[PATCH 0/3] block: drop BlockDriverState::read_only

[PATCH 0/3] block: drop BlockDriverState::read_only

[Vladimir Sementsov-Ogievskiy]

Hi all!

The field duplicates information in .open_flags. We have to carefully
sync these two fields everywhere. It's simple to introduce a bug by
forgetting it.

Let's drop the field, and fix users to call bdrv_is_read_only() and
bdrv_is_writable() instead.

Vladimir Sementsov-Ogievskiy (3):
  block: consistently use bdrv_is_read_only()
  block: drop BlockDriverState::read_only
  block: drop BlockBackendRootState::read_only

 include/block/block_int.h        |  2 --
 block.c                          | 16 +++++++---------
 block/block-backend.c            | 10 ++--------
 block/commit.c                   |  2 +-
 block/io.c                       |  4 ++--
 block/qapi.c                     |  2 +-
 block/qcow2-snapshot.c           |  2 +-
 block/qcow2.c                    |  4 ++--
 block/snapshot.c                 |  2 +-
 block/vhdx-log.c                 |  2 +-
 blockdev.c                       |  4 +++-
 tests/unit/test-block-iothread.c |  6 ------
 12 files changed, 21 insertions(+), 35 deletions(-)

-- 
2.29.2

[PATCH 1/3] block: consistently use bdrv_is_read_only()

[Vladimir Sementsov-Ogievskiy]

It's better to use accessor function instead of bs->read_only directly.
In some places use bdrv_is_writable() instead of
checking both BDRV_O_RDWR set and BDRV_O_INACTIVE not set.

In bdrv_open_common() it's a bit strange to add one more variable, but
we are going to drop bs->read_only in the next patch, so new ro local
variable substitutes it here.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block.c                | 11 +++++++----
 block/block-backend.c  |  2 +-
 block/commit.c         |  2 +-
 block/io.c             |  4 ++--
 block/qapi.c           |  2 +-
 block/qcow2-snapshot.c |  2 +-
 block/qcow2.c          |  4 ++--
 block/snapshot.c       |  2 +-
 block/vhdx-log.c       |  2 +-
 9 files changed, 17 insertions(+), 14 deletions(-)

diff --git a/block.c b/block.c
index 0dc97281dc..0270b609c4 100644
--- a/block.c
+++ b/block.c
@@ -1720,6 +1720,7 @@ static int bdrv_open_common(BlockDriverState *bs, BlockBackend *file,
     QemuOpts *opts;
     BlockDriver *drv;
     Error *local_err = NULL;
+    bool ro;
 
     assert(bs->file == NULL);
     assert(options != NULL && bs->options != options);
@@ -1772,15 +1773,17 @@ static int bdrv_open_common(BlockDriverState *bs, BlockBackend *file,
 
     bs->read_only = !(bs->open_flags & BDRV_O_RDWR);
 
-    if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv, bs->read_only)) {
-        if (!bs->read_only && bdrv_is_whitelisted(drv, true)) {
+    ro = bdrv_is_read_only(bs);
+
+    if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv, ro)) {
+        if (!ro && bdrv_is_whitelisted(drv, true)) {
             ret = bdrv_apply_auto_read_only(bs, NULL, NULL);
         } else {
             ret = -ENOTSUP;
         }
         if (ret < 0) {
             error_setg(errp,
-                       !bs->read_only && bdrv_is_whitelisted(drv, true)
+                       !ro && bdrv_is_whitelisted(drv, true)
                        ? "Driver '%s' can only be used for read-only devices"
                        : "Driver '%s' is not whitelisted",
                        drv->format_name);
@@ -1792,7 +1795,7 @@ static int bdrv_open_common(BlockDriverState *bs, BlockBackend *file,
     assert(qatomic_read(&bs->copy_on_read) == 0);
 
     if (bs->open_flags & BDRV_O_COPY_ON_READ) {
-        if (!bs->read_only) {
+        if (!ro) {
             bdrv_enable_copy_on_read(bs);
         } else {
             error_setg(errp, "Can't use copy-on-read on read-only device");
diff --git a/block/block-backend.c b/block/block-backend.c
index de5496af66..21b834e9df 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2269,7 +2269,7 @@ void blk_update_root_state(BlockBackend *blk)
     assert(blk->root);
 
     blk->root_state.open_flags    = blk->root->bs->open_flags;
-    blk->root_state.read_only     = blk->root->bs->read_only;
+    blk->root_state.read_only     = bdrv_is_read_only(blk->root->bs);
     blk->root_state.detect_zeroes = blk->root->bs->detect_zeroes;
 }
 
diff --git a/block/commit.c b/block/commit.c
index b89bb20b75..b7f0c7c061 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -453,7 +453,7 @@ int bdrv_commit(BlockDriverState *bs)
         return -EBUSY;
     }
 
-    ro = backing_file_bs->read_only;
+    ro = bdrv_is_read_only(backing_file_bs);
 
     if (ro) {
         if (bdrv_reopen_set_read_only(backing_file_bs, false, NULL)) {
diff --git a/block/io.c b/block/io.c
index 1e826ba9e8..323854d063 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1973,7 +1973,7 @@ bdrv_co_write_req_prepare(BdrvChild *child, int64_t offset, int64_t bytes,
 
     bdrv_check_request(offset, bytes, &error_abort);
 
-    if (bs->read_only) {
+    if (bdrv_is_read_only(bs)) {
         return -EPERM;
     }
 
@@ -3406,7 +3406,7 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset, bool exact,
     if (new_bytes) {
         bdrv_make_request_serialising(&req, 1);
     }
-    if (bs->read_only) {
+    if (bdrv_is_read_only(bs)) {
         error_setg(errp, "Image is read-only");
         ret = -EACCES;
         goto out;
diff --git a/block/qapi.c b/block/qapi.c
index 943e7b15ad..dc69341bfe 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -59,7 +59,7 @@ BlockDeviceInfo *bdrv_block_device_info(BlockBackend *blk,
 
     info = g_malloc0(sizeof(*info));
     info->file                   = g_strdup(bs->filename);
-    info->ro                     = bs->read_only;
+    info->ro                     = bdrv_is_read_only(bs);
     info->drv                    = g_strdup(bs->drv->format_name);
     info->encrypted              = bs->encrypted;
 
diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index 2e98c7f4b6..71ddb08c21 100644
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -1026,7 +1026,7 @@ int qcow2_snapshot_load_tmp(BlockDriverState *bs,
     int new_l1_bytes;
     int ret;
 
-    assert(bs->read_only);
+    assert(bdrv_is_read_only(bs));
 
     /* Search the snapshot */
     snapshot_index = find_snapshot_by_id_and_name(bs, snapshot_id, name);
diff --git a/block/qcow2.c b/block/qcow2.c
index 39b91ef940..d39d9f4e9b 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1724,7 +1724,7 @@ static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
     /* Clear unknown autoclear feature bits */
     update_header |= s->autoclear_features & ~QCOW2_AUTOCLEAR_MASK;
     update_header =
-        update_header && !bs->read_only && !(flags & BDRV_O_INACTIVE);
+        update_header && bdrv_is_writable(bs);
     if (update_header) {
         s->autoclear_features &= QCOW2_AUTOCLEAR_MASK;
     }
@@ -1811,7 +1811,7 @@ static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
     bs->supported_truncate_flags = BDRV_REQ_ZERO_WRITE;
 
     /* Repair image if dirty */
-    if (!(flags & (BDRV_O_CHECK | BDRV_O_INACTIVE)) && !bs->read_only &&
+    if (!(flags & BDRV_O_CHECK) && bdrv_is_writable(bs) &&
         (s->incompatible_features & QCOW2_INCOMPAT_DIRTY)) {
         BdrvCheckResult result = {0};
 
diff --git a/block/snapshot.c b/block/snapshot.c
index e8ae9a28c1..6702c75e42 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -415,7 +415,7 @@ int bdrv_snapshot_load_tmp(BlockDriverState *bs,
         error_setg(errp, "snapshot_id and name are both NULL");
         return -EINVAL;
     }
-    if (!bs->read_only) {
+    if (!bdrv_is_read_only(bs)) {
         error_setg(errp, "Device is not readonly");
         return -EINVAL;
     }
diff --git a/block/vhdx-log.c b/block/vhdx-log.c
index 404fb5f3cb..7672161d95 100644
--- a/block/vhdx-log.c
+++ b/block/vhdx-log.c
@@ -801,7 +801,7 @@ int vhdx_parse_log(BlockDriverState *bs, BDRVVHDXState *s, bool *flushed,
     }
 
     if (logs.valid) {
-        if (bs->read_only) {
+        if (bdrv_is_read_only(bs)) {
             bdrv_refresh_filename(bs);
             ret = -EPERM;
             error_setg(errp,
-- 
2.29.2

Re: [PATCH 1/3] block: consistently use bdrv_is_read_only()

[Kevin Wolf]

Am 26.05.2021 um 23:15 hat Vladimir Sementsov-Ogievskiy geschrieben:
> It's better to use accessor function instead of bs->read_only directly.
> In some places use bdrv_is_writable() instead of
> checking both BDRV_O_RDWR set and BDRV_O_INACTIVE not set.
> 
> In bdrv_open_common() it's a bit strange to add one more variable, but
> we are going to drop bs->read_only in the next patch, so new ro local
> variable substitutes it here.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

> diff --git a/block/qcow2.c b/block/qcow2.c
> index 39b91ef940..d39d9f4e9b 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1724,7 +1724,7 @@ static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
>      /* Clear unknown autoclear feature bits */
>      update_header |= s->autoclear_features & ~QCOW2_AUTOCLEAR_MASK;
>      update_header =
> -        update_header && !bs->read_only && !(flags & BDRV_O_INACTIVE);
> +        update_header && bdrv_is_writable(bs);

This statement fits on a single line now.

Kevin

[PATCH 2/3] block: drop BlockDriverState::read_only

[Vladimir Sementsov-Ogievskiy]

This variable is just a cache for !(bs->open_flags & BDRV_O_RDWR),
which we have to synchronize everywhere. Let's just drop it and
consistently use bdrv_is_read_only().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/block_int.h        | 1 -
 block.c                          | 7 +------
 tests/unit/test-block-iothread.c | 6 ------
 3 files changed, 1 insertion(+), 13 deletions(-)

diff --git a/include/block/block_int.h b/include/block/block_int.h
index b2c8b09d0f..09661a134b 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -843,7 +843,6 @@ struct BlockDriverState {
      * locking needed during I/O...
      */
     int open_flags; /* flags used to open the file, re-used for re-open */
-    bool read_only; /* if true, the media is read only */
     bool encrypted; /* if true, the media is encrypted */
     bool sg;        /* if true, the device is a /dev/sg* */
     bool probed;    /* if true, format was probed rather than specified */
diff --git a/block.c b/block.c
index 0270b609c4..d36329e712 100644
--- a/block.c
+++ b/block.c
@@ -265,7 +265,7 @@ void bdrv_parse_filename_strip_prefix(const char *filename, const char *prefix,
  * image is inactivated. */
 bool bdrv_is_read_only(BlockDriverState *bs)
 {
-    return bs->read_only;
+    return !(bs->open_flags & BDRV_O_RDWR);
 }
 
 int bdrv_can_set_read_only(BlockDriverState *bs, bool read_only,
@@ -317,7 +317,6 @@ int bdrv_apply_auto_read_only(BlockDriverState *bs, const char *errmsg,
         goto fail;
     }
 
-    bs->read_only = true;
     bs->open_flags &= ~BDRV_O_RDWR;
 
     return 0;
@@ -1549,7 +1548,6 @@ static int bdrv_open_driver(BlockDriverState *bs, BlockDriver *drv,
     }
 
     bs->drv = drv;
-    bs->read_only = !(bs->open_flags & BDRV_O_RDWR);
     bs->opaque = g_malloc0(drv->instance_size);
 
     if (drv->bdrv_file_open) {
@@ -1771,8 +1769,6 @@ static int bdrv_open_common(BlockDriverState *bs, BlockBackend *file,
     trace_bdrv_open_common(bs, filename ?: "", bs->open_flags,
                            drv->format_name);
 
-    bs->read_only = !(bs->open_flags & BDRV_O_RDWR);
-
     ro = bdrv_is_read_only(bs);
 
     if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv, ro)) {
@@ -4548,7 +4544,6 @@ static void bdrv_reopen_commit(BDRVReopenState *reopen_state)
     bs->explicit_options   = reopen_state->explicit_options;
     bs->options            = reopen_state->options;
     bs->open_flags         = reopen_state->flags;
-    bs->read_only = !(reopen_state->flags & BDRV_O_RDWR);
     bs->detect_zeroes      = reopen_state->detect_zeroes;
 
     if (reopen_state->replace_backing_bs) {
diff --git a/tests/unit/test-block-iothread.c b/tests/unit/test-block-iothread.c
index 8cf172cb7a..c39e70b2f5 100644
--- a/tests/unit/test-block-iothread.c
+++ b/tests/unit/test-block-iothread.c
@@ -194,13 +194,11 @@ static void test_sync_op_truncate(BdrvChild *c)
     g_assert_cmpint(ret, ==, -EINVAL);
 
     /* Error: Read-only image */
-    c->bs->read_only = true;
     c->bs->open_flags &= ~BDRV_O_RDWR;
 
     ret = bdrv_truncate(c, 65536, false, PREALLOC_MODE_OFF, 0, NULL);
     g_assert_cmpint(ret, ==, -EACCES);
 
-    c->bs->read_only = false;
     c->bs->open_flags |= BDRV_O_RDWR;
 }
 
@@ -236,13 +234,11 @@ static void test_sync_op_flush(BdrvChild *c)
     g_assert_cmpint(ret, ==, 0);
 
     /* Early success: Read-only image */
-    c->bs->read_only = true;
     c->bs->open_flags &= ~BDRV_O_RDWR;
 
     ret = bdrv_flush(c->bs);
     g_assert_cmpint(ret, ==, 0);
 
-    c->bs->read_only = false;
     c->bs->open_flags |= BDRV_O_RDWR;
 }
 
@@ -256,13 +252,11 @@ static void test_sync_op_blk_flush(BlockBackend *blk)
     g_assert_cmpint(ret, ==, 0);
 
     /* Early success: Read-only image */
-    bs->read_only = true;
     bs->open_flags &= ~BDRV_O_RDWR;
 
     ret = blk_flush(blk);
     g_assert_cmpint(ret, ==, 0);
 
-    bs->read_only = false;
     bs->open_flags |= BDRV_O_RDWR;
 }
 
-- 
2.29.2

[PATCH 3/3] block: drop BlockBackendRootState::read_only

[Vladimir Sementsov-Ogievskiy]

Instead of keeping additional boolean field, let's store the
information in BDRV_O_RDWR bit of BlockBackendRootState::open_flags.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/block_int.h |  1 -
 block/block-backend.c     | 10 ++--------
 blockdev.c                |  4 +++-
 3 files changed, 5 insertions(+), 10 deletions(-)

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 09661a134b..057d88b1fc 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -1007,7 +1007,6 @@ struct BlockDriverState {
 
 struct BlockBackendRootState {
     int open_flags;
-    bool read_only;
     BlockdevDetectZeroesOptions detect_zeroes;
 };
 
diff --git a/block/block-backend.c b/block/block-backend.c
index 21b834e9df..d1a33a2c8e 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1852,7 +1852,7 @@ bool blk_supports_write_perm(BlockBackend *blk)
     if (bs) {
         return !bdrv_is_read_only(bs);
     } else {
-        return !blk->root_state.read_only;
+        return blk->root_state.open_flags & BDRV_O_RDWR;
     }
 }
 
@@ -2269,7 +2269,6 @@ void blk_update_root_state(BlockBackend *blk)
     assert(blk->root);
 
     blk->root_state.open_flags    = blk->root->bs->open_flags;
-    blk->root_state.read_only     = bdrv_is_read_only(blk->root->bs);
     blk->root_state.detect_zeroes = blk->root->bs->detect_zeroes;
 }
 
@@ -2288,12 +2287,7 @@ bool blk_get_detect_zeroes_from_root_state(BlockBackend *blk)
  */
 int blk_get_open_flags_from_root_state(BlockBackend *blk)
 {
-    int bs_flags;
-
-    bs_flags = blk->root_state.read_only ? 0 : BDRV_O_RDWR;
-    bs_flags |= blk->root_state.open_flags & ~BDRV_O_RDWR;
-
-    return bs_flags;
+    return blk->root_state.open_flags;
 }
 
 BlockBackendRootState *blk_get_root_state(BlockBackend *blk)
diff --git a/blockdev.c b/blockdev.c
index 834c2304a1..c36884c691 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -576,6 +576,9 @@ static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
     }
 
     read_only = qemu_opt_get_bool(opts, BDRV_OPT_READ_ONLY, false);
+    if (!read_only) {
+        bdrv_flags |= BDRV_O_RDWR;
+    }
 
     /* init */
     if ((!file || !*file) && !qdict_size(bs_opts)) {
@@ -584,7 +587,6 @@ static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
         blk = blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
         blk_rs = blk_get_root_state(blk);
         blk_rs->open_flags    = bdrv_flags;
-        blk_rs->read_only     = read_only;
         blk_rs->detect_zeroes = detect_zeroes;
 
         qobject_unref(bs_opts);
-- 
2.29.2

Re: [PATCH 3/3] block: drop BlockBackendRootState::read_only

[Kevin Wolf]

Am 26.05.2021 um 23:15 hat Vladimir Sementsov-Ogievskiy geschrieben:
> Instead of keeping additional boolean field, let's store the
> information in BDRV_O_RDWR bit of BlockBackendRootState::open_flags.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

> diff --git a/blockdev.c b/blockdev.c
> index 834c2304a1..c36884c691 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -576,6 +576,9 @@ static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
>      }
>  
>      read_only = qemu_opt_get_bool(opts, BDRV_OPT_READ_ONLY, false);
> +    if (!read_only) {
> +        bdrv_flags |= BDRV_O_RDWR;
> +    }
>  
>      /* init */
>      if ((!file || !*file) && !qdict_size(bs_opts)) {
> @@ -584,7 +587,6 @@ static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
>          blk = blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
>          blk_rs = blk_get_root_state(blk);
>          blk_rs->open_flags    = bdrv_flags;
> -        blk_rs->read_only     = read_only;
>          blk_rs->detect_zeroes = detect_zeroes;
>  
>          qobject_unref(bs_opts);

That we're now using different flags for blk_new_open() looked a bit
suspicious, though the new state intuitively makes more sense than the
old one.

Anyway, this patch breaks ahci-test:

# starting QEMU: exec build/x86_64-softmmu/qemu-system-x86_64 -qtest unix:/tmp/qtest-54104.sock -qtest-log /dev/null -chardev socket,path=/tmp/qtest-54104.qmp,id=char0 -mon chardev=char0,mode=control -display none -drive file=/tmp/qtest.mGtxeg,if=none,id=drive0,cache=writeback,rerror=stop,werror=stop,format=raw -M q35 -device ide-hd,drive=drive0 -incoming unix:/tmp/qtest-migration.XVRZZi -accel qtest
qemu-system-x86_64: -drive file=/tmp/qtest.mGtxeg,if=none,id=drive0,cache=writeback,rerror=stop,werror=stop,format=raw: Block node is read-only

Kevin

Re: [PATCH 3/3] block: drop BlockBackendRootState::read_only

[Vladimir Sementsov-Ogievskiy]

27.05.2021 17:46, Kevin Wolf wrote:
> Am 26.05.2021 um 23:15 hat Vladimir Sementsov-Ogievskiy geschrieben:
>> Instead of keeping additional boolean field, let's store the
>> information in BDRV_O_RDWR bit of BlockBackendRootState::open_flags.
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> 
>> diff --git a/blockdev.c b/blockdev.c
>> index 834c2304a1..c36884c691 100644
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -576,6 +576,9 @@ static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
>>       }
>>   
>>       read_only = qemu_opt_get_bool(opts, BDRV_OPT_READ_ONLY, false);
>> +    if (!read_only) {
>> +        bdrv_flags |= BDRV_O_RDWR;
>> +    }
>>   
>>       /* init */
>>       if ((!file || !*file) && !qdict_size(bs_opts)) {
>> @@ -584,7 +587,6 @@ static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
>>           blk = blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
>>           blk_rs = blk_get_root_state(blk);
>>           blk_rs->open_flags    = bdrv_flags;
>> -        blk_rs->read_only     = read_only;
>>           blk_rs->detect_zeroes = detect_zeroes;
>>   
>>           qobject_unref(bs_opts);
> 
> That we're now using different flags for blk_new_open() looked a bit
> suspicious, though the new state intuitively makes more sense than the
> old one.
> 
> Anyway, this patch breaks ahci-test:
> 
> # starting QEMU: exec build/x86_64-softmmu/qemu-system-x86_64 -qtest unix:/tmp/qtest-54104.sock -qtest-log /dev/null -chardev socket,path=/tmp/qtest-54104.qmp,id=char0 -mon chardev=char0,mode=control -display none -drive file=/tmp/qtest.mGtxeg,if=none,id=drive0,cache=writeback,rerror=stop,werror=stop,format=raw -M q35 -device ide-hd,drive=drive0 -incoming unix:/tmp/qtest-migration.XVRZZi -accel qtest
> qemu-system-x86_64: -drive file=/tmp/qtest.mGtxeg,if=none,id=drive0,cache=writeback,rerror=stop,werror=stop,format=raw: Block node is read-only
> 

Interesting (me again not running make check, sorry for this :\. And that's fixed by

diff --git a/blockdev.c b/blockdev.c
index c36884c691..2f3f9bc746 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -609,7 +609,7 @@ static BlockBackend *blockdev_init(const char *file, QDict *bs_opts,
              bdrv_flags |= BDRV_O_INACTIVE;
          }
  
-        blk = blk_new_open(file, NULL, bs_opts, bdrv_flags, errp);
+        blk = blk_new_open(file, NULL, bs_opts, bdrv_flags & ~BDRV_O_RDWR, errp);
          if (!blk) {
              goto err_no_bs_opts;
          }


so to make a RW node we should not pass both option and flag ??

Most probably that's because in blk_new_open we do

         if (flags & BDRV_O_RDWR) {
             perm |= BLK_PERM_WRITE;
         }


Probably this should be refactored somehow, as it's strange that we tread read-only option and O_RDWR flags in a different ways.. Still not here. I'll resend so that this patch modifies only BlockBackendRootState, not the logic of blk_new_open() call.

-- 
Best regards,
Vladimir