Patchwork [v3,2/3] block: Add bdrv_change_backing_file

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Submitter Kevin Wolf
Date Dec. 17, 2009, 2:40 p.m.
Message ID <1261060822-23219-3-git-send-email-kwolf@redhat.com>
Download mbox | patch
Permalink /patch/41320/
State New
Headers show

Comments

Kevin Wolf - Dec. 17, 2009, 2:40 p.m.
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>
---
 block.c       |   20 +++++++++++++
 block.h       |    2 +
 block/qcow2.c |   88 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 block_int.h   |    3 ++
 4 files changed, 113 insertions(+), 0 deletions(-)

Patch

diff --git a/block.c b/block.c
index 20a0de2..faff40e 100644
--- a/block.c
+++ b/block.c
@@ -597,6 +597,26 @@  int bdrv_commit(BlockDriverState *bs)
     return 0;
 }
 
+/*
+ * Return values:
+ * 0        - success
+ * -EINVAL  - backing format specified, but no file
+ * -ENOSPC  - can't update the backing file because no space is left in the
+ *            image file header
+ * -ENOTSUP - format driver doesn't support changing the backing file
+ */
+int bdrv_change_backing_file(BlockDriverState *bs,
+    const char *backing_file, const char *backing_fmt)
+{
+    BlockDriver *drv = bs->drv;
+
+    if (drv->bdrv_change_backing_file != NULL) {
+        return drv->bdrv_change_backing_file(bs, backing_file, backing_fmt);
+    } else {
+        return -ENOTSUP;
+    }
+}
+
 static int bdrv_check_byte_request(BlockDriverState *bs, int64_t offset,
                                    size_t size)
 {
diff --git a/block.h b/block.h
index f660d5f..bee9ec5 100644
--- a/block.h
+++ b/block.h
@@ -83,6 +83,8 @@  int64_t bdrv_getlength(BlockDriverState *bs);
 void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr);
 void bdrv_guess_geometry(BlockDriverState *bs, int *pcyls, int *pheads, int *psecs);
 int bdrv_commit(BlockDriverState *bs);
+int bdrv_change_backing_file(BlockDriverState *bs,
+    const char *backing_file, const char *backing_fmt);
 void bdrv_register(BlockDriver *bdrv);
 
 /* async block I/O */
diff --git a/block/qcow2.c b/block/qcow2.c
index 984264b..7b29094 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -658,6 +658,92 @@  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};
+
+    /* 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;
+    }
+
+    bdrv_pwrite(s->hd, sizeof(QCowHeader), buf, ext_size);
+
+    /* 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);
+
+    bdrv_pwrite(s->hd, offsetof(QCowHeader, backing_file_offset),
+        &be_backing_file_offset, sizeof(uint64_t));
+    bdrv_pwrite(s->hd, offsetof(QCowHeader, backing_file_size),
+        &be_backing_file_size, sizeof(uint32_t));
+
+    return 0;
+}
+
+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 +1223,8 @@  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,
 };
diff --git a/block_int.h b/block_int.h
index 9a3b2e0..a0ebd90 100644
--- a/block_int.h
+++ b/block_int.h
@@ -98,6 +98,9 @@  struct BlockDriver {
     int (*bdrv_load_vmstate)(BlockDriverState *bs, uint8_t *buf,
                              int64_t pos, int size);
 
+    int (*bdrv_change_backing_file)(BlockDriverState *bs,
+        const char *backing_file, const char *backing_fmt);
+
     /* removable device specific */
     int (*bdrv_is_inserted)(BlockDriverState *bs);
     int (*bdrv_media_changed)(BlockDriverState *bs);