Re: [PATCH v3] hugetlbfs: check for pgoff value overflow

[Michal Hocko]

On Fri 16-03-18 09:19:07, Mike Kravetz wrote:
> On 03/16/2018 03:17 AM, Michal Hocko wrote:
> > On Thu 08-03-18 16:27:26, Mike Kravetz wrote:
> > 
> > OK, looks good to me. Hairy but seems to be the easiest way around this.
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > 
> <snip>
> >> +/*
> >> + * Mask used when checking the page offset value passed in via system
> >> + * calls.  This value will be converted to a loff_t which is signed.
> >> + * Therefore, we want to check the upper PAGE_SHIFT + 1 bits of the
> >> + * value.  The extra bit (- 1 in the shift value) is to take the sign
> >> + * bit into account.
> >> + */
> >> +#define PGOFF_LOFFT_MAX (PAGE_MASK << (BITS_PER_LONG - (2 * PAGE_SHIFT) - 1))
> 
> Thanks Michal,
> 
> However, kbuild found a problem with this definition on certain configs.
> Consider a config where,
> BITS_PER_LONG = 32 (32bit config)
> PAGE_SHIFT = 16 (64K pages)
> This results in the negative shift value.
> Not something I would not immediately think of, but a valid config.

Well, 64K pages on 32b doesn't sound even remotely sane to me but what
ever.

> The definition has been changed to,
> #define PGOFF_LOFFT_MAX \
> 	(((1UL << (PAGE_SHIFT + 1)) - 1) <<  (BITS_PER_LONG - (PAGE_SHIFT + 1)))
> 
> as discussed here,
> http://lkml.kernel.org/r/432fb2a3-b729-9c3a-7d60-890b8f9b10dd@xxxxxxxxxx

This looks more wild but seems correct as well. You can keep my acked-by

Thanks!
-- 
Michal Hocko
SUSE Labs