    def->memory == VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL) {
+                    align_location = VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL;
+                    align_match = false;
+                }
+                if (virDomainSnapshotAlignDisks(def, align_location,
+                                                align_match) < 0)
+                    goto cleanup;
             }
-            if (virDomainSnapshotAlignDisks(def, align_location,
-                                            align_match) < 0)
-                goto cleanup;
         }
     } else {
         /* Easiest way to clone inactive portion of vm->def is via
@@ -11463,11 +11483,12 @@ qemuDomainSnapshotCreateXML(virDomainPtr domain,
             goto cleanup;
     }

-    if (snap)
-        snap->def = def;
-    else if (!(snap = virDomainSnapshotAssignDef(vm->snapshots, def)))
-        goto cleanup;
-    def = NULL;
+    if (!snap) {
+        if (!(snap = virDomainSnapshotAssignDef(vm->snapshots, def)))
+            goto cleanup;
+
+        def = NULL;
+    }

     if (update_current)
         snap->def->current = true;
-- 
1.8.0.2