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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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).

And _if_ this here is the route to go, I'd clearly see this and your
earlier patch to be folded into just one (dealing with both leaks
in one go).

Jan

> Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
> ---
> 
> !! Not compile tested !!
> 
>  drivers/block/xen-blkback/xenbus.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/xenbus.c 
> b/drivers/block/xen-blkback/xenbus.c
> index a6585a4..8650b19 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -502,7 +502,7 @@ 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 *mode, *device_type;
>  
>       DPRINTK("");
>  
> @@ -528,13 +528,15 @@ static void backend_changed(struct xenbus_watch *watch,
>               return;
>       }
>  
> -     be->mode = xenbus_read(XBT_NIL, dev->nodename, "mode", NULL);
> -     if (IS_ERR(be->mode)) {
> -             err = PTR_ERR(be->mode);
> +     mode = xenbus_read(XBT_NIL, dev->nodename, "mode", NULL);
> +     kfree(be->mode);
> +     if (IS_ERR(mode)) {
> +             err = PTR_ERR(mode);
>               be->mode = NULL;
>               xenbus_dev_fatal(dev, err, "reading mode");
>               return;
>       }
> +     be->mode = mode;
>  
>       device_type = xenbus_read(XBT_NIL, dev->otherend, "device-type", NULL);
>       if (!IS_ERR(device_type)) {
> @@ -555,7 +557,7 @@ static void backend_changed(struct xenbus_watch *watch,
>               be->minor = minor;
>  
>               err = xen_vbd_create(be->blkif, handle, major, minor,
> -                              (NULL == strchr(be->mode, 'w')), cdrom);
> +                              (NULL == strchr(mode, 'w')), cdrom);
>               if (err) {
>                       be->major = 0;
>                       be->minor = 0;
> -- 
> 1.8.0.1




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