Re: [Xen-devel] [PATCH] xen-kbdfront: correct return value checks on xenbus_scanf()

[Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>]

On Thu, Jul 07, 2016 at 01:53:40AM -0600, Jan Beulich wrote:
> Only a positive return value indicates success.

If I am reading this correctly xenbus_scanf() guarantees that it will
not return 0. From include/xen/xenbus.h:

/* Single read and scanf: returns -errno or num scanned if > 0. */
__scanf(4, 5)
int xenbus_scanf(struct xenbus_transaction t,
                 const char *dir, const char *node, const char *fmt, ...);

and the code matches:

        ...
        /* Distinctive errno. */
        if (ret == 0)
                return -ERANGE;
        return ret;

> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
>  drivers/input/misc/xen-kbdfront.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> --- 4.7-rc6-xenbus_scanf.orig/drivers/input/misc/xen-kbdfront.c
> +++ 4.7-rc6-xenbus_scanf/drivers/input/misc/xen-kbdfront.c
> @@ -127,7 +127,8 @@ static int xenkbd_probe(struct xenbus_de
>       if (!info->page)
>               goto error_nomem;
>  
> -     if (xenbus_scanf(XBT_NIL, dev->otherend, "feature-abs-pointer", "%d", 
> &abs) < 0)
> +     if (xenbus_scanf(XBT_NIL, dev->otherend,
> +                      "feature-abs-pointer", "%d", &abs) <= 0)
>               abs = 0;
>       if (abs) {
>               ret = xenbus_printf(XBT_NIL, dev->nodename,
> @@ -324,7 +325,7 @@ static void xenkbd_backend_changed(struc
>  InitWait:
>               ret = xenbus_scanf(XBT_NIL, info->xbdev->otherend,
>                                  "feature-abs-pointer", "%d", &val);
> -             if (ret < 0)
> +             if (ret <= 0)
>                       val = 0;
>               if (val) {
>                       ret = xenbus_printf(XBT_NIL, info->xbdev->nodename,
> 
> 

Thanks.

-- 
Dmitry

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