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

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Date:   Sat, 3 Jun 2017 20:18:32 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Colin Ian King <colin.king@...onical.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 Sat, Jun 3, 2017 at 7:35 PM, Colin Ian King <colin.king@...onical.com> wrote:
> 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;

Looks cleaner, though I was thinking about changing type of entry as
Julia suggested. Needs to be checked carefully anyway.

> 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

With refined grep I'e got only 56 so far. And I see it's spread only
in 45 modules, most cases are single use cases.
So, I hardly can tell this is an idiom in kernel.

-- 
With Best Regards,
Andy Shevchenko