[Qemu-devel] [PATCH 2/2] qcow2: Keep unknown header extension when rewriting header

[Kevin Wolf <kwolf@redhat.com>]

If we want header extensions to work as compatible extensions, we can't
destroy yet unknown header extensions when rewriting the header (e.g.
for changing the backing file). Save all unknown header extensions in a
list of blobs and include them in a new header.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2.c |   44 ++++++++++++++++++++++++++++++++++++++++++--
 block/qcow2.h |    8 ++++++++
 2 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 9f8c2de..3692b45 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -77,8 +77,10 @@ static int qcow2_probe(const uint8_t *buf, int buf_size, const char *filename)
 static int qcow2_read_extensions(BlockDriverState *bs, uint64_t start_offset,
                                  uint64_t end_offset)
 {
+    BDRVQcowState *s = bs->opaque;
     QCowExtension ext;
     uint64_t offset;
+    int ret;
 
 #ifdef DEBUG_EXT
     printf("qcow2_read_extensions: start=%ld end=%ld\n", start_offset, end_offset);
@@ -129,8 +131,22 @@ static int qcow2_read_extensions(BlockDriverState *bs, uint64_t start_offset,
             break;
 
         default:
-            /* unknown magic -- just skip it */
-            offset = ((offset + ext.len + 7) & ~7);
+            /* unknown magic - save it in case we need to rewrite the header */
+            {
+                Qcow2UnknownHeaderExtension *uext;
+
+                uext = g_malloc0(sizeof(*uext)  + ext.len);
+                uext->magic = ext.magic;
+                uext->len = ext.len;
+                QLIST_INSERT_HEAD(&s->unknown_header_ext, uext, next);
+
+                ret = bdrv_pread(bs->file, offset , uext->data, uext->len);
+                if (ret < 0) {
+                    return ret;
+                }
+
+                offset = ((offset + ext.len + 7) & ~7);
+            }
             break;
         }
     }
@@ -138,6 +154,16 @@ static int qcow2_read_extensions(BlockDriverState *bs, uint64_t start_offset,
     return 0;
 }
 
+static void cleanup_unknown_header_ext(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    Qcow2UnknownHeaderExtension *uext, *next;
+
+    QLIST_FOREACH_SAFE(uext, &s->unknown_header_ext, next, next) {
+        QLIST_REMOVE(uext, next);
+        g_free(uext);
+    }
+}
 
 static int qcow2_open(BlockDriverState *bs, int flags)
 {
@@ -291,6 +317,7 @@ static int qcow2_open(BlockDriverState *bs, int flags)
     return ret;
 
  fail:
+    cleanup_unknown_header_ext(bs);
     qcow2_free_snapshots(bs);
     qcow2_refcount_close(bs);
     g_free(s->l1_table);
@@ -632,6 +659,7 @@ static void qcow2_close(BlockDriverState *bs)
     qcow2_cache_destroy(bs, s->l2_table_cache);
     qcow2_cache_destroy(bs, s->refcount_block_cache);
 
+    cleanup_unknown_header_ext(bs);
     g_free(s->cluster_cache);
     qemu_vfree(s->cluster_data);
     qcow2_refcount_close(bs);
@@ -705,6 +733,7 @@ int qcow2_update_header(BlockDriverState *bs)
     int ret;
     uint64_t total_size;
     uint32_t refcount_table_clusters;
+    Qcow2UnknownHeaderExtension *uext;
 
     buf = qemu_blockalign(bs, buflen);
     memset(buf, 0, s->cluster_size);
@@ -752,6 +781,17 @@ int qcow2_update_header(BlockDriverState *bs)
         buflen -= ret;
     }
 
+    /* Keep unknown header extensions */
+    QLIST_FOREACH(uext, &s->unknown_header_ext, next) {
+        ret = header_ext_add(buf, uext->magic, uext->data, uext->len, buflen);
+        if (ret < 0) {
+            goto fail;
+        }
+
+        buf += ret;
+        buflen -= ret;
+    }
+
     /* End of header extensions */
     ret = header_ext_add(buf, QCOW2_EXT_MAGIC_END, NULL, 0, buflen);
     if (ret < 0) {
diff --git a/block/qcow2.h b/block/qcow2.h
index aae5f89..fc35838 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -87,6 +87,13 @@ typedef struct QCowSnapshot {
 struct Qcow2Cache;
 typedef struct Qcow2Cache Qcow2Cache;
 
+typedef struct Qcow2UnknownHeaderExtension {
+    uint32_t magic;
+    uint32_t len;
+    QLIST_ENTRY(Qcow2UnknownHeaderExtension) next;
+    uint8_t data[];
+} Qcow2UnknownHeaderExtension;
+
 typedef struct BDRVQcowState {
     int cluster_bits;
     int cluster_size;
@@ -127,6 +134,7 @@ typedef struct BDRVQcowState {
     QCowSnapshot *snapshots;
 
     int flags;
+    QLIST_HEAD(, Qcow2UnknownHeaderExtension) unknown_header_ext;
 } BDRVQcowState;
 
 /* XXX: use std qcow open function ? */
-- 
1.7.6.5