[Qemu-devel] [PATCH] qmp: fix object-add assert() without props

[Qemu-devel] [PATCH] qmp: fix object-add assert() without props

[Marc-André Lureau]

Since commit ad739706bbadee49, user_creatable_add_type() expects to be
given a qdict. However, if object-add is called without props, you reach
the assert: "qemu/qom/object_interfaces.c:115: user_creatable_add_type:
Assertion `qdict' failed.", because the qdict isn't created in this
case (it's optional).

Furthermore, qmp_input_visitor_new() is not meant to be called without a
dict, and a further commit will assert in this situation.

If none given, create an empty qdict in qmp to avoid the
user_creatable_add_type() assert(qdict).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 qmp.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/qmp.c b/qmp.c
index 6733463..8078038 100644
--- a/qmp.c
+++ b/qmp.c
@@ -665,7 +665,7 @@ void qmp_add_client(const char *protocol, const char *fdname,
 void qmp_object_add(const char *type, const char *id,
                     bool has_props, QObject *props, Error **errp)
 {
-    const QDict *pdict = NULL;
+    QDict *pdict;
     Visitor *v;
     Object *obj;
 
@@ -675,14 +675,19 @@ void qmp_object_add(const char *type, const char *id,
             error_setg(errp, QERR_INVALID_PARAMETER_TYPE, "props", "dict");
             return;
         }
+    } else {
+        pdict = qdict_new();
     }
 
-    v = qmp_input_visitor_new(props, true);
+    v = qmp_input_visitor_new(QOBJECT(pdict), true);
     obj = user_creatable_add_type(type, id, pdict, v, errp);
     visit_free(v);
     if (obj) {
         object_unref(obj);
     }
+    if (!props) {
+        qobject_decref(QOBJECT(pdict));
+    }
 }
 
 void qmp_object_del(const char *id, Error **errp)
-- 
2.10.0

Re: [Qemu-devel] [PATCH] qmp: fix object-add assert() without props

[Eric Blake]

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On 09/21/2016 02:41 PM, Marc-André Lureau wrote:
> Since commit ad739706bbadee49, user_creatable_add_type() expects to be
> given a qdict. However, if object-add is called without props, you reach
> the assert: "qemu/qom/object_interfaces.c:115: user_creatable_add_type:
> Assertion `qdict' failed.", because the qdict isn't created in this
> case (it's optional).
> 
> Furthermore, qmp_input_visitor_new() is not meant to be called without a
> dict, and a further commit will assert in this situation.
> 
> If none given, create an empty qdict in qmp to avoid the
> user_creatable_add_type() assert(qdict).
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  qmp.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 

>      if (obj) {
>          object_unref(obj);
>      }
> +    if (!props) {
> +        qobject_decref(QOBJECT(pdict));

This can be written:

QDECREF(pdict);

With that simplification,
Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [PATCH] qmp: fix object-add assert() without props

[Paolo Bonzini]

On 21/09/2016 21:41, Marc-André Lureau wrote:
> Since commit ad739706bbadee49, user_creatable_add_type() expects to be
> given a qdict. However, if object-add is called without props, you reach
> the assert: "qemu/qom/object_interfaces.c:115: user_creatable_add_type:
> Assertion `qdict' failed.", because the qdict isn't created in this
> case (it's optional).
> 
> Furthermore, qmp_input_visitor_new() is not meant to be called without a
> dict, and a further commit will assert in this situation.
> 
> If none given, create an empty qdict in qmp to avoid the
> user_creatable_add_type() assert(qdict).
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  qmp.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/qmp.c b/qmp.c
> index 6733463..8078038 100644
> --- a/qmp.c
> +++ b/qmp.c
> @@ -665,7 +665,7 @@ void qmp_add_client(const char *protocol, const char *fdname,
>  void qmp_object_add(const char *type, const char *id,
>                      bool has_props, QObject *props, Error **errp)
>  {
> -    const QDict *pdict = NULL;
> +    QDict *pdict;
>      Visitor *v;
>      Object *obj;
>  
> @@ -675,14 +675,19 @@ void qmp_object_add(const char *type, const char *id,
>              error_setg(errp, QERR_INVALID_PARAMETER_TYPE, "props", "dict");
>              return;
>          }
> +    } else {
> +        pdict = qdict_new();
>      }
>  
> -    v = qmp_input_visitor_new(props, true);
> +    v = qmp_input_visitor_new(QOBJECT(pdict), true);
>      obj = user_creatable_add_type(type, id, pdict, v, errp);
>      visit_free(v);
>      if (obj) {
>          object_unref(obj);
>      }
> +    if (!props) {
> +        qobject_decref(QOBJECT(pdict));

Or QDECREF(pdict).

In any case,

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

> +    }
>  }
>  
>  void qmp_object_del(const char *id, Error **errp)
> 

Re: [Qemu-devel] [PATCH] qmp: fix object-add assert() without props

[Daniel P. Berrange]

On Wed, Sep 21, 2016 at 11:41:26PM +0400, Marc-André Lureau wrote:
> Since commit ad739706bbadee49, user_creatable_add_type() expects to be
> given a qdict. However, if object-add is called without props, you reach
> the assert: "qemu/qom/object_interfaces.c:115: user_creatable_add_type:
> Assertion `qdict' failed.", because the qdict isn't created in this
> case (it's optional).
> 
> Furthermore, qmp_input_visitor_new() is not meant to be called without a
> dict, and a further commit will assert in this situation.
> 
> If none given, create an empty qdict in qmp to avoid the
> user_creatable_add_type() assert(qdict).
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  qmp.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/qmp.c b/qmp.c
> index 6733463..8078038 100644
> --- a/qmp.c
> +++ b/qmp.c
> @@ -665,7 +665,7 @@ void qmp_add_client(const char *protocol, const char *fdname,
>  void qmp_object_add(const char *type, const char *id,
>                      bool has_props, QObject *props, Error **errp)
>  {
> -    const QDict *pdict = NULL;
> +    QDict *pdict;
>      Visitor *v;
>      Object *obj;
>  
> @@ -675,14 +675,19 @@ void qmp_object_add(const char *type, const char *id,
>              error_setg(errp, QERR_INVALID_PARAMETER_TYPE, "props", "dict");
>              return;
>          }
> +    } else {
> +        pdict = qdict_new();
>      }
>  
> -    v = qmp_input_visitor_new(props, true);
> +    v = qmp_input_visitor_new(QOBJECT(pdict), true);
>      obj = user_creatable_add_type(type, id, pdict, v, errp);
>      visit_free(v);
>      if (obj) {
>          object_unref(obj);
>      }
> +    if (!props) {
> +        qobject_decref(QOBJECT(pdict));
> +    }
>  }
>  
>  void qmp_object_del(const char *id, Error **errp)

I'd like to see us add a unit test case to cover this scenario.

AFAICT, we don't currently have unit tests for specific QMP commands, just
the command framework, but we could still do it in tests/test-qmp-commands.c

Regards,
Daniel
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Re: [Qemu-devel] [PATCH] qmp: fix object-add assert() without props

[Markus Armbruster]

Marc-André Lureau <marcandre.lureau@redhat.com> writes:

> Since commit ad739706bbadee49, user_creatable_add_type() expects to be
> given a qdict. However, if object-add is called without props, you reach
> the assert: "qemu/qom/object_interfaces.c:115: user_creatable_add_type:
> Assertion `qdict' failed.", because the qdict isn't created in this
> case (it's optional).
>
> Furthermore, qmp_input_visitor_new() is not meant to be called without a
> dict, and a further commit will assert in this situation.
>
> If none given, create an empty qdict in qmp to avoid the
> user_creatable_add_type() assert(qdict).
>
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  qmp.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/qmp.c b/qmp.c
> index 6733463..8078038 100644
> --- a/qmp.c
> +++ b/qmp.c
> @@ -665,7 +665,7 @@ void qmp_add_client(const char *protocol, const char *fdname,
>  void qmp_object_add(const char *type, const char *id,
>                      bool has_props, QObject *props, Error **errp)
>  {
> -    const QDict *pdict = NULL;
> +    QDict *pdict;
>      Visitor *v;
>      Object *obj;
>  
> @@ -675,14 +675,19 @@ void qmp_object_add(const char *type, const char *id,
>              error_setg(errp, QERR_INVALID_PARAMETER_TYPE, "props", "dict");
>              return;
>          }
> +    } else {
> +        pdict = qdict_new();
>      }
>  
> -    v = qmp_input_visitor_new(props, true);
> +    v = qmp_input_visitor_new(QOBJECT(pdict), true);
>      obj = user_creatable_add_type(type, id, pdict, v, errp);
>      visit_free(v);
>      if (obj) {
>          object_unref(obj);
>      }
> +    if (!props) {
> +        qobject_decref(QOBJECT(pdict));
> +    }
>  }
>  
>  void qmp_object_del(const char *id, Error **errp)

The reference counting here is needlessly complex.

Case props != NULL:

* The caller holds a reference to props

* pdict = qobject_to_qdict(props) is a weak reference (not counted)

Case props == NULL:

* pdict = qdict_new() is a counted reference

* When it goes out of scope, we need to decrement the reference count.

I recommend to avoid the dual nature of pdict, by converting the weak
reference into a strong one, then decrement the reference count
unconditionally:

    if (props) {
        pdict = qobject_to_qdict(props);
        if (!pdict) {
            error_setg(errp, QERR_INVALID_PARAMETER_TYPE, "props", "dict");
            return;
        }
        QINCREF(pdict);
    } else {
        pdict = qdict_new();
    }

    v = qmp_input_visitor_new(QOBJECT(pdict), true);
    obj = user_creatable_add_type(type, id, pdict, v, errp);
    visit_free(v);
    if (obj) {
        object_unref(obj);
    }
    QDECREF(pdict);