block: Fix open flags with BDRV_O_SNAPSHOT

The immediately visible effect of this patch is that it fixes committing a temporary snapshot to its backing file. Previously, it would fail with a "permission denied" error because bdrv_inherited_flags() forced the backing file to be read-only, ignoring the r/w reopen of bdrv_commit().

The bigger problem this releaved is that the original open flags must actually only be applied to the temporary snapshot, and the original image file must be treated as a backing file of the temporary snapshot and get the right flags for that.

Reported-by Jan Kiszka <[email protected]>

Signed-off-by Kevin Wolf <[email protected]>

---
 block.c                    | 34 +++++++++++++++++++---------------
 include/block/block.h      |  2 +-
 tests/qemu-iotests/051     |  4 ++++
 tests/qemu-iotests/051.out | 10 ++++++++++
 4 files changed, 34 insertions(+), 16 deletions(-)

block.c +19 -15

index b749d31..c90c71a 100644
@@ -775,6 +775,16 @@ void bdrv_disable_copy_on_read(BlockDriverState *bs)
 }
 
 /*
+ * Returns the flags that a temporary snapshot should get, based on the
+ * originally requested flags (the originally requested image will have flags
+ * like a backing file)
+ */
+static int bdrv_temp_snapshot_flags(int flags)
+{
+    return (flags & ~BDRV_O_SNAPSHOT) | BDRV_O_TEMPORARY;
+}
+
+/*
  * Returns the flags that bs->file should get, based on the given flags for
  * the parent BDS
  */

@@ -787,11 +797,6 @@ static int bdrv_inherited_flags(int flags)
      * so we can enable both unconditionally on lower layers. */
     flags |= BDRV_O_CACHE_WB | BDRV_O_UNMAP;
 
-    /* The backing file of a temporary snapshot is read-only */
-    if (flags & BDRV_O_SNAPSHOT) {
-        flags &= ~BDRV_O_RDWR;
-    }
-
     /* Clear flags that only apply to the top layer */
     flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING | BDRV_O_COPY_ON_READ);
 

@@ -817,11 +822,6 @@ static int bdrv_open_flags(BlockDriverState *bs, int flags)
 {
     int open_flags = flags | BDRV_O_CACHE_WB;
 
-    /* The backing file of a temporary snapshot is read-only */
-    if (flags & BDRV_O_SNAPSHOT) {
-        open_flags &= ~BDRV_O_RDWR;
-    }
-
     /*
      * Clear flags that are internal to the block layer before opening the
      * image.

@@ -1206,7 +1206,7 @@ done:
     return ret;
 }
 
-void bdrv_append_temp_snapshot(BlockDriverState *bs, Error **errp)
+void bdrv_append_temp_snapshot(BlockDriverState *bs, int flags, Error **errp)
 {
     /* TODO: extra byte is a hack to ensure MAX_PATH space on Windows. */
     char *tmp_filename = g_malloc0(PATH_MAX + 1);

@@ -1262,8 +1262,7 @@ void bdrv_append_temp_snapshot(BlockDriverState *bs, Error **errp)
     bs_snapshot = bdrv_new("", &error_abort);
 
     ret = bdrv_open(&bs_snapshot, NULL, NULL, snapshot_options,
-                    (bs->open_flags & ~BDRV_O_SNAPSHOT) | BDRV_O_TEMPORARY,
-                    bdrv_qcow2, &local_err);
+                    flags, bdrv_qcow2, &local_err);
     if (ret < 0) {
         error_propagate(errp, local_err);
         goto out;

@@ -1298,6 +1297,7 @@ int bdrv_open(BlockDriverState **pbs, const char *filename,
     BlockDriverState *file = NULL, *bs;
     const char *drvname;
     Error *local_err = NULL;
+    int snapshot_flags = 0;
 
     assert(pbs);
 

@@ -1358,6 +1358,10 @@ int bdrv_open(BlockDriverState **pbs, const char *filename,
     if (flags & BDRV_O_RDWR) {
         flags |= BDRV_O_ALLOW_RDWR;
     }
+    if (flags & BDRV_O_SNAPSHOT) {
+        snapshot_flags = bdrv_temp_snapshot_flags(flags);
+        flags = bdrv_backing_flags(flags);
+    }
 
     assert(file == NULL);
     ret = bdrv_open_image(&file, filename, options, "file",

@@ -1417,8 +1421,8 @@ int bdrv_open(BlockDriverState **pbs, const char *filename,
 
     /* For snapshot=on, create a temporary qcow2 overlay. bs points to the
      * temporary snapshot afterwards. */
-    if (flags & BDRV_O_SNAPSHOT) {
-        bdrv_append_temp_snapshot(bs, &local_err);
+    if (snapshot_flags) {
+        bdrv_append_temp_snapshot(bs, snapshot_flags, &local_err);
         if (local_err) {
             error_propagate(errp, local_err);
             goto close_and_fail;

include/block/block.h +1 -1

index 467fb2b..2fda81c 100644
@@ -191,7 +191,7 @@ int bdrv_open_image(BlockDriverState **pbs, const char *filename,
                     QDict *options, const char *bdref_key, int flags,
                     bool allow_none, Error **errp);
 int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp);
-void bdrv_append_temp_snapshot(BlockDriverState *bs, Error **errp);
+void bdrv_append_temp_snapshot(BlockDriverState *bs, int flags, Error **errp);
 int bdrv_open(BlockDriverState **pbs, const char *filename,
               const char *reference, QDict *options, int flags,
               BlockDriver *drv, Error **errp);

tests/qemu-iotests/051 +4 -0

index 073dc7a..c4af131 100755
@@ -233,6 +233,10 @@ echo 'qemu-io ide0-hd0 "write -P 0x22 0 4k"' | run_qemu -drive file="$TEST_IMG",
 
 $QEMU_IO -c "read -P 0x22 0 4k" "$TEST_IMG" | _filter_qemu_io
 
+echo -e 'qemu-io ide0-hd0 "write -P 0x33 0 4k"\ncommit ide0-hd0' | run_qemu -drive file="$TEST_IMG",snapshot=on | _filter_qemu_io
+
+$QEMU_IO -c "read -P 0x33 0 4k" "$TEST_IMG" | _filter_qemu_io
+
 # success, all done
 echo "*** done"
 rm -f $seq.full

tests/qemu-iotests/051.out +3 -0

index 01b0384..31e329e 100644
@@ -358,4 +358,14 @@ wrote 4096/4096 bytes at offset 0
 
 read 4096/4096 bytes at offset 0
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Testing: -drive file=TEST_DIR/t.qcow2,snapshot=on
+QEMU X.Y.Z monitor - type 'help' for more information
+(qemu) qqeqemqemuqemu-qemu-iqemu-ioqemu-io qemu-io iqemu-io idqemu-io ideqemu-io ide0qemu-io ide0-qemu-io ide0-hqemu-io ide0-hdqemu-io ide0-hd0qemu-io ide0-hd0 qemu-io ide0-hd0 "qemu-io ide0-hd0 "wqemu-io ide0-hd0 "wrqemu-io ide0-hd0 "wri!
 qemu-io ide0-hd0 "writqemu-io ide0-hd0 "writeqemu-io ide0-hd0 "write qemu-io ide0-hd0 "write -qemu-io ide0-hd0 "write -Pqemu-io ide0-hd0 "write -P qemu-io ide0-hd0 "write -P 0qemu-io ide0-hd0 "write -P 0xqemu-io ide0-hd0 "write -P 0x3qemu-io ide0-hd0 "w!
 rite -P 0x33

[Dqemu-io ide0-hd0 "write -P 0x33 qemu-io ide0-hd0 "write -P 0x33 0qemu-io ide0-hd0 "write -P 0x33 0 qemu-io ide0-hd0 "write -P 0x33 0 4qemu-io ide0-hd0 "write -P 0x33 0 4kqemu-io ide0-hd0 "write -P 0x33 0 4k"

Quoted 18 lines · Kevin Wolf

The immediately visible effect of this patch is that it fixes committing a temporary snapshot to its backing file. Previously, it would fail with a "permission denied" error because bdrv_inherited_flags() forced the backing file to be read-only, ignoring the r/w reopen of bdrv_commit().

The bigger problem this releaved is that the original open flags must actually only be applied to the temporary snapshot, and the original image file must be treated as a backing file of the temporary snapshot and get the right flags for that.

Reported-by Jan Kiszka <[email protected]>

Signed-off-by Kevin Wolf <[email protected]>

---
  block.c                    | 34 +++++++++++++++++++---------------
  include/block/block.h      |  2 +-
  tests/qemu-iotests/051     |  4 ++++
  tests/qemu-iotests/051.out | 10 ++++++++++
  4 files changed, 34 insertions(+), 16 deletions(-)

Quoted 15 lines

tests/qemu-iotests/051.out +3 -0

index 01b0384..31e329e 100644
@@ -358,4 +358,14 @@ wrote 4096/4096 bytes at offset 0
 
  read 4096/4096 bytes at offset 0
  4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Testing: -drive file=TEST_DIR/t.qcow2,snapshot=on
+QEMU X.Y.Z monitor - type 'help' for more information
+(qemu) qqeqemqemuqemu-qemu-iqemu-ioqemu-io qemu-io iqemu-io idqemu-io ideqemu-io ide0qemu-io ide0-qemu-io ide0-hqemu-io ide0-hdqemu-io ide0-hd0qemu-io ide0-hd0 qemu-io ide0-hd0 "qemu-io ide0-hd0 "wqemu-io ide0-hd0 "wrqemu-io ide0-hd0 "wri!
  qemu-io ide0-hd0 "writqemu-io ide0-hd0 "writeqemu-io ide0-hd0 "write qemu-io ide0-hd0 "write -qemu-io ide0-hd0 "write -Pqemu-io ide0-hd0 "write -P qemu-io ide0-hd0 "write -P 0qemu-io ide0-hd0 "write -P 0xqemu-io ide0-hd0 "write -P 0x3qemu-io ide0-hd0 "w!
  rite -P 0x33

[Dqemu-io ide0-hd0 "write -P 0x33 qemu-io ide0-hd0 "write -P 0x33 0qemu-io ide0-hd0 "write -P 0x33 0 qemu-io ide0-hd0 "write -P 0x33 0 4qemu-io ide0-hd0 "write -P 0x33 0 4kqemu-io ide0-hd0 "write -P 0x33 0 4k"

tests/qemu-iotests/051.out

Quoted 8 lines

tests/qemu-iotests/051.out +7 -0

+wrote 4096/4096 bytes at offset 0
+4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+(qemu) ccocomcommcommicommitcommit commit icommit idcommit idecommit ide0commit ide0-commit ide0-hcommit ide0-hdcommit ide0-hd0
+(qemu) qququiquit
+
+read 4096/4096 bytes at offset 0
+4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
  *** done

Works fine here!

(For unknown reason, applying the iotest hunk didn't work for me, though.)

Quoted 19 lines · Kevin Wolf

Quoted 18 lines

The immediately visible effect of this patch is that it fixes committing a temporary snapshot to its backing file. Previously, it would fail with a "permission denied" error because bdrv_inherited_flags() forced the backing file to be read-only, ignoring the r/w reopen of bdrv_commit().

The bigger problem this releaved is that the original open flags must actually only be applied to the temporary snapshot, and the original image file must be treated as a backing file of the temporary snapshot and get the right flags for that.

Reported-by Jan Kiszka <[email protected]>

Signed-off-by Kevin Wolf <[email protected]>

---
 block.c                    | 34 +++++++++++++++++++---------------
 include/block/block.h      |  2 +-
 tests/qemu-iotests/051     |  4 ++++
 tests/qemu-iotests/051.out | 10 ++++++++++
 4 files changed, 34 insertions(+), 16 deletions(-)

Quoted 3 lines

Works fine here!

(For unknown reason, applying the iotest hunk didn't work for me, though.)

The lines are too long and therefore split in this mail, they need to be joined manually before applying the patch.

Perhaps the monitor should be changed to avoid printing so many useless control characters, then we'd hit the limit less often...

Stefan, didn't you plan to do something like this? Or was it unrelated?

The immediately visible effect of this patch is that it fixes committing a temporary snapshot to its backing file. Previously, it would fail with a "permission denied" error because bdrv_inherited_flags() forced the backing file to be read-only, ignoring the r/w reopen of bdrv_commit().

The bigger problem this releaved is that the original open flags must actually only be applied to the temporary snapshot, and the original image file must be treated as a backing file of the temporary snapshot and get the right flags for that.

Reported-by Jan Kiszka <[email protected]>

Signed-off-by Kevin Wolf <[email protected]>

---
  block.c                    | 34 +++++++++++++++++++---------------
  include/block/block.h      |  2 +-
  tests/qemu-iotests/051     |  4 ++++
  tests/qemu-iotests/051.out | 10 ++++++++++
  4 files changed, 34 insertions(+), 16 deletions(-)

The immediately visible effect of this patch is that it fixes committing a temporary snapshot to its backing file. Previously, it would fail with a "permission denied" error because bdrv_inherited_flags() forced the backing file to be read-only, ignoring the r/w reopen of bdrv_commit().

The bigger problem this releaved is that the original open flags must actually only be applied to the temporary snapshot, and the original image file must be treated as a backing file of the temporary snapshot and get the right flags for that.

Reported-by Jan Kiszka <[email protected]>

Signed-off-by Kevin Wolf <[email protected]>

---
 block.c                    | 34 +++++++++++++++++++---------------
 include/block/block.h      |  2 +-
 tests/qemu-iotests/051     |  4 ++++
 tests/qemu-iotests/051.out | 10 ++++++++++
 4 files changed, 34 insertions(+), 16 deletions(-)