Re: [PATCH] qom-qmp-cmds: fix two memleaks in qmp_object_add

[Pan Nengyuan]

On 3/9/2020 6:15 PM, Daniel P. Berrangé wrote:
> On Mon, Mar 09, 2020 at 10:51:45AM +0100, Igor Mammedov wrote:
>> On Mon, 9 Mar 2020 17:22:12 +0800
>> Pan Nengyuan <address@hidden> wrote:
>>
>>> 'type/id' forgot to free in qmp_object_add, this patch fix that.
>>>
>>> The leak stack:
>>> Direct leak of 84 byte(s) in 6 object(s) allocated from:
>>>     #0 0x7fe2a5ebf768 in __interceptor_malloc (/lib64/libasan.so.5+0xef768)
>>>     #1 0x7fe2a5044445 in g_malloc (/lib64/libglib-2.0.so.0+0x52445)
>>>     #2 0x7fe2a505dd92 in g_strdup (/lib64/libglib-2.0.so.0+0x6bd92)
>>>     #3 0x56344954e692 in qmp_object_add 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/qom/qom-qmp-cmds.c:258
>>>     #4 0x563449960f5a in do_qmp_dispatch 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/qapi/qmp-dispatch.c:132
>>>     #5 0x563449960f5a in qmp_dispatch 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/qapi/qmp-dispatch.c:175
>>>     #6 0x563449498a30 in monitor_qmp_dispatch 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/monitor/qmp.c:145
>>>     #7 0x56344949a64f in monitor_qmp_bh_dispatcher 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/monitor/qmp.c:234
>>>     #8 0x563449a92a3a in aio_bh_call 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/util/async.c:136
>>>
>>> Direct leak of 54 byte(s) in 6 object(s) allocated from:
>>>     #0 0x7fe2a5ebf768 in __interceptor_malloc (/lib64/libasan.so.5+0xef768)
>>>     #1 0x7fe2a5044445 in g_malloc (/lib64/libglib-2.0.so.0+0x52445)
>>>     #2 0x7fe2a505dd92 in g_strdup (/lib64/libglib-2.0.so.0+0x6bd92)
>>>     #3 0x56344954e6c4 in qmp_object_add 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/qom/qom-qmp-cmds.c:267
>>>     #4 0x563449960f5a in do_qmp_dispatch 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/qapi/qmp-dispatch.c:132
>>>     #5 0x563449960f5a in qmp_dispatch 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/qapi/qmp-dispatch.c:175
>>>     #6 0x563449498a30 in monitor_qmp_dispatch 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/monitor/qmp.c:145
>>>     #7 0x56344949a64f in monitor_qmp_bh_dispatcher 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/monitor/qmp.c:234
>>>     #8 0x563449a92a3a in aio_bh_call 
>>> /mnt/sdb/qemu-new/qemu_test/qemu/util/async.c:136
>>>
>>> Fixes: 5f07c4d60d091320186e7b0edaf9ed2cc16b2d1e
>>> Reported-by: Euler Robot <address@hidden>
>>> Signed-off-by: Pan Nengyuan <address@hidden>
>>> ---
>>>  qom/qom-qmp-cmds.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/qom/qom-qmp-cmds.c b/qom/qom-qmp-cmds.c
>>> index 49db926fcc..ac59ba1aa8 100644
>>> --- a/qom/qom-qmp-cmds.c
>>> +++ b/qom/qom-qmp-cmds.c
>>> @@ -247,8 +247,8 @@ void qmp_object_add(QDict *qdict, QObject **ret_data, 
>>> Error **errp)
>>>      QDict *pdict;
>>>      Visitor *v;
>>>      Object *obj;
>>> -    const char *type;
>>> -    const char *id;
>>> +    g_autofree const char *type = NULL;
>>> +    g_autofree const char *id = NULL;
>>
>> not sure that's it's correct
>>
>> qdict_get_try_str() returns the reference to a string within
>> the QDict, so caller who provided qdict should take care of
>> freeing it.
> 
> This is correct, but two lines later we have  "type = g_strdup(type)".
> 
> IOW, the code is storing both a const and non-const string in the
> same variable which is gross.
> 
> So there's definitely a leak, but this is also not the right way
> to fix it.
> 
> To fix it, notice that g_strdup says
> 
>    "Duplicates a string. If str is NULL it returns NULL. The 
>     returned string should be freed with g_free() when no longer
>     needed."
> 
> IOW, instead of
> 
>     const char *type;
> 
>     type = qdict_get_try_str(qdict, "qom-type");
>     if (!type) {
>         error_setg(errp, QERR_MISSING_PARAMETER, "qom-type");
>         return;
>     } else {
>         type = g_strdup(type);
>         qdict_del(qdict, "qom-type");
>     }
> 
> we want
> 
>     g_autofree char *type = NULL;
> 
>     type = g_strdup(qdict_get_try_str(qdict, "qom-type"));
>     if (!type) {
>         error_setg(errp, QERR_MISSING_PARAMETER, "qom-type");
>         return;
>     }
> 
>     qdict_del(qdict, "qom-type");

Good points. Looks very clear.

Thanks.

> 
>>>      type = qdict_get_try_str(qdict, "qom-type");
>>>      if (!type) {
> 
> 
> 
> 
> Regards,
> Daniel
>