Re: [Xen-devel] [PATCH] xen/blkback: prevent leak of mode during multiple backend_changed calls

[Olaf Hering <olaf@xxxxxxxxx>]

On Wed, Dec 05, Jan Beulich wrote:

> >>> On 05.12.12 at 11:01, Olaf Hering <olaf@xxxxxxxxx> wrote:
> > backend_changed might be called multiple times, which will leak
> > be->mode. free the previous value before storing the current mode value.
> 
> As said before - this is one possible route to take. But did you
> consider at all the alternative of preventing the function from
> getting called more than once for a given device? As also said
> before, I think that would have other bad effects, and hence
> should be preferred (and would likely also result in a smaller
> patch).

Maybe it could be done like this, adding a flag to the backend device
and exit early if its called twice.

Olaf


diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index a6585a4..2822e73 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -28,6 +28,7 @@ struct backend_info {
        unsigned                major;
        unsigned                minor;
        char                    *mode;
+       unsigned                alive;
 };
 
 static struct kmem_cache *xen_blkif_cachep;
@@ -506,6 +507,9 @@ static void backend_changed(struct xenbus_watch *watch,
 
        DPRINTK("");
 
+       if (be->alive)
+               return;
+
        err = xenbus_scanf(XBT_NIL, dev->nodename, "physical-device", "%x:%x",
                           &major, &minor);
        if (XENBUS_EXIST_ERR(err)) {
@@ -548,8 +552,11 @@ static void backend_changed(struct xenbus_watch *watch,
                char *p = strrchr(dev->otherend, '/') + 1;
                long handle;
                err = strict_strtoul(p, 0, &handle);
-               if (err)
+               if (err) {
+                       kfree(be->mode);
+                       be->mode = NULL;
                        return;
+               }
 
                be->major = major;
                be->minor = minor;
@@ -560,6 +567,8 @@ static void backend_changed(struct xenbus_watch *watch,
                        be->major = 0;
                        be->minor = 0;
                        xenbus_dev_fatal(dev, err, "creating vbd structure");
+                       kfree(be->mode);
+                       be->mode = NULL;
                        return;
                }
 
@@ -569,10 +578,13 @@ static void backend_changed(struct xenbus_watch *watch,
                        be->major = 0;
                        be->minor = 0;
                        xenbus_dev_fatal(dev, err, "creating sysfs entries");
+                       kfree(be->mode);
+                       be->mode = NULL;
                        return;
                }
 
                /* We're potentially connected now */
+               be->alive = 1;
                xen_update_blkif_status(be->blkif);
        }
 }
-- 
1.8.0.1


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