qcow2: Update whole header at once

In order to switch the backing file, qcow2 issues multiple write
requests that only changed a part of the image header. Any failure after
the first one would leave the header in an corrupted state. With this
patch, the whole header is written at once, so we can't fail in the
middle.

At the same time, this gives us a reusable functions that updates all
fields of the qcow2 header and not only the backing file.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -669,103 +669,137 @@ static void qcow2_invalidate_cache(BlockDriverState *bs)
     }
 }
 
-/*
- * 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)
+static size_t header_ext_add(char *buf, uint32_t magic, const void *s,
+    size_t len, size_t buflen)
 {
-    size_t backing_file_len = 0;
-    size_t backing_fmt_len = 0;
-    BDRVQcowState *s = bs->opaque;
-    QCowExtension ext_backing_fmt = {0, 0};
-    int ret;
+    QCowExtension *ext_backing_fmt = (QCowExtension*) buf;
+    size_t ext_len = sizeof(QCowExtension) + ((len + 7) & ~7);
 
-    /* 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(QCOW2_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) {
+    if (buflen < ext_len) {
         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;
+    *ext_backing_fmt = (QCowExtension) {
+        .magic  = cpu_to_be32(magic),
+        .len    = cpu_to_be32(len),
+    };
+    memcpy(buf + sizeof(QCowExtension), s, len);
 
-    if (backing_file) {
-        if (backing_fmt) {
-            int padding = backing_fmt_len -
-                (sizeof(ext_backing_fmt) + strlen(backing_fmt));
+    return ext_len;
+}
 
-            memcpy(buf + offset, &ext_backing_fmt, sizeof(ext_backing_fmt));
-            offset += sizeof(ext_backing_fmt);
+/*
+ * Updates the qcow2 header, including the variable length parts of it, 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.
+ */
+int qcow2_update_header(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    QCowHeader *header;
+    char *buf;
+    size_t buflen = s->cluster_size;
+    int ret;
+    uint64_t total_size;
+    uint32_t refcount_table_clusters;
 
-            memcpy(buf + offset, backing_fmt, strlen(backing_fmt));
-            offset += strlen(backing_fmt);
+    buf = qemu_blockalign(bs, buflen);
+    memset(buf, 0, s->cluster_size);
 
-            memset(buf + offset, 0, padding);
-            offset += padding;
+    /* Header structure */
+    header = (QCowHeader*) buf;
+
+    if (buflen < sizeof(*header)) {
+        ret = -ENOSPC;
+        goto fail;
+    }
+
+    total_size = bs->total_sectors * BDRV_SECTOR_SIZE;
+    refcount_table_clusters = s->refcount_table_size >> (s->cluster_bits - 3);
+
+    *header = (QCowHeader) {
+        .magic                  = cpu_to_be32(QCOW_MAGIC),
+        .version                = cpu_to_be32(QCOW_VERSION),
+        .backing_file_offset    = 0,
+        .backing_file_size      = 0,
+        .cluster_bits           = cpu_to_be32(s->cluster_bits),
+        .size                   = cpu_to_be64(total_size),
+        .crypt_method           = cpu_to_be32(s->crypt_method_header),
+        .l1_size                = cpu_to_be32(s->l1_size),
+        .l1_table_offset        = cpu_to_be64(s->l1_table_offset),
+        .refcount_table_offset  = cpu_to_be64(s->refcount_table_offset),
+        .refcount_table_clusters = cpu_to_be32(refcount_table_clusters),
+        .nb_snapshots           = cpu_to_be32(s->nb_snapshots),
+        .snapshots_offset       = cpu_to_be64(s->snapshots_offset),
+    };
+
+    buf += sizeof(*header);
+    buflen -= sizeof(*header);
+
+    /* Backing file format header extension */
+    if (*bs->backing_format) {
+        ret = header_ext_add(buf, QCOW2_EXT_MAGIC_BACKING_FORMAT,
+                             bs->backing_format, strlen(bs->backing_format),
+                             buflen);
+        if (ret < 0) {
+            goto fail;
         }
 
-        memcpy(buf + offset, backing_file, backing_file_len);
-        backing_file_offset = sizeof(QCowHeader) + offset;
+        buf += ret;
+        buflen -= ret;
     }
 
-    ret = bdrv_pwrite_sync(bs->file, sizeof(QCowHeader), buf, ext_size);
+    /* End of header extensions */
+    ret = header_ext_add(buf, QCOW2_EXT_MAGIC_END, NULL, 0, buflen);
     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);
+    buf += ret;
+    buflen -= ret;
 
-    ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, backing_file_offset),
-        &be_backing_file_offset, sizeof(uint64_t));
-    if (ret < 0) {
-        goto fail;
+    /* Backing file name */
+    if (*bs->backing_file) {
+        size_t backing_file_len = strlen(bs->backing_file);
+
+        if (buflen < backing_file_len) {
+            ret = -ENOSPC;
+            goto fail;
+        }
+
+        strncpy(buf, bs->backing_file, buflen);
+
+        header->backing_file_offset = cpu_to_be64(buf - ((char*) header));
+        header->backing_file_size   = cpu_to_be32(backing_file_len);
     }
 
-    ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, backing_file_size),
-        &be_backing_file_size, sizeof(uint32_t));
+    /* Write the new header */
+    ret = bdrv_pwrite(bs->file, 0, header, s->cluster_size);
     if (ret < 0) {
         goto fail;
     }
 
     ret = 0;
 fail:
+    qemu_vfree(header);
     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);
+    /* Backing file format doesn't make sense without a backing file */
+    if (backing_fmt && !backing_file) {
+        return -EINVAL;
+    }
+
+    pstrcpy(bs->backing_file, sizeof(bs->backing_file), backing_file ?: "");
+    pstrcpy(bs->backing_format, sizeof(bs->backing_format), backing_fmt ?: "");
+
+    return qcow2_update_header(bs);
 }
 
 static int preallocate(BlockDriverState *bs)

block/qcow2.h

@@ -178,6 +178,7 @@ static inline int64_t align_offset(int64_t offset, int n)
 /* qcow2.c functions */
 int qcow2_backing_read1(BlockDriverState *bs, QEMUIOVector *qiov,
                   int64_t sector_num, int nb_sectors);
+int qcow2_update_header(BlockDriverState *bs);
 
 /* qcow2-refcount.c functions */
 int qcow2_refcount_init(BlockDriverState *bs);