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

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Message-ID: <alpine.DEB.2.20.1706031847280.2014@hadrien>
Date:   Sat, 3 Jun 2017 18:53:36 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     Colin Ian King <colin.king@...onical.com>
cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        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, 3 Jun 2017, Colin Ian King 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;
>
> 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

Does entry really have to be unsigned?  The jumbo_frm function returns an
int, not an unsigned int, so it seems unpleasant to make it unsigned
prematurely just to put a cast afterwards.  The remaining computation
seems to involve only small numbers.

julia