[Qemu-block] [PATCH for-4.2 08/13] qcow2: Fix broken snapshot table entries

[Max Reitz]

The only case where we currently reject snapshot table entries is when
they have too much extra data.  Fix them with qemu-img check -r all by
counting it as a corruption, reducing their extra_data_size, and then
letting qcow2_check_fix_snapshot_table() do the rest.

Signed-off-by: Max Reitz <address@hidden>
---
 block/qcow2-snapshot.c | 69 ++++++++++++++++++++++++++++++++++--------
 1 file changed, 56 insertions(+), 13 deletions(-)

diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index 58609b6ea4..9956c32964 100644
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -44,7 +44,23 @@ void qcow2_free_snapshots(BlockDriverState *bs)
     s->nb_snapshots = 0;
 }
 
-int qcow2_read_snapshots(BlockDriverState *bs, Error **errp)
+/*
+ * If @repair is true, try to repair a broken snapshot table instead
+ * of just returning an error:
+ *
+ * - If there were snapshots with too much extra metadata, increment
+ *   *extra_data_dropped for each.
+ *   This requires the caller to eventually rewrite the whole snapshot
+ *   table, which requires cluster allocation.  Therefore, this should
+ *   be done only after qcow2_check_refcounts() made sure the refcount
+ *   structures are valid.
+ *   (In the meantime, the image is still valid because
+ *   qcow2_check_refcounts() does not do anything with snapshots'
+ *   extra data.)
+ */
+static int qcow2_do_read_snapshots(BlockDriverState *bs, bool repair,
+                                   int *extra_data_dropped,
+                                   Error **errp)
 {
     BDRVQcow2State *s = bs->opaque;
     QCowSnapshotHeader h;
@@ -64,6 +80,8 @@ int qcow2_read_snapshots(BlockDriverState *bs, Error **errp)
     s->snapshots = g_new0(QCowSnapshot, s->nb_snapshots);
 
     for(i = 0; i < s->nb_snapshots; i++) {
+        bool discard_unknown_extra_data = false;
+
         /* Read statically sized part of the snapshot header */
         offset = ROUND_UP(offset, 8);
         ret = bdrv_pread(bs->file, offset, &h, sizeof(h));
@@ -86,10 +104,21 @@ int qcow2_read_snapshots(BlockDriverState *bs, Error 
**errp)
         name_size = be16_to_cpu(h.name_size);
 
         if (sn->extra_data_size > QCOW_MAX_SNAPSHOT_EXTRA_DATA) {
-            ret = -EFBIG;
-            error_setg(errp, "Too much extra metadata in snapshot table "
-                       "entry %i", i);
-            goto fail;
+            if (!repair) {
+                ret = -EFBIG;
+                error_setg(errp, "Too much extra metadata in snapshot table "
+                           "entry %i", i);
+                error_append_hint(errp, "You can force-remove this extra "
+                                  "metadata with qemu-img check -r all\n");
+                goto fail;
+            }
+
+            fprintf(stderr, "Discarding too much extra metadata in snapshot "
+                    "table entry %i (%" PRIu32 " > %u)\n",
+                    i, sn->extra_data_size, QCOW_MAX_SNAPSHOT_EXTRA_DATA);
+
+            (*extra_data_dropped)++;
+            discard_unknown_extra_data = true;
         }
 
         /* Read known extra data */
@@ -112,16 +141,22 @@ int qcow2_read_snapshots(BlockDriverState *bs, Error 
**errp)
         }
 
         if (sn->extra_data_size > sizeof(extra)) {
-            /* Store unknown extra data */
             size_t unknown_extra_data_size =
                 sn->extra_data_size - sizeof(extra);
 
-            sn->unknown_extra_data = g_malloc(unknown_extra_data_size);
-            ret = bdrv_pread(bs->file, offset, sn->unknown_extra_data,
-                             unknown_extra_data_size);
-            if (ret < 0) {
-                error_setg_errno(errp, -ret, "Failed to read snapshot table");
-                goto fail;
+            if (discard_unknown_extra_data) {
+                /* Discard unknown extra data */
+                sn->extra_data_size = sizeof(extra);
+            } else {
+                /* Store unknown extra data */
+                sn->unknown_extra_data = g_malloc(unknown_extra_data_size);
+                ret = bdrv_pread(bs->file, offset, sn->unknown_extra_data,
+                                 unknown_extra_data_size);
+                if (ret < 0) {
+                    error_setg_errno(errp, -ret,
+                                     "Failed to read snapshot table");
+                    goto fail;
+                }
             }
             offset += unknown_extra_data_size;
         }
@@ -162,6 +197,11 @@ fail:
     return ret;
 }
 
+int qcow2_read_snapshots(BlockDriverState *bs, Error **errp)
+{
+    return qcow2_do_read_snapshots(bs, false, NULL, errp);
+}
+
 /* add at the end of the file a new list of snapshots */
 int qcow2_write_snapshots(BlockDriverState *bs)
 {
@@ -327,6 +367,7 @@ int coroutine_fn 
qcow2_check_read_snapshot_table(BlockDriverState *bs,
 {
     BDRVQcow2State *s = bs->opaque;
     Error *local_err = NULL;
+    int extra_data_dropped = 0;
     int ret;
     struct {
         uint32_t nb_snapshots;
@@ -362,7 +403,8 @@ int coroutine_fn 
qcow2_check_read_snapshot_table(BlockDriverState *bs,
     }
 
     qemu_co_mutex_unlock(&s->lock);
-    ret = qcow2_read_snapshots(bs, &local_err);
+    ret = qcow2_do_read_snapshots(bs, fix & BDRV_FIX_ERRORS,
+                                  &extra_data_dropped, &local_err);
     qemu_co_mutex_lock(&s->lock);
     if (ret < 0) {
         result->check_errors++;
@@ -375,6 +417,7 @@ int coroutine_fn 
qcow2_check_read_snapshot_table(BlockDriverState *bs,
 
         return ret;
     }
+    result->corruptions += extra_data_dropped;
 
     return 0;
 }
-- 
2.21.0