[PULL 18/21] qcow2: do not clear the dirty bit when reopening a read-only node

[Kevin Wolf <kwolf@redhat.com>]

From: "Denis V. Lunev" <den@openvz.org>

qcow2_reopen_prepare() clears the dirty bit whenever the node is
reopened read-only, with an unguarded header write. A read-only node
can still be dirty, inherited from an earlier writable session, and it
holds no BLK_PERM_WRITE to resolve that. A read-only to read-only
reopen of a dirty image therefore fails outright:

  $ qemu-io -r -f qcow2 dirty.qcow2 <<< $'reopen -r\nquit'
  qemu-io: failed while preparing to reopen image 'dirty.qcow2'

Where the file node below is writable the write is not refused early,
and bdrv_co_write_req_prepare() aborts on its BLK_PERM_WRITE assertion
instead.

Clear it only for a node that is writable now, the predicate
qcow2_do_open() already uses for the repair. bdrv_is_writable() also
excludes an inactive node, whose header must not be touched either.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Hanna Reitz <hreitz@redhat.com>
CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Cc: qemu-stable@nongnu.org
Message-ID: <20260824133729.1141990-2-den@openvz.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2.c              |  8 +++---
 tests/qemu-iotests/039     | 50 ++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/039.out | 20 +++++++++++++++
 3 files changed, 75 insertions(+), 3 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 7292dd036c9..1543255eba8 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2102,9 +2102,11 @@ qcow2_reopen_prepare(BDRVReopenState *state,BlockReopenQueue *queue,
             goto fail;
         }
 
-        ret = qcow2_mark_clean(state->bs);
-        if (ret < 0) {
-            goto fail;
+        if (bdrv_is_writable(state->bs)) {
+            ret = qcow2_mark_clean(state->bs);
+            if (ret < 0) {
+                goto fail;
+            }
         }
     }
 
diff --git a/tests/qemu-iotests/039 b/tests/qemu-iotests/039
index 8fe677ce731..686c6da50fd 100755
--- a/tests/qemu-iotests/039
+++ b/tests/qemu-iotests/039
@@ -93,6 +93,40 @@ $QEMU_IMG info --image-opts \
 # The dirty bit must still be set: this open never wrote any guest data
 _qcow2_dump_header | grep incompatible_features
 
+echo
+echo "== Read-only reopen must not clear the dirty bit =="
+
+# A read-only node cannot write the header, and must keep the dirty bit
+$QEMU_IO -r -c "reopen -r" -c "read -P 0x5a 0 512" "$TEST_IMG" \
+    | _filter_qemu_io
+
+# The dirty bit must still be set
+_qcow2_dump_header | grep incompatible_features
+
+echo
+echo "== Read-only reopen must not write through a writable file node =="
+
+# The write the header update needs is refused by the permission system
+echo "{'execute': 'qmp_capabilities'}
+      {'execute': 'blockdev-reopen',
+       'arguments': {'options': [{'node-name': 'drive',
+                                  'driver': 'qcow2',
+                                  'read-only': true,
+                                  'file': 'prot'}]}}
+      {'execute': 'quit'}" \
+    | $QEMU -qmp stdio -nographic -nodefaults \
+            -blockdev "{'node-name': 'prot',
+                        'driver': 'file',
+                        'filename': '$TEST_IMG'}" \
+            -blockdev "{'node-name': 'drive',
+                        'driver': 'qcow2',
+                        'file': 'prot',
+                        'read-only': true}" \
+    | _filter_qmp
+
+# The dirty bit must still be set
+_qcow2_dump_header | grep incompatible_features
+
 echo
 echo "== Repairing the image file must succeed =="
 
@@ -106,6 +140,22 @@ echo "== Data should still be accessible after repair =="
 
 $QEMU_IO -c "read -P 0x5a 0 512" "$TEST_IMG" | _filter_qemu_io
 
+echo
+echo "== A read-write to read-only reopen must clear the dirty bit =="
+
+_make_test_img -o "compat=1.1,lazy_refcounts=on" $size
+
+# The kill keeps the close from clearing the bit, so the header shows what
+# the reopen did with it
+_NO_VALGRIND \
+$QEMU_IO -c "write -P 0x5a 0 512" \
+         -c "reopen -r" \
+         -c "sigraise $(kill -l KILL)" "$TEST_IMG" 2>&1 \
+    | _filter_qemu_io
+
+# The dirty bit must not be set
+_qcow2_dump_header | grep incompatible_features
+
 echo
 echo "== Opening a dirty image read/write should repair it =="
 
diff --git a/tests/qemu-iotests/039.out b/tests/qemu-iotests/039.out
index c66361128f6..ce8ee577216 100644
--- a/tests/qemu-iotests/039.out
+++ b/tests/qemu-iotests/039.out
@@ -27,6 +27,19 @@ incompatible_features     [0]
 == Read-only open must not crash on close ==
 incompatible_features     [0]
 
+== Read-only reopen must not clear the dirty bit ==
+read 512/512 bytes at offset 0
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+incompatible_features     [0]
+
+== Read-only reopen must not write through a writable file node ==
+QMP_VERSION
+{"return": {}}
+{"return": {}}
+{"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "SHUTDOWN", "data": {"guest": false, "reason": "host-qmp-quit"}}
+{"return": {}}
+incompatible_features     [0]
+
 == Repairing the image file must succeed ==
 ERROR cluster 5 refcount=0 reference=1
 Rebuilding refcount structure
@@ -45,6 +58,13 @@ incompatible_features     []
 read 512/512 bytes at offset 0
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 
+== A read-write to read-only reopen must clear the dirty bit ==
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
+wrote 512/512 bytes at offset 0
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+./common.rc: Killed ( VALGRIND_QEMU="${VALGRIND_QEMU_IO}" _qemu_proc_exec "${VALGRIND_LOGFILE}" "$QEMU_IO_PROG" $QEMU_IO_ARGS "$@" )
+incompatible_features     []
+
 == Opening a dirty image read/write should repair it ==
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
 wrote 512/512 bytes at offset 0
-- 
2.56.0