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

[Dan Carpenter]

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?

regards,
dan carpenter