linux-kernel - Re: [PATCH] net: stmmac: xgmac: add missing parentheses to fix precendence error

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Message-ID: <20191002140733.GQ29696@kadam>
Date:   Wed, 2 Oct 2019 17:07:33 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Colin Ian King <colin.king@...onical.com>
Cc:     Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S . Miller" <davem@...emloft.net>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: stmmac: xgmac: add missing parentheses to fix
 precendence error

On Wed, Oct 02, 2019 at 02:53:17PM +0100, Colin Ian King wrote:
> On 02/10/2019 14:42, Dan Carpenter wrote:
> > On Wed, Oct 02, 2019 at 04:33:57PM +0300, Dan Carpenter wrote:
> >> On Wed, Oct 02, 2019 at 12:08:49PM +0100, Colin King wrote:
> >>> From: Colin Ian King <colin.king@...onical.com>
> >>>
> >>> The expression !(hw_cap & XGMAC_HWFEAT_RAVSEL) >> 10 is always zero, so
> >>> the masking operation is incorrect. Fix this by adding the missing
> >>> parentheses to correctly bind the negate operator on the entire expression.
> >>>
> >>> Addresses-Coverity: ("Operands don't affect result")
> >>> Fixes: c2b69474d63b ("net: stmmac: xgmac: Correct RAVSEL field interpretation")
> >>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> >>> ---
> >>>  drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c | 2 +-
> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c b/drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c
> >>> index 965cbe3e6f51..2e814aa64a5c 100644
> >>> --- a/drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c
> >>> +++ b/drivers/net/ethernet/stmicro/stmmac/dwxgmac2_dma.c
> >>> @@ -369,7 +369,7 @@ static void dwxgmac2_get_hw_feature(void __iomem *ioaddr,
> >>>  	dma_cap->eee = (hw_cap & XGMAC_HWFEAT_EEESEL) >> 13;
> >>>  	dma_cap->atime_stamp = (hw_cap & XGMAC_HWFEAT_TSSEL) >> 12;
> >>>  	dma_cap->av = (hw_cap & XGMAC_HWFEAT_AVSEL) >> 11;
> >>> -	dma_cap->av &= !(hw_cap & XGMAC_HWFEAT_RAVSEL) >> 10;
> >>> +	dma_cap->av &= !((hw_cap & XGMAC_HWFEAT_RAVSEL) >> 10);
> >>
> >> There is no point to the shift at all.
> > 
> > Sorry I meant to say it should be a bitwise NOT, right?  I was just
> > looking at some other dma_cap stuff that did this same thing...  I can't
> > find it now...
> 
> In drivers/net/ethernet/stmicro/stmmac/stmmac_tc.c it is being used like
> a boolean and not a bitmask'd value:
> 
>         if (!priv->dma_cap.av)
> 
> so the original logic is to do boolean flag merging rather than bit-wise
> logic.

Oh yeah.  Thanks.  This code is hard to read.

It would be better to just write it like this:

	if (hw_cap & XGMAC_HWFEAT_AVSEL) && !(hw_cap & XGMAC_HWFEAT_RAVSEL)
		dma_cap->av = true;
	else
		dma_cap->av = false;

All these very shifts are concise but they introduce bugs like this one
you have found.

regards,
dan carpenter