linux-kernel - Re: [PATCH] net: stmmac: ensure jumbo_frm error return is correctly checked for -ve value

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Message-ID: <40331b09-5829-9041-f815-7f1426960bd8@canonical.com>
Date:   Sat, 3 Jun 2017 17:35:22 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        netdev <netdev@...r.kernel.org>, kernel-janitors@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: stmmac: ensure jumbo_frm error return is correctly
 checked for -ve value

On 03/06/17 16:55, Andy Shevchenko wrote:
> On Fri, Jun 2, 2017 at 5:58 PM, Colin King <colin.king@...onical.com> wrote:
>> The current comparison of entry < 0 will never be true since entry is an
>> unsigned integer. Cast entry to an int to ensure -ve error return values
>> from the call to jumbo_frm are correctly being caught.
> 
>>         if (unlikely(is_jumbo) && likely(priv->synopsys_id <
>>                                          DWMAC_CORE_4_00)) {
>>                 entry = priv->hw->mode->jumbo_frm(tx_q, skb, csum_insertion);
>> -               if (unlikely(entry < 0))
>> +               if (unlikely((int)entry < 0))
> 
> It feels like a hiding some other issue.
> 

The alternative is:

		int rc = priv->hw->mode->jumbo_frm(tx_q, skb, csum_insertion);
		if (unlikely(rc < 0))
			goto dma_map_err;

		entry = rc;

however, that is effectively the same. The cast I'm using is a well used
idiom in the kernel, it used in almost a hundred similar cases.

git grep "< 0" | grep "(int)" | wc -l
95

Colin