linux-kernel - Re: [PATCH] scsi: mvumi: fix 32 bit shift of a 32 bit unsigned int

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Date:   Mon, 18 Feb 2019 18:47:03 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     James Bottomley <jejb@...ux.ibm.com>
Cc:     Walter Harms <wharms@....de>,
        Colin King <colin.king@...onical.com>,
        Jianyun Li <jyli@...vell.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scsi: mvumi: fix 32 bit shift of a 32 bit unsigned int

On Mon, Feb 18, 2019 at 07:32:05AM -0800, James Bottomley wrote:
> On Mon, 2019-02-18 at 12:37 +0300, Dan Carpenter wrote:
> > On Sat, Feb 16, 2019 at 05:27:16PM +0100, Walter Harms wrote:
> > > Am 16.02.2019 15:44, schrieb Colin King:
> > > > From: Colin Ian King <colin.king@...onical.com>
> > > > 
> > > > Currently m_sg->baseaddr_h (a 32 bit unsigned int) is being
> > > > shifted by a
> > > > total of 32 bits; this always produces a 0 result.  Fix this by
> > > > casting
> > > > it to a dma_addr_t (a 64 bit unsigned int) before performing the
> > > > shift.
> > > > 
> > > > Detected by CoverityScan, CID#147270 ("Operands don't affect
> > > > result")
> > > > 
> > > > Fixes: f0c568a478f0 ("[SCSI] mvumi: Add Marvell UMI driver")
> > > > Signed-off-by: Colin Ian King <colin.king@...onical.com>
> > > > ---
> > > >  drivers/scsi/mvumi.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/scsi/mvumi.c b/drivers/scsi/mvumi.c
> > > > index 36f64205ecfa..d3582accfd09 100644
> > > > --- a/drivers/scsi/mvumi.c
> > > > +++ b/drivers/scsi/mvumi.c
> > > > @@ -313,7 +313,7 @@ static void mvumi_delete_internal_cmd(struct
> > > > mvumi_hba
> > > > *mhba,
> > > >  			sgd_getsz(mhba, m_sg, size);
> > > >  
> > > >  			phy_addr = (dma_addr_t) m_sg->baseaddr_l 
> > > > |
> > > > -				(dma_addr_t) ((m_sg->baseaddr_h
> > > > << 16) << 16);
> > > > +				(((dma_addr_t) m_sg->baseaddr_h
> > > > << 16) << 16);
> > > >  
> > > >  			dma_free_coherent(&mhba->pdev->dev,
> > > > size, cmd->data_buf,
> > > >  								
> > > > phy_addr);
> > > 
> > > i would suggest to try a version with less casts to make it more
> > > readable
> > > like this untested suggestion:
> > > 
> > > phy_addr =(m_sg->baseaddr_h << 16)| m_sg->baseaddr_l;
> > > phy_addr <<= 16;
> > > 
> > 
> > That would be a behavior change but it also might be a bugfix?  Why
> > doesn't the code just do:
> > 
> > 	phy_addr = ((dma_addr_t)m_sg->baseaddr_h << 32) | m_sg-
> > >baseaddr_l;
> > 
> > (Probably they broke it up into two shifts to silence a GCC warning
> > that the shift was wrong because of the missing cast?)
> 
> No because dma_addr_t can be 32 bits and the warning would then always
> appear on some builds.  The << 16 << 16 makes sure it doesn't.
> 

Yeah.  You're right.  Thanks.

The original patch is the right fix.  Although it sort of feels like the
double shift should be a macro.

/*
 * The dma_addr_t type can be either 32 or 64 bit.  Left shifting a 32
 * bit number is undefined so this do two 16 bit left shifts.
 *
 */
#define DMA_LSHIFT_32(val) (((dma_addr_t)(val) << 16) << 16)

regards,
dan carpenter