Re: [PATCH] staging: vc04_services: shut up out-of-range warning

[Phil Elwell]

Hi Dan,

On 27/09/2021 13:26, Dan Carpenter wrote:
> On Mon, Sep 27, 2021 at 01:36:56PM +0200, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@xxxxxxxx>
> >
> > The comparison against SIZE_MAX produces a harmless warning on 64-bit
> > architectures:
> >
> > drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c:185:16: error: result of comparison of constant 419244183493398898 with expression of type 'unsigned int' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
> >          if (num_pages > (SIZE_MAX - sizeof(struct pagelist) -
> >              ~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > Shut up that warning by adding a cast to a longer type.
> >
> > Fixes: ca641bae6da9 ("staging: vc04_services: prevent integer overflow in create_pagelist()")
> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> > ---
> >   drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> > index b25369a13452..967f10b9582a 100644
> > --- a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> > +++ b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> > @@ -182,7 +182,7 @@ create_pagelist(char *buf, char __user *ubuf,
> >   		offset = (uintptr_t)ubuf & (PAGE_SIZE - 1);
> >   	num_pages = DIV_ROUND_UP(count + offset, PAGE_SIZE);
> >   
> > -	if (num_pages > (SIZE_MAX - sizeof(struct pagelist) -
> > +	if ((size_t)num_pages > (SIZE_MAX - sizeof(struct pagelist) -
> >   			 sizeof(struct vchiq_pagelist_info)) /
> >   			(sizeof(u32) + sizeof(pages[0]) +
> >   			 sizeof(struct scatterlist)))
>
> The temptation would be to declare "num_pages" as size_t instead of
> adding this cost.  But then something will complain about the
> "pagelistinfo->num_pages = num_pages;" assignment because
> "pagelistinfo->num_pages" is a u32.
>
> The next temptation is to change the SIZE_MAX to UINT_MAX.  I didn't
> do that originally because I can't test this and I was trying not to
> break things...  We probably still don't want to break things, but maybe
> there is someone who is more familiar with this who knows if UINT_MAX is
> okay?

The VPU can't address more than 1GB directly, so UINT_MAX is more than sufficient.

Phil