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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Mar 09, 2020 at 10:51:45AM +0100, Igor Mammedov wrote:
> On Mon, 9 Mar 2020 17:22:12 +0800
> Pan Nengyuan <pannengyuan@huawei.com> 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 <euler.robot@huawei.com>
> > Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
> > ---
> >  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");

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




Regards,
Daniel
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