linux-kernel - Re: [Xen-devel] [PATCH 1/1] xen-netfront: do not cast grant table reference to signed short

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Date:   Mon, 31 Oct 2016 01:47:56 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Dongli Zhang" <dongli.zhang@...cle.com>
Cc:     <david.vrabel@...rix.com>, <xen-devel@...ts.xenproject.org>,
        <boris.ostrovsky@...cle.com>, "Juergen Gross" <JGross@...e.com>,
        <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 1/1] xen-netfront: do not cast grant
 table reference to signed short

>>> On 31.10.16 at 06:38, <dongli.zhang@...cle.com> wrote:
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -304,7 +304,7 @@ static void xennet_alloc_rx_buffers(struct netfront_queue *queue)
>  		queue->rx_skbs[id] = skb;
>  
>  		ref = gnttab_claim_grant_reference(&queue->gref_rx_head);
> -		BUG_ON((signed short)ref < 0);
> +		WARN_ON_ONCE(IS_ERR_VALUE((unsigned long)ref));

You really need to cast to plain (or signed) long here - casting to
unsigned long will work only in 32-bit configurations, as otherwise
you lose the sign of the value.

And then just issuing a warning here is insufficient, I think: Either
you follow David's line of thought assuming that no failure here is
possible at all (in which case the BUG_ON() can be ditched without
replacement), or you follow your original one (which matches mine)
that we can't exclude an error here because of a bug elsewhere,
in which case this either needs to stay BUG_ON() or should be
followed by some form of bailing out (so that the bad ref won't get
stored, preventing its later use from causing further damage).

Jan