Re: [Qemu-devel] [PATCH 3/3] block/gluster: add support for multiple gluster servers

[Peter Krempa]

On Thu, Nov 05, 2015 at 07:45:50 -0500, Prasanna Kumar Kalever wrote:
> On Thursday, November 5, 2015 6:07:06 PM, Prasanna Kumar Kalever wrote:
> > This patch adds a way to specify multiple volfile servers to the gluster
> > block backend of QEMU with tcp|rdma transport types and their port numbers.
> > 
> > Problem:
> > 
> > Currently VM Image on gluster volume is specified like this:
> 

[...]

> > @@ -345,7 +676,7 @@ static int qemu_gluster_open(BlockDriverState *bs, QDict
> > *options,
> >  
> >  out:
> >      qemu_opts_del(opts);
> > -    qemu_gluster_gconf_free(gconf);
> > +    qapi_free_BlockdevOptionsGluster(gconf);
> 
> Can some one help me please ?
> This leads to crash in the second iteration i.e. while freeing 
> "gconf->servers->next->value"

So, prior to this you allocate a array of the data structures as:

+    gsconf = g_new0(GlusterServer, num_servers);
+
+    ptr = qemu_opt_get(opts, GLUSTER_OPT_VOLUME);
+    if (!ptr) {
+        error_setg(&local_err, "Error: qemu_gluster: please provide 'volume' "
+                               "option");
+        goto out;                        
+    }

Then you use the following code to fill the linked list:

+      if (gconf->servers == NULL) {
+            gconf->servers = g_new0(GlusterServerList, 1);
+            gconf->servers->value = &gsconf[i];

So here you set the value. For a i of 0 the '&gsconf[i]' expression will
be a pointer with equal address to 'gsconf'. For explanation:

'gsconf[i]' can be written as '*(gsconf + i)', so
'&gsconf[i]' becomes basically '&(*(gsconf + i))'

This can be also simplified to:
'gsconf + i'. For a i of 0 this becomes the same pointer as 'gsconf'

And once you use that with free(), the whole gsconf array will be freed.
All the other pointers that you've filled to the linked list become
invalid, since they were pointing into the same array that was
completely freed in the first iteration.

Peter

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