Subject: Re: [PATCH] powerpc/powernv/npu: Allocate enough memory in pnv_try_setup_npu_table_group()

From: Dan Carpenter <[email-masked]>
Date: Sat, 12 Jan 2019 13:51:05 +0300

On Sat, Jan 12, 2019 at 01:34:42AM -0600, Segher Boessenkool wrote:
> On Sat, Jan 12, 2019 at 08:44:26AM +0300, Dan Carpenter wrote:
> > On Sat, Jan 12, 2019 at 11:30:35AM +1100, Balbir Singh wrote:
> > > On Wed, Jan 09, 2019 at 01:23:29PM +0300, Dan Carpenter wrote:
> > > > There is a typo so we accidentally allocate enough memory for a pointer
> > > > when we wanted to allocate enough for a struct.
> > > > 
> > > > Fixes: 0bd971676e68 ("powerpc/powernv/npu: Add compound IOMMU groups")
> > > > Signed-off-by: Dan Carpenter <[email-masked]>
> > > > ---
> > > >  arch/powerpc/platforms/powernv/npu-dma.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/arch/powerpc/platforms/powernv/npu-dma.c b/arch/powerpc/platforms/powernv/npu-dma.c
> > > > index d7f742ed48ba..3f58c7dbd581 100644
> > > > --- a/arch/powerpc/platforms/powernv/npu-dma.c
> > > > +++ b/arch/powerpc/platforms/powernv/npu-dma.c
> > > > @@ -564,7 +564,7 @@ struct iommu_table_group *pnv_try_setup_npu_table_group(struct pnv_ioda_pe *pe)
> > > >  		}
> > > >  	} else {
> > > >  		/* Create a group for 1 GPU and attached NPUs for POWER8 */
> > > > -		pe->npucomp = kzalloc(sizeof(pe->npucomp), GFP_KERNEL);
> > > > +		pe->npucomp = kzalloc(sizeof(*pe->npucomp), GFP_KERNEL);
> > > 
> > > To avoid these in the future, I wonder if instead of sizeof(pe->npucomp), we insist on
> > > sizeof structure
> > > 
> > > pe->npucomp = kzalloc(sizeof(struct npucomp), GFP_KERNEL);
> > > 
> > 
> > The latest kernel fashion is sizeof(*ptr).  It can go wrong either way.
> > I don't have strong feelings about it.  These sorts of bugs don't last
> > long because they're caught in testing or with static analysis.
> 
> And it is easy to see someone forgot the * in "sizeof *ptr", and with
> experience it will just automatically look wrong if it is forgotten; but
> it isn't obvious at all if the wrong struct is used, which cannot happen
> with the *ptr form, but happens frequently with the "sizeof(struct x)"
> form.

It doesn't happen very frequently.  I look for this with Smatch.  The
results I see are when the first few members of a struct are a header
and the actual size can vary.

	hdr = alloc(sizeof(struct larger_struct));

regards,
dan carpenter