block: Add bdrv_change_backing_file

Introduce the functions needed to change the backing file of an image. The
function is implemented for qcow2.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

1 files changed, 101 insertions, 0 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index 984264b3ea..4f9ef91715 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -658,6 +658,105 @@ static void qcow_close(BlockDriverState *bs)
     bdrv_delete(s->hd);
 }
 
+/*
+ * Updates the variable length parts of the qcow2 header, i.e. the backing file
+ * name and all extensions. qcow2 was not designed to allow such changes, so if
+ * we run out of space (we can only use the first cluster) this function may
+ * fail.
+ *
+ * Returns 0 on success, -errno in error cases.
+ */
+static int qcow2_update_ext_header(BlockDriverState *bs,
+    const char *backing_file, const char *backing_fmt)
+{
+    size_t backing_file_len = 0;
+    size_t backing_fmt_len = 0;
+    BDRVQcowState *s = bs->opaque;
+    QCowExtension ext_backing_fmt = {0, 0};
+    int ret;
+
+    /* Backing file format doesn't make sense without a backing file */
+    if (backing_fmt && !backing_file) {
+        return -EINVAL;
+    }
+
+    /* Prepare the backing file format extension if needed */
+    if (backing_fmt) {
+        ext_backing_fmt.len = cpu_to_be32(strlen(backing_fmt));
+        ext_backing_fmt.magic = cpu_to_be32(QCOW_EXT_MAGIC_BACKING_FORMAT);
+        backing_fmt_len = ((sizeof(ext_backing_fmt)
+            + strlen(backing_fmt) + 7) & ~7);
+    }
+
+    /* Check if we can fit the new header into the first cluster */
+    if (backing_file) {
+        backing_file_len = strlen(backing_file);
+    }
+
+    size_t header_size = sizeof(QCowHeader) + backing_file_len
+        + backing_fmt_len;
+
+    if (header_size > s->cluster_size) {
+        return -ENOSPC;
+    }
+
+    /* Rewrite backing file name and qcow2 extensions */
+    size_t ext_size = header_size - sizeof(QCowHeader);
+    uint8_t buf[ext_size];
+    size_t offset = 0;
+    size_t backing_file_offset = 0;
+
+    if (backing_file) {
+        if (backing_fmt) {
+            int padding = backing_fmt_len -
+                (sizeof(ext_backing_fmt) + strlen(backing_fmt));
+
+            memcpy(buf + offset, &ext_backing_fmt, sizeof(ext_backing_fmt));
+            offset += sizeof(ext_backing_fmt);
+
+            memcpy(buf + offset, backing_fmt, strlen(backing_fmt));
+            offset += strlen(backing_fmt);
+
+            memset(buf + offset, 0, padding);
+            offset += padding;
+        }
+
+        memcpy(buf + offset, backing_file, backing_file_len);
+        backing_file_offset = sizeof(QCowHeader) + offset;
+    }
+
+    ret = bdrv_pwrite(s->hd, sizeof(QCowHeader), buf, ext_size);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    /* Update header fields */
+    uint64_t be_backing_file_offset = cpu_to_be64(backing_file_offset);
+    uint32_t be_backing_file_size = cpu_to_be32(backing_file_len);
+
+    ret = bdrv_pwrite(s->hd, offsetof(QCowHeader, backing_file_offset),
+        &be_backing_file_offset, sizeof(uint64_t));
+    if (ret < 0) {
+        goto fail;
+    }
+
+    ret = bdrv_pwrite(s->hd, offsetof(QCowHeader, backing_file_size),
+        &be_backing_file_size, sizeof(uint32_t));
+    if (ret < 0) {
+        goto fail;
+    }
+
+    ret = 0;
+fail:
+    return ret;
+}
+
+static int qcow2_change_backing_file(BlockDriverState *bs,
+    const char *backing_file, const char *backing_fmt)
+{
+    return qcow2_update_ext_header(bs, backing_file, backing_fmt);
+}
+
 static int get_bits_from_size(size_t size)
 {
     int res = 0;
@@ -1137,6 +1236,8 @@ static BlockDriver bdrv_qcow2 = {
     .bdrv_save_vmstate    = qcow_save_vmstate,
     .bdrv_load_vmstate    = qcow_load_vmstate,
 
+    .bdrv_change_backing_file   = qcow2_change_backing_file,
+
     .create_options = qcow_create_options,
     .bdrv_check = qcow_check,
 };