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Re: [PATCH v11 04/13] copy-on-read: pass overlay base node name to COR d

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH v11 04/13] copy-on-read: pass overlay base node name to COR driver
Date: Wed, 14 Oct 2020 19:18:28 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.2

14.10.2020 19:08, Andrey Shinkevich wrote:
On 14.10.2020 14:09, Max Reitz wrote:
On 12.10.20 19:43, Andrey Shinkevich wrote:
We are going to use the COR-filter for a block-stream job.
To limit COR operations by the base node in the backing chain during
stream job, pass the name of overlay base node to the copy-on-read
driver as base node itself may change due to possible concurrent jobs.
The rest of the functionality will be implemented in the patch that

Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
  block/copy-on-read.c | 14 ++++++++++++++
  1 file changed, 14 insertions(+)

Is there a reason why you didn’t add this option to QAPI (as part of a
yet-to-be-created BlockdevOptionsCor)?  Because I’d really like it there.

I agree that passing a base overlay under the base option looks clumsy. We 
could pass the base node name and find its overlay ourselves here in 
cor_open(). In that case, we can use the existing QAPI.

Actually, there is no existing QAPI: if you don't modify qapi/*.json, user is 
not able to pass the option through QAPI. It's still possible to pass the 
option through command-line, or when create the filter internally (like we are 
going to do in block-stream), but not through QAPI. So, it's better to make a 
new QAPI parameter, to make the new option available for QMP interface.

The reason I used the existing QAPI is to make it easier for a user to operate 
with the traditional options and to keep things simple. So, the user shouldn't 
think what overlay or above-base node to pass.
If we introduce the specific BlockdevOptionsCor, what other options may come 

diff --git a/block/copy-on-read.c b/block/copy-on-read.c
index bcccf0f..c578b1b 100644
--- a/block/copy-on-read.c
+++ b/block/copy-on-read.c
@@ -24,19 +24,24 @@
  #include "block/block_int.h"
  #include "qemu/module.h"
  #include "qapi/error.h"
+#include "qapi/qmp/qerror.h"
  #include "qapi/qmp/qdict.h"
  #include "block/copy-on-read.h"
  typedef struct BDRVStateCOR {
      bool active;
+    BlockDriverState *base_overlay;
  } BDRVStateCOR;
  static int cor_open(BlockDriverState *bs, QDict *options, int flags,
                      Error **errp)
+    BlockDriverState *base_overlay = NULL;
      BDRVStateCOR *state = bs->opaque;
+    /* We need the base overlay node rather than the base itself */
+    const char *base_overlay_node = qdict_get_try_str(options, "base");

Shouldn’t it be called base-overlay or above-base then?

The base_overlay identifier is used below as the pointer to BS. The 
base_overlay_node stands for the name of the node. I used that identifier to 
differ between the types. And the above_base has another meaning per 
block/stream.c - it can be a temporary filter with a JSON-name.

      bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
                                 BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
@@ -52,7 +57,16 @@ static int cor_open(BlockDriverState *bs, QDict *options, 
int flags,
+    if (base_overlay_node) {
+        qdict_del(options, "base");
+        base_overlay = bdrv_lookup_bs(NULL, base_overlay_node, errp);

I think this is a use-after-free.  The storage @base_overlay_node points
to belongs to a QString, which is referenced only by @options; so
deleting that element of @options should free that string.


I will swap those two function calls (bdrv_lookup_bs(); qdict_del();).
Thank you.


+        if (!base_overlay) {
+            error_setg(errp, QERR_BASE_NOT_FOUND, base_overlay_node);
+            return -EINVAL;
+        }
+    }
      state->active = true;
+    state->base_overlay = base_overlay;
       * We don't need to call bdrv_child_refresh_perms() now as the permissions

Best regards,

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