@@ -691,6 +691,10 @@ struct BlockDriverState {
char filename[PATH_MAX];
char backing_file[PATH_MAX]; /* if non zero, the image is a diff of
this file image */
+ /* The backing filename indicated by the image header; if we ever
+ * open this file, then this is replaced by the resulting BDS's
+ * filename (i.e. after a bdrv_refresh_filename() run). */
+ char auto_backing_file[PATH_MAX];
char backing_format[16]; /* if non-zero and backing_file exists */
QDict *full_open_options;
@@ -2279,6 +2279,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
char *bdref_key_dot;
const char *reference = NULL;
int ret = 0;
+ bool implicit_backing = false;
BlockDriverState *backing_hd;
QDict *options;
QDict *tmp_parent_options = NULL;
@@ -2314,6 +2315,16 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
qobject_unref(options);
goto free_exit;
} else {
+ if (qdict_size(options) == 0) {
+ /* If the user specifies options that do not modify the
+ * backing file's behavior, we might still consider it the
+ * implicit backing file. But it's easier this way, and
+ * just specifying some of the backing BDS's options is
+ * only possible with -drive anyway (otherwise the QAPI
+ * schema forces the user to specify everything). */
+ implicit_backing = !strcmp(bs->auto_backing_file, bs->backing_file);
+ }
+
bdrv_get_full_backing_filename(bs, backing_filename, PATH_MAX,
&local_err);
if (local_err) {
@@ -2347,6 +2358,12 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
}
bdrv_set_aio_context(backing_hd, bdrv_get_aio_context(bs));
+ if (implicit_backing) {
+ bdrv_refresh_filename(backing_hd);
+ pstrcpy(bs->auto_backing_file, sizeof(bs->auto_backing_file),
+ backing_hd->filename);
+ }
+
/* Hook up the backing file link; drop our reference, bs owns the
* backing_hd reference now */
bdrv_set_backing_hd(bs, backing_hd, &local_err);
@@ -3660,6 +3677,8 @@ int bdrv_change_backing_file(BlockDriverState *bs,
if (ret == 0) {
pstrcpy(bs->backing_file, sizeof(bs->backing_file), backing_file ?: "");
pstrcpy(bs->backing_format, sizeof(bs->backing_format), backing_fmt ?: "");
+ pstrcpy(bs->auto_backing_file, sizeof(bs->auto_backing_file),
+ backing_file ?: "");
}
return ret;
}
@@ -31,6 +31,7 @@
#include "qemu/module.h"
#include "qemu/option.h"
#include "qemu/bswap.h"
+#include "qemu/cutils.h"
#include <zlib.h>
#include "qapi/qmp/qdict.h"
#include "qapi/qmp/qstring.h"
@@ -298,11 +299,13 @@ static int qcow_open(BlockDriverState *bs, QDict *options, int flags,
goto fail;
}
ret = bdrv_pread(bs->file, header.backing_file_offset,
- bs->backing_file, len);
+ bs->auto_backing_file, len);
if (ret < 0) {
goto fail;
}
- bs->backing_file[len] = '\0';
+ bs->auto_backing_file[len] = '\0';
+ pstrcpy(bs->backing_file, sizeof(bs->backing_file),
+ bs->auto_backing_file);
}
/* Disable migration when qcow images are used */
@@ -1461,13 +1461,15 @@ static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
goto fail;
}
ret = bdrv_pread(bs->file, header.backing_file_offset,
- bs->backing_file, len);
+ bs->auto_backing_file, len);
if (ret < 0) {
error_setg_errno(errp, -ret, "Could not read backing file name");
goto fail;
}
- bs->backing_file[len] = '\0';
- s->image_backing_file = g_strdup(bs->backing_file);
+ bs->auto_backing_file[len] = '\0';
+ pstrcpy(bs->backing_file, sizeof(bs->backing_file),
+ bs->auto_backing_file);
+ s->image_backing_file = g_strdup(bs->auto_backing_file);
}
/* Internal snapshots */
@@ -2456,6 +2458,8 @@ static int qcow2_change_backing_file(BlockDriverState *bs,
return -EINVAL;
}
+ pstrcpy(bs->auto_backing_file, sizeof(bs->auto_backing_file),
+ backing_file ?: "");
pstrcpy(bs->backing_file, sizeof(bs->backing_file), backing_file ?: "");
pstrcpy(bs->backing_format, sizeof(bs->backing_format), backing_fmt ?: "");
@@ -454,11 +454,14 @@ static int coroutine_fn bdrv_qed_do_open(BlockDriverState *bs, QDict *options,
}
ret = qed_read_string(bs->file, s->header.backing_filename_offset,
- s->header.backing_filename_size, bs->backing_file,
- sizeof(bs->backing_file));
+ s->header.backing_filename_size,
+ bs->auto_backing_file,
+ sizeof(bs->auto_backing_file));
if (ret < 0) {
return ret;
}
+ pstrcpy(bs->backing_file, sizeof(bs->backing_file),
+ bs->auto_backing_file);
if (s->header.features & QED_F_BACKING_FORMAT_NO_PROBE) {
pstrcpy(bs->backing_format, sizeof(bs->backing_format), "raw");
@@ -380,12 +380,14 @@ static int vmdk_parent_open(BlockDriverState *bs)
ret = -EINVAL;
goto out;
}
- if ((end_name - p_name) > sizeof(bs->backing_file) - 1) {
+ if ((end_name - p_name) > sizeof(bs->auto_backing_file) - 1) {
ret = -EINVAL;
goto out;
}
- pstrcpy(bs->backing_file, end_name - p_name + 1, p_name);
+ pstrcpy(bs->auto_backing_file, end_name - p_name + 1, p_name);
+ pstrcpy(bs->backing_file, sizeof(bs->backing_file),
+ bs->auto_backing_file);
}
out:
If the backing file is overridden, this most probably does change the guest-visible data of a BDS. Therefore, we will need to consider this in bdrv_refresh_filename(). To see whether it has been overridden, we might want to compare bs->backing_file and bs->backing->bs->filename. However, bs->backing_file is changed by bdrv_set_backing_hd() (which is just used to change the backing child at runtime, without modifying the image header), so bs->backing_file most of the time simply contains a copy of bs->backing->bs->filename anyway, so it is useless for such a comparison. This patch adds an auto_backing_file BDS field which contains the backing file path as indicated by the image header, which is not changed by bdrv_set_backing_hd(). Because of bdrv_refresh_filename() magic, however, a BDS's filename may differ from what has been specified during bdrv_open(). Then, the comparison between bs->auto_backing_file and bs->backing->bs->filename may fail even though bs->backing was opened from bs->auto_backing_file. To mitigate this, we can copy the real BDS's filename (after the whole bdrv_open() and bdrv_refresh_filename() process) into bs->auto_backing_file, if we know the former has been opened based on the latter. This is only possible if no options modifying the backing file's behavior have been specified, though. To simplify things, this patch only copies the filename from the backing file if no options have been specified for it at all. Furthermore, there are cases where an overlay is created by qemu which already contains a BDS's filename (e.g. in blockdev-snapshot-sync). We do not need to worry about updating the overlay's bs->auto_backing_file there, because we actually wrote a post-bdrv_refresh_filename() filename into the image header. So all in all, there will be false negatives where (as of a future patch) bdrv_refresh_filename() will assume that the backing file differs from what was specified in the image header, even though it really does not. However, these cases should be limited to where (1) the user actually did override something in the backing chain (e.g. by specifying options for the backing file), or (2) the user executed a QMP command to change some node's backing file (e.g. change-backing-file or block-commit with @backing-file given) where the given filename does not happen to coincide with qemu's idea of the backing BDS's filename. Then again, (1) really is limited to -drive. With -blockdev or blockdev-add, you have to adhere to the schema, so a user cannot give partial "unimportant" options (e.g. by just setting backing.node-name and leaving the rest to the image header). Therefore, trying to fix this would mean trying to fix something for -drive only. To improve on (2), we would need a full infrastructure to "canonicalize" an arbitrary filename (+ options), so it can be compared against another. That seems a bit over the top, considering that filenames nowadays are there mostly for the user's entertainment. Signed-off-by: Max Reitz <mreitz@redhat.com> --- include/block/block_int.h | 4 ++++ block.c | 19 +++++++++++++++++++ block/qcow.c | 7 +++++-- block/qcow2.c | 10 +++++++--- block/qed.c | 7 +++++-- block/vmdk.c | 6 ++++-- 6 files changed, 44 insertions(+), 9 deletions(-)