Re: [Xen-devel] [PATCH 1/1] xen-netfront: do not cast grant table reference to signed short
From: Jan Beulich <hidden>
Date: 2016-10-31 07:48:02
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On 31.10.16 at 06:38, [off-list ref] 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