linux-kernel - Re: [PATCH][next] iommu/mediatek: Fix unsigned domid comparison with less than zero

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Date:   Tue, 9 Feb 2021 12:19:23 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Will Deacon <will@...nel.org>
Cc:     Colin King <colin.king@...onical.com>,
        Joerg Roedel <joro@...tes.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Anan sun <anan.sun@...iatek.com>,
        Yong Wu <yong.wu@...iatek.com>,
        Chao Hao <chao.hao@...iatek.com>,
        Tomasz Figa <tfiga@...omium.org>,
        iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] iommu/mediatek: Fix unsigned domid comparison with
 less than zero

On Thu, Feb 04, 2021 at 09:25:58AM +0000, Will Deacon wrote:
> On Wed, Feb 03, 2021 at 01:59:36PM +0000, Colin King wrote:
> > From: Colin Ian King <colin.king@...onical.com>
> > 
> > Currently the check for domid < 0 is always false because domid
> > is unsigned.  Fix this by making it signed.
> > 
> > Addresses-CoverityL ("Unsigned comparison against 0")
> 
> Typo here ('L' instead of ':')
> 
> > Fixes: ab1d5281a62b ("iommu/mediatek: Add iova reserved function")
> > Signed-off-by: Colin Ian King <colin.king@...onical.com>
> > ---
> >  drivers/iommu/mtk_iommu.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/iommu/mtk_iommu.c b/drivers/iommu/mtk_iommu.c
> > index 0ad14a7604b1..823d719945b2 100644
> > --- a/drivers/iommu/mtk_iommu.c
> > +++ b/drivers/iommu/mtk_iommu.c
> > @@ -640,7 +640,7 @@ static void mtk_iommu_get_resv_regions(struct device *dev,
> >  				       struct list_head *head)
> >  {
> >  	struct mtk_iommu_data *data = dev_iommu_priv_get(dev);
> > -	unsigned int domid = mtk_iommu_get_domain_id(dev, data->plat_data), i;
> > +	int domid = mtk_iommu_get_domain_id(dev, data->plat_data), i;
> 
> Not sure if it's intentional, but this also makes 'i' signed. It probably
> should remain 'unsigned' to match 'iova_region_nr' in
> 'struct mtk_iommu_plat_data'.


iova_region_nr is either 1 or 5 so unsigned doesn't matter.

I once almost introduced a bug where the iterator was supposed to be
size_t.  I fixed a bug by making it signed but I ended up introducing a
new bug.  But generally that's pretty rare.  The more common case is
that making iterators unsigned introduces bugs.

It's better to default to "int i;" and if more complicated types are
required that should stand out.  "size_t pg_idx;" or whatever.

regards,
dan carpenter