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

[Olaf Hering <olaf@xxxxxxxxx>]

On Thu, Dec 06, Jan Beulich wrote:

> >>> On 06.12.12 at 17:23, Olaf Hering <olaf@xxxxxxxxx> wrote:
> > 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.
> 
> Maybe, but it looks odd to me. But then again I had hoped Konrad
> would have an opinion here...

Looking at this some more, if backend_changed is supposed to be called
exactly once then major/minor checks can be removed because they will be
always zero, like this:


 drivers/block/xen-blkback/xenbus.c | 68 ++++++++++++++++++--------------------
 1 file changed, 32 insertions(+), 36 deletions(-)

diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index a6585a4..5ca77c3 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;
@@ -502,10 +503,14 @@ static void backend_changed(struct xenbus_watch *watch,
                = container_of(watch, struct backend_info, backend_watch);
        struct xenbus_device *dev = be->dev;
        int cdrom = 0;
-       char *device_type;
+       char *device_type, *p;
+       long handle;
 
        DPRINTK("");
 
+       if (be->alive)
+               return;
+
        err = xenbus_scanf(XBT_NIL, dev->nodename, "physical-device", "%x:%x",
                           &major, &minor);
        if (XENBUS_EXIST_ERR(err)) {
@@ -521,12 +526,7 @@ static void backend_changed(struct xenbus_watch *watch,
                return;
        }
 
-       if ((be->major || be->minor) &&
-           ((be->major != major) || (be->minor != minor))) {
-               pr_warn(DRV_PFX "changing physical device (from %x:%x to %x:%x) 
not supported.\n",
-                       be->major, be->minor, major, minor);
-               return;
-       }
+       be->alive = 1;
 
        be->mode = xenbus_read(XBT_NIL, dev->nodename, "mode", NULL);
        if (IS_ERR(be->mode)) {
@@ -542,39 +542,35 @@ static void backend_changed(struct xenbus_watch *watch,
                kfree(device_type);
        }
 
-       if (be->major == 0 && be->minor == 0) {
-               /* Front end dir is a number, which is used as the handle. */
-
-               char *p = strrchr(dev->otherend, '/') + 1;
-               long handle;
-               err = strict_strtoul(p, 0, &handle);
-               if (err)
-                       return;
-
-               be->major = major;
-               be->minor = minor;
+       /* Front end dir is a number, which is used as the handle. */
+       p = strrchr(dev->otherend, '/') + 1;
+       err = strict_strtoul(p, 0, &handle);
+       if (err)
+               return;
 
-               err = xen_vbd_create(be->blkif, handle, major, minor,
-                                (NULL == strchr(be->mode, 'w')), cdrom);
-               if (err) {
-                       be->major = 0;
-                       be->minor = 0;
-                       xenbus_dev_fatal(dev, err, "creating vbd structure");
-                       return;
-               }
+       be->major = major;
+       be->minor = minor;
 
-               err = xenvbd_sysfs_addif(dev);
-               if (err) {
-                       xen_vbd_free(&be->blkif->vbd);
-                       be->major = 0;
-                       be->minor = 0;
-                       xenbus_dev_fatal(dev, err, "creating sysfs entries");
-                       return;
-               }
+       err = xen_vbd_create(be->blkif, handle, major, minor,
+                        (NULL == strchr(be->mode, 'w')), cdrom);
+       if (err) {
+               be->major = 0;
+               be->minor = 0;
+               xenbus_dev_fatal(dev, err, "creating vbd structure");
+               return;
+       }
 
-               /* We're potentially connected now */
-               xen_update_blkif_status(be->blkif);
+       err = xenvbd_sysfs_addif(dev);
+       if (err) {
+               xen_vbd_free(&be->blkif->vbd);
+               be->major = 0;
+               be->minor = 0;
+               xenbus_dev_fatal(dev, err, "creating sysfs entries");
+               return;
        }
+
+       /* We're potentially connected now */
+       xen_update_blkif_status(be->blkif);
 }
 
 
-- 
1.8.0.1



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