migration/savevm: Add a compatibility check for capabilities

It has always been possible to enable arbitrary migration capabilities
and attempt to take a snapshot of the VM with the savevm/loadvm
commands as well as their QMP counterparts
snapshot-save/snapshot-load.

Most migration capabilities are not meant to be used with snapshots
and there's a risk of crashing QEMU or producing incorrect
behavior. Ideally, every migration capability would either be
implemented for savevm or explicitly rejected.

Add a compatibility check routine and reject the snapshot command if
an incompatible capability is enabled. For now only act on the the two
that actually cause a crash: multifd and mapped-ram.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2881
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20251007184213.5990-1-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/options.c

@@ -445,11 +445,38 @@ INITIALIZE_MIGRATE_CAPS_SET(check_caps_background_snapshot,
     MIGRATION_CAPABILITY_VALIDATE_UUID,
     MIGRATION_CAPABILITY_ZERO_COPY_SEND);
 
+/* Snapshot compatibility check list */
+static const
+INITIALIZE_MIGRATE_CAPS_SET(check_caps_savevm,
+                            MIGRATION_CAPABILITY_MULTIFD,
+                            MIGRATION_CAPABILITY_MAPPED_RAM,
+);
+
 static bool migrate_incoming_started(void)
 {
     return !!migration_incoming_get_current()->transport_data;
 }
 
+bool migrate_can_snapshot(Error **errp)
+{
+    MigrationState *s = migrate_get_current();
+    int i;
+
+    for (i = 0; i < check_caps_savevm.size; i++) {
+        int incomp_cap = check_caps_savevm.caps[i];
+
+        if (s->capabilities[incomp_cap]) {
+            error_setg(errp,
+                       "Snapshots are not compatible with %s",
+                       MigrationCapability_str(incomp_cap));
+            return false;
+        }
+    }
+
+    return true;
+}
+
+
 bool migrate_rdma_caps_check(bool *caps, Error **errp)
 {
     if (caps[MIGRATION_CAPABILITY_XBZRLE]) {

migration/options.h

@@ -59,6 +59,7 @@ bool migrate_tls(void);
 
 bool migrate_rdma_caps_check(bool *caps, Error **errp);
 bool migrate_caps_check(bool *old_caps, bool *new_caps, Error **errp);
+bool migrate_can_snapshot(Error **errp);
 
 /* parameters */
 

migration/savevm.c

@@ -3322,6 +3322,10 @@ bool save_snapshot(const char *name, bool overwrite, const char *vmstate,
 
     GLOBAL_STATE_CODE();
 
+    if (!migrate_can_snapshot(errp)) {
+        return false;
+    }
+
     if (migration_is_blocked(errp)) {
         return false;
     }
@@ -3507,6 +3511,10 @@ bool load_snapshot(const char *name, const char *vmstate,
     int ret;
     MigrationIncomingState *mis = migration_incoming_get_current();
 
+    if (!migrate_can_snapshot(errp)) {
+        return false;
+    }
+
     if (!bdrv_all_can_snapshot(has_devices, devices, errp)) {
         return false;
     }